Heat Flux Processes in Streams and Their Impact on Coldwater and Coolwater Fishes

The deluge of rain that soaked the lower Susquehanna watershed during last week is now just a memory.  Streams to the west of the river, where the flooding courtesy of the remnants of Hurricane Debby was most severe, have reached their crest and receded.  Sliding away toward the Chesapeake and Atlantic is all that runoff, laden with a brew of pollutants including but not limited to: agricultural nutrients, sediment, petroleum products, sewage, lawn chemicals, tires, dog poop, and all that litter—paper, plastics, glass, Styrofoam, and more.  For aquatic organisms including our freshwater fish, these floods, particularly when they occur in summer, can compound the effects of the numerous stressors that already limit their ability to live, thrive, and reproduce.

(Environmental Protection Agency image)

One of those preexisting stressors, high water temperature, can be either intensified or relieved by summertime precipitation.  Runoff from forested or other densely vegetated ground normally has little impact on stream temperature.  But segments of waterways receiving significant volumes of runoff from areas of sun-exposed impervious ground will usually see increases during at least the early stages of a rain event.  Fortunately, projects implemented to address the negative impacts of stormwater flow and stream impairment can often have the additional benefit of helping to attenuate sudden rises in stream temperature.

Stream Subjected to Agricultural Runoff
While a row of trees along a creek can help provide protection from the thermal impact of the sun, a vegetative riparian buffer must be much wider to be effective for absorbing, cooling, and treating runoff from fields, lawns, and paved surfaces.  This buffer is too narrow to prevent surface runoff from polluting the water.

Of the fishes inhabiting the Lower Susquehanna River Watershed’s temperate streams, the least tolerant of summer warming are the trouts and sculpins—species often described as “coldwater fishes”.  Coldwater fishes require water temperatures below 70° Fahrenheit to thrive and reproduce.  The optimal temperature range is 50° to 65° F.  In the lower Susquehanna valley, few streams are able to sustain trouts and sculpins through the summer months—largely due to the effects of warm stormwater runoff and other forms of impairment.

Blue Ridge Sculpin
Sculpins, including the Blue Ridge Sculpin (Cottus caeruleomentum) seen here, are native coldwater fishes which, during the 11,000 years since the last glacial maximum, have had the availability of their favored habitat sharply reduced by warming water temperatures and a rising Atlantic.  During this interval, seawater has inundated the path of the “Late” Pleistocene lower Susquehanna which passed through the section of flooded river watershed we now call Chesapeake Bay and continued across the continental shelf to what was, during the glacial maximum, the river’s mouth at Norfolk Canyon.  Today, cut off from neighboring drainage basins, sculpins survive exclusively in cold headwaters, and only in those where human alterations including pollution, dams, channelization, and reduced base flow haven’t yet eliminated their isolated populations.  Formerly believed to be composed of two widespread North American species, the Slimy Sculpin (Cottus cognatus) and the Mottled Sculpin (Cottus bairdii), study in recent decades is discovering that sculpin populations in the present-day lower Susquehanna and neighboring Potomac headwaters consist of at least three newly delineated species: Blue Ridge Sculpin, Potomac Sculpin (Cottus gerardi), and Checkered Sculpin (Cottus sp.), the latter an as yet undescribed species found only in the refugium of limestone springs in the Potomac drainage in West Virginia; Frederick and Washington Counties, Maryland; and Franklin County, Pennsylvania.  (United States Geological Survey image)
Ice Age Susquehanna
Stare at this for a little while, you’ll figure it out…………More than 11,000 years ago, during the last glacial maximum, when sea level was about 275 feet lower than it is today, there was no Chesapeake Bay, just a great Susquehanna River that flowed to the edge of the continental shelf and its mouth at Norfolk Canyon.  It was a river draining taiga forests of pine, spruce , and fir, and it carried along the waters of all the present-day bay’s tributaries and more.  The section of the river’s watershed we presently call the lower Susquehanna was, at the time, the upper Susquehanna watershed.  Brook Trout and sculpins had the run of the river and its tributaries back then.  And the entire watershed was a coldwater fishery, with limestone and other groundwater springs providing not refuge from summer heat, but a place to escape freezing water.  (United States Geological Survey base image)
Norfolk Canyon, the mouth of the Susquehanna River during the most recent glacial maximum, now lies more than 275 feet below the surface of the ocean and plunges to more than a mile in depth along the finger of out wash from the gorge.  (United States Geological Survey image)
Rainbow. Brown, and Brook Trout
Tens of thousands of trout are raised in state-operated and cooperative nurseries for stocking throughout the lower Susquehanna valley.  These rearing facilities are located on spring-fed headwaters with sufficient flow to assure cold temperatures year round.  While the Rainbow Trout and Brown Trout (Salmo trutta) are the most commonly stocked species, the Brook Trout (Salvelinus fontinalis) is the only one native to American waters.  It is the least tolerant of stream warming and still reproduces in the wild only in a few pristine headwaters streams in the region.  During spring, all three of these species have been observed on rare occasions entering the fish lift facilities at the hydroelectric dams on the river, presumably returning to the Susquehanna as sea-run trout.

Coldwater fishes are generally found in small spring-fed creeks and  headwaters runs. Where stream gradient, substrate, dissolved oxygen, and other parameters are favorable, some species may be tolerant of water warmer than the optimal values.  In other words, these temperature classifications are not set in stone and nobody ever explained ichthyology to a fish, so there are exceptions.  The Brown Trout for example is sometimes listed as a “coldwater transition fish”, able to survive and reproduce in waters where stream quality is exceptionally good but the temperature may periodically reach the mid-seventies.

Eastern Blacknose Dace
The Eastern Blacknose Dace is sometimes classified as a “coldwater transition fish”.   It can be found in headwaters runs as well as in creeks with good water quality.
Longnose Dace
The Longnose Dace is another “coldwater transition fish” known only from clear, clean, flowing waters.

More tolerant of summer heat than the trouts, sculpins, and daces are the “coolwater fishes”—species able to feed, grow, and reproduce in streams with a temperature of less than 80° F, but higher than 60° F.  Coolwater fishes thrive in creeks and rivers that hover in the 65° to 70° F range during summer.

Creek Chubs
The Creek Chub is a familiar species of “coolwater fish” seldom found remaining in waters exceeding 80 degrees Fahrenheit.
The Yellow Perch (Perca flavescens) was perhaps the most frequently targeted coolwater “gamefish” in the Lower Susquehanna River Watershed prior to the introduction of the Northern Pike (Esox lucius) and Muskellunge (Esox masquinongy).  Today’s prevalence of warmwater streams and the dozens of species of non-native predatory fishes now naturalized within them have left the Yellow Perch populations greatly reduced and all but forgotten by anglers.  Out of sight, out of mind.  (National Park Service image)

What are the causes of modern-day reductions in coldwater and coolwater fish habitats in the lower Susquehanna River and its hundreds of miles of tributaries?  To answer that, let’s take a look at the atmospheric, cosmic, and hydrologic processes that impact water temperature.  Technically, these processes could be measured as heat flux—the rate of heat energy transfer per unit area per unit time, frequently expressed as watts per meter squared (W/m²).  Without getting too technical, we’ll just take a look at the practical impact these processes have on stream temperatures.

HEAT FLUX PROCESSES IN A SEGMENT OF STREAM

Heat Flux Processes on Stream and River Segments.  These processes could be measured as heat flux—the rate of heat energy transfer per unit area per unit time.  (Environmental Protection Agency image)
      • INCOMING TEMPERATURE AND FLOW—The baseline temperature of stream water entering a given segment of waterway is obviously the chief factor determining its temperature when exiting that segment.  Incoming temperature and flow also determine the water’s susceptibility to heat absorption or loss while transiting the segment.  Lower flows may subject the given volume of water to a greater loss or gain of heat energy during the time needed to pass through the segment than the same volume at a higher flow.  Lower flows may also reduce stream velocity and extend a given volume of water’s exposure time to the exchange of heat energy while moving through the segment.  Generally speaking…
        1. …the higher the stream flow, the less a given volume of that stream’s  water may be impacted by the effects of the heat flux processes within the segment.
        2. …the lower the stream flow, the more a given volume of that stream’s water may be impacted by the effects of the heat flux processes within that segment.
        3. …the temperature and flow rate of precipitation entering the segment are factors that determine the impact of its heat energy transfer to or from a given volume of the stream’s waters.
        4. …the temperature and flow rate of runoff and point-source discharges entering the segment are factors that determine the impact of their heat energy transfer to or from a given volume of the stream’s waters.
Stormwater Discharge into Channelized Creek
Stormwater from impervious surfaces including roads, parking lots, roofs, and lawns quickly impacts temperatures in small creeks.  Channelized  streams are availed few of the positive attributes provided by many of the heat flux processes we’re about to see.  They therefore suffer from severe impairment and are exposed to temperature extremes that few aquatic organisms can survive.  Runoff from sun-heated pavement during a summer thunderstorm can often exceed 100 degrees Fahrenheit and can, at sufficient flow rate, quickly raise the temperature of a small stream to well over 90 degrees.
Stormwater Runoff
Stormwater runoff not only poses a thermal threat to waterways, its a significant source of a wide variety of pollutants.
      • GROUNDWATER INPUT—In streams connected to the aquifer, the temperature in a flowing segment can be impacted by the influx of cold groundwater.  With temperatures ranging from about 52° to 60° Fahrenheit, groundwater will absorb heat from the stream in summer, and warm it in the winter.  In warmwater streams, coldwater and coolwater fishes will often seek areas of the substrate where groundwater is entering for use as refugium from the summer heat.  Yellow Perch in the lower Susquehanna are known to exhibit this behavior.
Creeks and rivers connected to the aquifer and receiving supplemental flow from it are known as “gaining streams”. These streams frequently feed water into the aquifer as well. (United States Geological Survey image)
When flowing through an area experiencing drought or an excessive removal of groundwater (lots of wells, etc.), a waterway can become a “losing stream”, one that surrenders a portion of its flow to recharge the aquifer.  Further downstream, the reduced flow can make such a creek or river more susceptible to the effects of heat flux processes.  (United States Geological Survey image)
Seriously depleted aquifers can lead to a “disconnected stream”.  Smaller waterways subjected to these conditions will sometimes lose all their flow to the ground, often causing a catastrophic failure of the aquatic ecosystem supported therein.  (United States Geological Survey image)
Urban Flooding and Dry Streambed
Urban runoff overwhelms this small stream with polluted water than can reach temperatures of 100 degrees or more (left), then lets it high and dry with no baseflow during periods of dry weather (right) as the waterway becomes disconnected from the much-depleted aquifer.
Stormwater Retention Basin
Well-designed and properly constructed stormwater retention basins not only recharge groundwater supplies for wells and streams, they can also help prevent thermal pollution in waterways.  Planted with native wetland species and allowed to thrive, they can become treasured wildlife islands in otherwise inhospitable environs.  The benefits don’t stop there; plants also help sequester nutrients contained in the runoff.
      • HYPORHEIC EXCHANGE—Related to groundwater input, hyporheic exchange is the slow movement of water through the rock, sand, gravel, and soils composing the streambed, saturated shoreline, shallow aquifer, and connected floodplain of a creek or river.  As a heat flux process, hyporheic exchange helps moderate extremes in seasonal water temperatures by conducting energy between the solid materials in the zone and the flowing water.  Hyporheic zones are important habitats for many species of aquatic invertebrates and spawning fish.  Natural chemical processes within these zones convert ammonia-producing wastes into nitrite, then nitrate, allowing it to be absorbed as food by plants growing in the stream or in the alluvium within the zone.  Vegetation removal, channelization, legacy sediments, silt deposits, and man-made walls and dams can negate the benefits of hyporheic exchange.
Exchange of surface and ground water within the hyporheic zone is most directly associated with high-gradient (left) and meandering (right) segments of streams. (United States Geological Survey image)
Legacy Sediments and Fill
Very common on streams in the lower Susquehanna valley are these accumulations of legacy sediments at the sites of former mill ponds.  After the dams were removed, the creeks began eroding their way down through the mire as they tried to reestablish their floodplains and find their native substrate.  These trapped waterways are not only cut off from their hyporheic zones, they’re now a major source of nutrient and sediment pollution.  Misguided landowners like this one frequently dump fill into these sites to “save their land” and “control flooding”.  The fill and materials added to “shore up the banks” do nothing to fix what ails the creek, but instead displace more water to make the impact of flooding even more widespread.
Flooplain and Stream Restoration
Rehabilitation projects that remove legacy sediments help restore hyporheic exchange by reconnecting the stream to its underlying geology, its floodplain, and its wetlands.  Rising waters remain in the floodplain where they get a good bio-scrubbing and help replenish the creek and groundwater supply.  As the experts say, “floodplains are for flooding.”
      • ATMOSPHERIC EXCHANGE (CONVECTION, EVAPORATION)—Primarily a process by which a stream loses heat energy and cools its waters, atmospheric exchange is also a means by which a warm air mass can relinquish heat to cooler waters and thus increase their temperature.  This phenomenon can be dramatically enhanced when a stream passes through a so-called urban heat island where air temperatures remain warm through the night.  Convection, the movement of heat energy through a fluid (liquid or gas), causes warmer, less-dense water to rise to the surface of a stream, particularly where there is minimal turbulence.  When the air above is cooler than the water’s surface layer, the stream will conduct heat energy across the water/atmosphere interface causing the warmed air molecules to rise in a convection column.  If the atmospheric relative humidity is less than 100%, some surface water will vaporize—a process that expends more of the stream’s heat energy.  The rate of convective and evaporative cooling in a given stream segment is directly related to the degree of difference between the water temperature and air temperature, and to the relative humidity in the air mass above the lake, creek, or river.  The mechanical action of stream turbulence including rapids, riffles, and falls increases the contact area between air and water to maximize the atmospheric exchange of heat energy.  The convective air current we call surface wind has a turbulent wave-producing effect on water that can also maximize atmospheric exchange; think of a cold autumn wind robbing heat energy from a warm lake or river or a hot summer wind imparting its heat to a cooler creek.  These exchanges are both conductive in nature (air-to-water/water-to-air) and evaporative, the latter being expedited by the movement of dry air over warm water.
Tessellated Darter
Usually classified as one of the coolwater fishes, the bottom-dwelling Tessellated Darter can thrive in the warmer creeks and in the main stem of the Susquehanna by inhabiting riffles where atmospheric exchange in the form of increased evaporation helps reduce temperatures and convective currents carry the cooler, well-oxygenated water to the streambed.
Three mile Island Unit 1 Cooling Towers
Humans utilize the concept of atmospheric exchange, adopting the phenomena of evaporation and convection to cool the hot waters produced during electric generation and other industrial processes before discharge into a lake or river.
      • STREAMBED CONDUCTIVE EXCHANGE—In the lower Susquehanna watershed, there may be no better natural example of streambed conductive exchange than the Triassic-Jurassic diabase pothole bedrocks of Conewago Falls on the river at the south end of Three Mile Island.
During sunny days, the massive diabase pothole rocks at Conewago Falls absorb solar (shortwave) radiation, then conduct that heat energy into the flowing water, often continuing to pass the accumulated warmth into the river during the night.  On cloudy days, the riverbed collects longwave atmospheric radiation, a heat flux process that yields significantly less energy for conduction into the rapids, riffles, and pools of the falls.  During periods of low river flow, the heating effect of streambed conductive exchange can become magnified.  Compared to conditions that prevail when torrents of turbid water are rushing through the falls, partially exposed bedrock surrounded by clear water collects radiated energy much more efficiently, then conducts the heat to a greatly reduced volume of passing water.  During summer and autumn, this process can create a mix of temperature zones within the falls with warmer water lingering in slow-moving pools and cooler water flowing in the deeper fast-moving channels.  Along the falls’ mile-long course, a haven is created for aquatic organisms including warmwater and some coolwater fishes, oft times attracting anglers and a variety of hungry migrating birds as well.
Fallfish
Classified as one of our coolwater fishes, the Fallfish finds favorable conditions for feeding, growing, and spawning in the well-oxygenated waters of Conewago Falls.
Northern Hog Sucker
Though the lower Susquehanna River is classified as a warmwater fishery, the Northern Hog Sucker (Hypentelium nigricans), another of our native coolwater fishes, finds the fast-moving waters of Conewago Falls to its liking.  Northern Hog Suckers are known to inhabit streams cold enough to host trout.  They exhibit remarkable home range fidelity, sometimes spending their entire lives occupying the same several hundred feet of waterway.  Northern Hog Suckers are often designated an indicator of good water quality, intolerant of many stream impairment parameters.  Their presence in Conewago Falls provides testament to the quality of the warmwater fishery there.
Severely Impaired Channelized Stream
An unnatural example.  The reduced base flow in this channelized and severely impaired creek has been rendered vulnerable to the negative impacts of several heat flux processes including streambed conductive exchange.  Urban stormwater/surfacewater inflow, solar (shortwave) radiation, and heat conducted into the stream from the masonry walls, curbs, and raceway can all conspire to cook aquatic organisms with life-quenching summer water temperatures exceeding 90 degrees Fahrenheit.
      • SOLAR (SHORTWAVE) RADIATION—The sun provides the energy that fuels the earth’s complex climate.  The primary heat flux process that heats our planet is the absorption of solar radiation in the shortwave spectrum, which includes ultraviolet, visible, and infrared frequencies at the upper end of the longwave spectrum.  Streams and other bodies of water absorb the greatest amounts of solar (shortwave) radiation during the weeks around summer solstice when the sun at mid-day is closer to zenith than at any other time of the year.  However, the heating impact of the radiation may be greatest when the volume of water in the creek, river, or lake is at its minimum for the year—often during early fall.
The rate, measured in watts per square meter, at which solar (shortwave) energy is directly radiated to a given area on the earth’s surface (including streams and other waters) is determined by: solar activity, the angle of the sun in the sky, aspect (slope) of the receiving surface, the opacity of the overlying atmosphere, and the distance of the earth from the sun.  The former varies with the year’s seasons, the time of day, and the latitude of a given area.  The latter is currently at its annual minimum when earth is at perihelion during the early days of January, thus providing the northern hemisphere with a little bump in radiation during the shortest days of the year when the sun is at its lowest angle in the sky.  (NASA image)
A varying portion of the solar (shortwave) radiation reaching the earth is reflected back into space by clouds.  A smaller share is absorbed by the atmosphere, thus heating it.  An even lesser quantity is reflected back into space by water and land.  The remainder of the energy is absorbed by the planet’s surfaces, its water and land. (NASA image)
      • INCIDENT SHORTWAVE RADIATION—Also known as insolation (incoming solar radiation), incident shortwave radiation is the sum total energy of both the direct solar radiation that travels to the earth’s surface unaffected by the atmosphere and the diffuse radiation, waves that have been weakened and scattered by constituents of the atmosphere before reaching the planet’s surface.  On a cloudy day, the warming of terrestrial surfaces including streams and other bodies of water is the result of diffuse radiation.  On days with any amount of sunshine at all, both direct and diffuse radiation heat our waters and lands.
Pumkinseed
Warmwater fishes such as the native Pumpkinseed (Lepomis gibbosus) thrive in sun-drenched 70-to-85-degree waters as long as other heat flux processes prevent sudden temperature increases and oxygen depletion.
Mowed Stream Bank
Mowed stream banks offer a waterway no protection from incoming solar (shortwave) radiation, nor terrestrial forms of impairment including nutrient-rich stormwater runoff and silt.
      • REFLECTED SHORTWAVE RADIATION—known as albedo, reflected solar (shortwave) radiation is energy directed away from the earth’s surface before being absorbed.  A surface’s albedo value is basically determined by its color, black having little reflective value, white and silvery surfaces reflecting nearly all solar (shortwave) radiation away.  A surface with no reflective properties has an albedo value of 0, while a totally reflective surface has a value of 1.  Clean snow with a value of about 0.85 to 0.9 (85% to 90%) is a highly reflective surface; yellow snow isn’t as good.  A stream, river, or lake blanketed with ice and snow will absorb very little solar energy and will rely upon other heat flux processes to trigger a melt and thaw.  The surface of open water has a varying albedo value determined mostly by the angle of the sun.  Solar radiation striking the water’s surface at a low angle is mostly reflected away, while that originating at an angle closer to zenith is more readily absorbed.
Forested Stream
To avoid the heating effects of solar (shortwave) and atmospheric longwave radiation, coldwater and coolwater fishes require streams offering protection from full exposure to direct sunlight and cloud cover.  Runs and creeks flowing beneath a closed canopy of forest trees are shielded from 25% or more of incoming radiation and are thus able to better maintain thermal stability during the most vulnerable period of the year for temperature-sensitive fishes, May through October.
      • LONGWAVE RADIATION—Radiation in the longwave spectrum is composed of infrared waves at frequencies lower than those of the shortwave spectrum.  Longwave radiation, sometimes just called infrared radiation, is produced by the earth and its atmosphere and is propagated in all directions, day and night.  It warms mostly the lower atmosphere which in turn warms the earth’s surface including its waters.  Some longwave energy can even be radiated into the waterway from its own streambed—and the stream can return the favor.  Other forms of mass surrounding  a stream such as a rocky shoreline or a man-made structure such as bridge pier can trade longwave radiation with a waterway.  The effect of these latter exchanges is largely trivial and never rivals the heat flux transfer of warm to cold provided by  conduction.
Longwave radiation emissions slow as the temperature of the emitting mass decreases, just as they also increase with temperature of the mass.  Longwave radiation emissions therefore decrease with altitude along with the temperature of the water vapor, carbon dioxide, methane, and other gases that produce them.  As such, the highest reaches of the atmosphere have a greatly reduced capability of shedding longwave radiation into space.  At ground level, lakes, creeks, and streams receive their greatest dose of longwave radiation while beneath the cover of low-lying clouds or fog.  (NASA image)
      • CANOPY RADIATION—Trees emit longwave radiation that may have a limited heat flux impact on waterway temperature.  This radiation is diffuse, of scattered effect, and scarcely detectable, particularly beneath multilayered dense canopies.  Some of the infrared energy transmitted by the tree canopy is radiated skyward as well.
      • WATER RADIATION—Water, like all earthly matter composed of vibrating molecules, emits longwave radiation.  This heat flux process provides an ongoing cooling effect to streams, rivers, lakes, and oceans—warmer ones shedding infrared energy at a faster rate than those that are cold.

Now that we have a basic understanding of the heat flux processes responsible for determining the water temperatures of our creeks and rivers, let’s venture a look at a few graphics from gauge stations on some of the lower Susquehanna’s tributaries equipped with appropriate United States Geological Survey monitoring devices.  While the data from each of these stations is clearly noted to be provisional, it can still be used to generate comparative graphics showing basic trends in easy-to-monitor parameters like temperature and stream flow.

Each image is self-labeled and plots stream temperature in degrees Fahrenheit (bold blue) and stream discharge in cubic feet per second (thin blue).

The West Conewago Creek drains much of the Gettysburg Basin’s Triassic redbeds in Adams and northern York Counties in Pennsylvania and includes a small headwaters area in northern Maryland.  The gauge station is located just a over a mile upstream from the waterway’s mouth on the Susquehanna just below Conewago Falls.  Right through the summer heatwave, this 90-day graph shows a consistent daily pattern of daytime rises in temperature and nighttime cooling.  To the right, a rapid cool down can be seen coinciding with two periods of high water, the first from a series of heavy thundershowers, the second from flooding caused by the remnants of Hurricane Debby.  Notice that the early August downpours were so heavy that they cooled the hot surface runoff and waterway quickly, without creating a rise in stream temperature at the gauging station.  Had this monitoring device been located on a small tributary in an area with an abundance of impervious surfaces, there would probably have been a brief rise in stream temperature prior to the cooldown.  (United States Geological Survey image)

The daily oscillations in temperature reflect the influence of several heat flux processes.  During the day, solar (shortwave) radiation and convection from summer air, especially those hot south winds, are largely responsible for the daily rises of about 5° F.  Longwave radiation has a round-the-clock influence—adding heat to the stream during the day and mostly shedding it at night.  Atmospheric exchange including evaporative cooling may help moderate the rise in stream temperatures during the day, and certainly plays a role in bringing them back down after sunset.  Along its course this summer, the West Conewago Creek absorbed enough heat to render it a warmwater fishery in the area of the gauging station.  The West Conewago is a shallow, low gradient stream over almost its entire course.  Its waters move very slowly, thus extending their exposure time to radiated heat flux and reducing the benefit of cooling by atmospheric exchange.  Fortunately for bass, catfish, and sunfish, these temperatures are in the ideal range for warmwater fishes to feed, grow, and reproduce—generally over 80° F, and ideally in the 70° to 85° F range.  Coolwater fishes though, would not find this stream segment favorable.  It was consistently above the 80° F maximum and the 60° to 70° F range preferred by these species.  And coldwater fishes, well, they wouldn’t be caught dead in this stream segment.  Wait, scratch that—the only way they would be caught in this segment is dead.  No trouts or sculpins here.

The Codorus Creek drains primarily the carbonate valleys of York County to the south of the West Conewago watershed.  This gauge station is located about a mile upstream from the creek’s mouth on the Susquehanna just below Haldeman Riffles.  The graphic pattern is very similar to that of the West Conewago’s: daily heating and cooling cycles and a noticeable drop in stream temperature in early August caused by a day of thundershowers followed by the remnants of Hurricane Debby.  (United States Geological Survey image)

Look closely and you’ll notice that although the temperature pattern on this chart closely resembles that of the West Conewago’s, the readings average about 5 degrees cooler.  This may seem surprising when one realizes that the Codorus follows a channelized path through the heart of York City and its urbanized suburbs—a heat island of significance to a stream this size.  Before that it passes through numerous impoundments where its waters are exposed to the full energy of the sun.  The tempering factor for the Codorus is its baseflow.  Despite draining a smaller watershed than its neighbor to the north, the Codorus’s baseflow (low flow between periods of rain) was 96 cubic feet per second on August 5th, nearly twice that of the West Conewago (51.1 cubic feet per second on August 5th).  Thus, the incoming heat energy was distributed over a greater mass in the Codorus and had a reduced impact on its temperature.  Though the Codorus is certainly a warmwater fishery in its lower reaches, coolwater and transitional fishes could probably inhabit its tributaries in segments located closer to groundwater sources without stress.  Several streams in its upper reaches are in fact classified as trout-stocked fisheries.

This is a zoomed-in look at the previous graph showing the impact of a rainfall event on the water temperatures in Codorus Creek.  Unlike the sharp declines accompanying the deluge of flood waters during the two events in early August, these lesser storms in late June generated just enough runoff to capture heat energy from impervious surfaces and warm the creek, temporarily breaking the daily heating/cooling cycle.  Upstream in the immediate area of the runoff, the impact on the stream and/or its tributaries was probably much more dramatic, certainly raising temperatures into the nineties or above.  (United States Geological Survey image)
Kreutz Creek drains a carbonate bedrock area of York County and flows parallel to the Lincoln Highway (US 30) to enter the Susquehanna at Wrightsville.  The gauging station is about one mile upstream from the creek’s mouth.   (United States Geological Survey image)

The Kreutz Creek gauge shows temperature patterns similar to those in the West Conewago and Codorus data sets, but notice the lower overall temperature trend and the flow.  Kreutz Creek is a much smaller stream than the other two, with a flow averaging less than one tenth that of the West Conewago and about one twentieth of that in the Codorus.  And most of the watershed is cropland or urban/suburban space.  Yet, the stream remains below 80° F through most of the summer.  The saving graces in Kreutz Creek are reduced exposure time and gradient.  The waters of Kreutz Creek tumble their way through a small watershed to enter the Susquehanna within twenty-four hours, barely time to go through a single daily heating and cooling cycle.  As a result, their is no chance for water to accumulate radiant and convective heat over multiple summer days.  The daily oscillations in temperature are less amplified than we find in the previous streams—a swing of about three degrees compared to five.  This indicates a better balance between heat flux processes that raise temperature and those that reduce it.  Atmospheric exchange in the stream’s riffles, forest cover, and good hyporheic exchange along its course could all be tempering factors in Kreutz Creek.  From a temperature perspective, Kreutz Creek provides suitable waters for coolwater fishes.

Muddy Creek drains portions of southern York County through rolling farmland and woodlots.  There are no large impoundments or widespread urban impacts in the watershed, which may help explain its slightly lower temperature trends.  (United States Geological Survey image)

Muddy Creek is a trout-stocked fishery, but it cannot sustain coldwater species through the summer heat.  Though temperatures in Muddy Creek may be suitable for coolwater fishes, silt, nutrients, low dissolved oxygen, and other factors could easily render it strictly a warmwater fishery, inhabited by species tolerant of significant stream impairment.

Chiques Creek drains mostly limestone farmland in northwestern Lancaster County.  The gauging station is located near the stream’s mouth on the Susquehanna at Chiques (Chickies) Rock.  Oscillations in temperature again resemble the other waterways, but daily highs remain almost entirely below 80 degrees.  (United States Geological Survey image)

A significant number of stream segments in the Chiques watershed have been rehabilitated to eliminate intrusion by grazing livestock, cropland runoff, and other sources of impairment.  Through partnerships between a local group of watershed volunteers and landowners, one tributary, Donegal Creek, has seen riparian buffers, exclusion fencing, and other water quality and habitat improvements installed along nearly ever inch of its run from Donegal Springs through high-intensity farmland to its mouth on the main stem of the Chiques just above its confluence with the Susquehanna.  The improved water quality parameters in the Donegal support native coldwater sculpins and an introduced population of reproducing Brown Trout.  While coldwater habitat is limited to the Donegal, the main stem of the Chiques and its largest tributary, the Little Chiques Creek, both provide suitable temperatures for coolwater fishes.

Limestone Formation on Little Chiques Creek
Streams in the Chiques Creek and similar limestone watersheds often pass through areas with significant bedrock formations.  Heat flux processes including groundwater input, hyporheic exchange, and streambed conductive exchange can have a greater influence on water temperature along these segments.
Eastern Blacknose Dace
A breeding condition Eastern Balcknose Dace, one of the coldwater transition fishes found in the Chiques and its tributaries.
Common Shiner
The Common Shiner (Luxilus cornutus), a fish tolerant of warmwater streams, prefers cool, clear waters for spawning.  For protection from late-spring and summer heat, breeding males may seek a section of creek with a streambed inflow of limestone groundwater to defend as their nesting territory.
A closeup of the Chiques Creek graph showing what appears to be a little bump in temperature caused by surface runoff during a couple of late-May showers.  Stream rehabilitation is an ongoing process and the pressures of land disturbances both old and new present challenges to those who make it their passion to fix the wrongs that have been inflicted upon our local waters.  Even the  exemplary Donegal Creek faces new threats from urbanization in one of its headwater areas several miles to the northwest of the historic springs.  (United States Geological Survey image)
Conewago Creek (East) drains primarily Triassic redbed farmlands in Dauphin, Lancaster, and Lebanon Counties.  Much of the headwaters area is forested but is experiencing an increasing rate of encroachment by housing and some commercial development.  Conewago Creek (East) enters the Susquehanna on the east side of Conewago Falls at Three Mile Island.  The watershed is equipped with three U.S.G.S. gauge stations capable of providing temperature data.  This first one is located just over a mile upstream of the creek’s mouth.  (United States Geological Survey image)

Despite its meander through and receipt of water from high-intensity farmland, the temperature of the lower Conewago (East) maxes out at about 85° F, making it ideal for warmwater fishes and even those species that are often considered coolwater transition fishes like introduced Smallmouth Bass, Rock Bass, Walleye, and native Margined Madtom.  This survivable temperature is a testament to the naturally occurring and planted forest buffers along much of the stream’s course, particularly on its main stem.  But the Conewago suffers serious baseflow problems compared to other streams we’ve looked at so far.  Just prior to the early August storms, flow was well below 10 cubic feet per second for a drainage area of more than fifty square miles.  While some of this reduced flow is the result of evaporation, much of it is anthropogenic in origin as the rate of groundwater removal continues to increase  and a recent surge in stream withdraws for irrigation reaches its peak during the hottest days of summer.

Juvenile Rock Bass
A juvenile Rock Bass.
A juvenile Margined Madtom.
A juvenile Margined Madtom.
A closer look at the Conewago Creek (East) graphic shows the temperature drop associated with a series of thundershowers and the remnants of Hurricane Debby in early August.  Despite the baseflow being below five cubic feet per second, the cooling effect of the downpours as measured in the area of the gauge was significant enough to overwhelm any heating of runoff that may have occurred as precipitation drained across hardened soils or man-made impervious surfaces.  (United States Geological Survey image)

A little side note—the flow rate on the Conewago at the Falmouth gauge climbed to about 160 cubic feet per second as a result of the remnants of Hurricane Debby while the gauge on the West Conewago at Manchester skyrocketed to about 20,000 cubic feet per second.  Although the West Conewago’s watershed (drainage area) is larger than that of the Conewago on the east shore, it’s larger only by a multiple of two or three, not 125.  That’s a dramatic difference in rainfall!

The Bellaire monitoring station on Conewago Creek (East) is located on the stream’s main stem just downstream from the mouth of Little Conewago Creek, a tributary with its origins in farmland and woodlots.  (United States Geological Survey image)

The temperatures at the Bellaire monitoring station, which is located upstream of the Conewago’s halfway point between its headwaters in Mount Gretna and its mouth, are quite comparable to those at the Falmouth gauge.  Although a comparison between these two sets of data indicate a low net increase in heat absorption along the stream’s course between the two points, it also suggests sources of significant warming upstream in the areas between the Bellaire gauge and the headwaters.

Data from the gauge site on the Little Conewago Creek shows a temperature averaging about five degrees cooler than the gauge several miles downstream on the main stem of the Conewago at Bellaire.  (United States Geological Survey image)

The waters of the Little Conewago are protected within planted riparian buffers and mature woodland along much of their course to the confluence with the Conewago’s main stem just upstream of Bellaire.  This tributary certainly isn’t responsible for raising the temperature of the creek, but is instead probably helping to cool it with what little flow it has.

Juvenile Eastern Blacknose Dace (top) and a juvenile Longnose Dace.
A stream like the Little Conewago Creek with daily temperatures that remain mostly below 80 degrees and retreat to 75 degrees or less during the night can be suitable for coldwater transition fishes like these juvenile Eastern Blacknose Dace (top) and Longnose Dace.

Though mostly passing through natural and planted forest buffers above its confluence with the Little Conewago, the main stem’s critically low baseflow makes it particularly susceptible to heat flux processes that raise stream temperatures in segments within the two or three large agricultural properties where owners have opted not to participate in partnerships to rehabilitate the waterway.  The headwaters area, while largely within Pennsylvania State Game Lands, is interspersed with growing residential communities where potable water is sourced from hundreds of private and community wells—every one of them removing groundwater and contributing to the diminishing baseflow of the creek.  Some of that water is discharged into the stream after treatment at the two municipal sewer plants in the upper Conewago.  This effluent can become quite warm during processing and may have significant thermal impact when the stream is at a reduced rate of flow.  A sizeable headwaters lake is seasonally flooded for recreation in Mount Gretna.  Such lakes can function as effective mid-day collectors of solar (shortwave) radiation that both warms the water and expedites atmospheric exchange.

The Conewago Creek (East) Watershed from the Bellaire U.S.G.S. Gauging Station (lower left) upstream to the headwaters in Mount Gretna.  (United States Geological Survey image)

Though Conewago Creek (East) is classified as a trout-stocked fishery in its upper reaches in Lebanon County, its low baseflow and susceptibility to warming render it inhospitable to these coldwater fishes by late-spring/early summer.

River Chub
Despite being considered a warmwater fish, the River Chub (Nocomis micropogon) will ascend streams like the Conewago to seek cooler, gravel-bottomed waters for spawning.  Reduced baseflow has probably rendered the stream currently too small for this species on Pennsylvania State Game Lands in Colebrook where this specimen was photographed in 2018.
Juvenile Golden Shiner
The Golden Shiner, another warmwater fish, often ascends streams to enter cooler water. Juvenile Golden Shiners like this one will move into shallower headwaters not only to seek reduced temperatures, but to escape large predatory fishes as well.
Irrigation using stream water.
Irrigation of agricultural fields using a large portion of the already diminished baseflow in the Conewago Creek (East) just downstream of the Bellaire gauging station.  Despite millions of dollars in investment to rehabilitate this Susquehanna valley stream, the riparian buffers and other practices can have little effect when the creek gets sucked down to just a trickle.  Low baseflow is a hard nut to crack.  It’s best prevented, not corrected.
Hammer Creek, a trout-stocked fishery, originates, in part, within Triassic conglomerate in the Furnace Hills of Lebanon County, then flows north into the limestone Lebanon Valley where it picks up significant flow from other tributaries before working its way south back through the Furnace Hills into the limestone farmlands of Lancaster County.  From there the stream merges with the Cocalico Creek, then the Conestoga River, and at last the Susquehanna.  Note the tremendous daily temperature oscillations on this headwaters stream as it surges about 15 degrees each day before recovering back close to groundwater temperature by sunrise the next day.  (United States Geological Survey image)
Headwaters of Hammer Creek including Buffalo Springs, a significant source of cold groundwater feeding the western leg of the stream.  The large dams on this section that created the Lebanon and Rexmont Reservoirs have been removed.  (United States Geological Survey base image)

The removal of two water supply dams on the headwaters of Hammer Creek at Rexmont eliminated a large source of temperature fluctuation on the waterway, but did little to address the stream’s exposure to radiant and convective heat flux processes as it meanders largely unprotected out of the forest cover of Pennsylvania State Game Lands and through high-intensity farmlands in the Lebanon Valley.  Moderating the temperature to a large degree is the influx of karst water from Buffalo Springs, located about two miles upstream from this gauging station, and other limestone springs that feed tributaries which enter the Hammer from the east and north.  Despite the cold water, the impact of the stream’s nearly total exposure to radiative and other warming heat flux processes can readily be seen in the graphic.  Though still a coldwater fishery by temperature standards, it is rather obvious that rapid heating and other forms of impairment await these waters as they continue flowing through segments with few best management practices in place for mitigating pollutants.  By the time Hammer Creek passes back through the Furnace Hills and Pennsylvania State Game Lands, it is leaning toward classification as a coolwater fishery with significant accumulations of sediment and nutrients.  But this creek has a lot going for it—mainly, sources of cold water.  A core group of enthusiastic landowners could begin implementing the best management practices and undertaking the necessary water quality improvement projects that could turn this stream around and make it a coldwater treasure.  An organized effort is currently underway to do just that.  Visit Trout Unlimited’s Don Fritchey Chapter and Donegal Chapter to learn more.  Better yet, join them as a volunteer or cooperating landowner!

Male Creek Chub
The male Creek Chub, one of our coolwater fishes, develops head tubercles and becomes flushed with color during spawning season.  Hammer Creek not only provides a home for the Creek Chub, its cold headwaters provide refuge for a population of native Brook Trout too.
Like no other example we’ve looked at so far, this closeup of the Hammer Creek graphic shows temperature bumps correlating with the stormwater runoff from early August’s rains.  Because the stream flow is small and the precipitation rate was not as great at this location, the effect of heat flux from runoff is more readily apparent.  (United States Geological Survey image)
Brook Trout adult and juvenile.  (United States Fish and Wildlife Service image by Ryan Hagerty)

For coldwater fishes, the thousands of years since the most recent glacial maximum have seen their range slowly contract from nearly the entirety of the once much larger Susquehanna watershed to the headwaters of only our most pristine streams.  Through no fault of their own, they had the misfortune of bad timing—humans arrived and found coldwater streams and the groundwater that feeds them to their liking.  Some of the later arrivals even built their houses right on top of the best-flowing springs.  Today, populations of these fishes in the region we presently call the Lower Susquehanna River Watershed are seriously disconnected and the prospect for survival of these species here is not good.  Stream rehabilitation, groundwater management, and better civil planning and land/water stewardship are the only way coldwater fishes, and very possibly coolwater fishes as well, will survive.  For some streams like Hammer Creek, it’s not too late to make spectacular things happen.  It mostly requires a cadre of citizens, local government, project specialists, and especially stakeholders to step up and be willing to remain focused upon project goals so that the many years of work required to turn a failing stream around can lead to success.

Riparian Buffer
Riparian buffers with fences to exclude livestock can immediately begin improving water quality.  With establishment of such vegetative buffers, the effects of stressors that otherwise eliminate coldwater and coolwater fishes from these segments will begin to diminish.
Riparian Buffer
Within five to ten years, a riparian buffer planted with native trees is not only helping to reduce nutrient and sediment loads in the stream, it is also shielding the waters from heat flux processes including the solar (shortwave) radiation that raises water temperatures to levels not tolerated by coldwater and coolwater fishes.
Riparian Buffer
A well-established riparian buffer.
Forested Stream
A forested stream.

You’re probably glad this look at heat flux processes in streams has at last come to an end.  That’s good, because we’ve got a lot of work to do.

Add one more benefit to the wildflower meadow, it infiltrates stormwater to recharge the aquifer much better than mowed grass.  And another related plus, it reduces runoff and its thermal pollution.  Besides, you don’t have time to mow grass, because we have work to do!
Potomac Sculpin
Our native coldwater fishes including the Potomac Sculpin will survive only if we protect and expand the scattered few habitats where they have taken refuge.  They have no choice but to live in these seriously threatened places, but we do.  So let’s give ’em some space.  How ’bout it?  (United States Fish and Wildlife Service image by Ryan Hagerty)

Three Mile Island and Agnes: Fifty Years Later

Fifty years ago this week, the remnants of Hurricane Agnes drifted north through the Susquehanna River basin as a tropical storm and saturated the entire watershed with wave after wave of torrential rains.  The storm caused catastrophic flooding along the river’s main stem and along many major tributaries.  The nuclear power station at Three Mile Island, then under construction, received its first major flood.  Here are some photos taken during the climax of that flood on June 24, 1972.  The river stage as measured just upstream of Three Mile Island at the Harrisburg gauge crested at 33.27 feet, more than 10 feet above flood stage and almost 30 feet higher than the stage at present.  At Three Mile Island and Conewago Falls, the river was receiving additional flow from the raging Swatara Creek, which drains much of the anthracite coal region of eastern Schuylkill County—where rainfall from Agnes may have been the heaviest.

Three Mile Island flooding from Agnes 1972.
1972-  From the river’s east shore at the mouth of Conewago Creek, Three Mile Island’s “south bridge” crosses the Susquehanna along the upstream edge of Conewago Falls.  The flood crested just after covering the roadway on the span.  Floating debris including trees, sections of buildings, steel drums, and rubbish began accumulating against the railings on the bridge’s upstream side, leading observers to speculate that the span would fail.  When a very large fuel tank, thousands of gallons in capacity, was seen approaching, many thought it would be the straw that would break the camel’s back.  It wasn’t, but the crashing sounds it made as it struck the bridge then turned and began rolling against the rails was unforgettable.  (Larry L. Coble, Sr. image)
Three Mile Island flooding from Agnes 1972.
1972-  In this close-up of the preceding photo, the aforementioned piles of junk can be seen along the upstream side of the bridge (behind the sign on the right).  The fuel tank struck and was rolling on the far side of this pile.  (Larry L. Coble, Sr. image)
2022-  Three Mile Island’s “south bridge” as it appeared this morning, June 24,2022.
Three Mile Island flooding from Agnes 1972.
1972-  The railroad along the east shore at Three Mile Island’s “south bridge” was inundated by rising water.  This flooded automobile was one of many found in the vicinity.  Some of these vehicles were overtaken by rising water while parked, others were stranded while being driven, and still others floated in from points unknown.  (Larry L. Coble, Sr. image)
2022-  A modern view of the same location.
Three Mile Island flooding from Agnes 1972.
1972-  At the north end of Three Mile Island, construction on Unit 1 was halted.  The completed cooling towers can be seen to the right and the round reactor building can be seen behind the generator building to the left.  The railroad grade along the river’s eastern shore opposite the north end of the island was elevated enough for this train to stop and shelter there for the duration of the flood.  (Larry L. Coble, Sr. image)
2022-  Three Mile Island Unit 1 as it appears today: shut down, defueled, and in the process of deconstruction.
Three Mile Island flooding from Agnes 1972.
1972-  In March of 1979, the world would come to know of Three Mile Island Unit 2.  During Agnes in June of 1972, flood waters surrounding the plant resulted in a delay of its construction.  In the foreground, note the boxcar from the now defunct Penn Central Railroad.  (Larry L. Coble, Sr. image)
2022-  A current look at T.M.I. Unit 2, shut down since the accident and partial meltdown in 1979.

Pictures capture just a portion of the experience of witnessing a massive flood.  Sometimes the sounds and smells of the muddy torrents tell us more than photographs can show.

Aside from the booming noise of the fuel tank banging along the rails of the south bridge, there was the persistent roar of floodwaters, at the rate of hundreds of thousands of cubic feet per second, tumbling through Conewago Falls on the downstream side of the island.   The sound of the rapids during a flood can at times carry for more than two miles.  It’s a sound that has accompanied the thousands of floods that have shaped the falls and its unique diabase “pothole rocks” using abrasives that are suspended in silty waters after being eroded from rock formations in the hundreds of square miles of drainage basin upstream.  This natural process, the weathering of rock and the deposition of the material closer to the coast, has been the prevailing geologic cycle in what we now call the Lower Susquehanna River Watershed since the end of the Triassic Period, more than two hundred million years ago.

More than the sights and sounds, it was the smell of the Agnes flood that warned witnesses of the dangers of the non-natural, man-made contamination—the pollution—in the waters then flowing down the Susquehanna.

Because they float, gasoline and other fuels leaked from flooded vehicles, storage tanks, and containers were most apparent.  The odor of their vapors was widespread along not only along the main stem of the river, but along most of the tributaries that at any point along their course passed through human habitations.

Blended with the strong smell of petroleum was the stink of untreated excrement.  Flooded treatment plants, collection systems overwhelmed by stormwater, and inundated septic systems all discharged raw sewage into the river and many of its tributaries.  This untreated wastewater, combined with ammoniated manure and other farm runoff, gave a damaging nutrient shock to the river and Chesapeake Bay.

Adding to the repugnant aroma of the flood was a mix of chemicals, some percolated from storage sites along watercourses, and yet others leaking from steel drums seen floating in the river.  During the decades following World War II, stacks and stacks of drums, some empty, some containing material that is very dangerous, were routinely stored in floodplains at businesses and industrial sites throughout the Susquehanna basin.  Many were lifted up and washed away during the record-breaking Agnes flood.  Still others were “allowed” to be carried away by the malicious pigs who see a flooding stream as an opportunity to “get rid of stuff”.  Few of these drums were ever recovered, and hundreds were stranded along the shoreline and in the woods and wetlands of the floodplain below Conewago Falls.  There, they rusted away during the next three decades, some leaking their contents into the surrounding soils and waters.  Today, there is little visible trace of any.

During the summer of ’72, the waters surrounding Three Mile Island were probably viler and more polluted than at any other time during the existence of the nuclear generating station there.  And little, if any of that pollution originated at the facility itself.

The Susquehanna’s floodplain and water quality issues that had been stashed in the corner, hidden out back, and swept under the rug for years were flushed out by Agnes, and she left them stuck in the stinking mud.

2018 Migration Count Summary: Rainout

If you were a regular visitor to this website during the autumn of 2017, you will recall the proliferation of posts detailing the bird migration at Conewago Falls during the season.  The lookout site among the Pothole Rocks remained high and dry for most of the count’s duration. 

In the fall of 2018, those lookout rocks were never to be seen. There was to be no safe perch for a would-be observer. There was no attempt to conduct a tally of passing migrants. If you live in the lower Susquehanna River drainage basin, you know why—rain—record setting rain.

Annual precipitation during 2018 as indicated by radar.  Note the extensive areas in pink.  They received in excess of 70 inches of precipitation during 2018, much of it during the second half of the year.  (NOAA/National Weather Service image)
Average annual rainfall.  Most of the lower Susquehanna drainage basin receives an average of just over 40 inches of rain each year.  (NOAA/National Weather Service image)
Departure from normal annual precipitation totals.  Note the extensive areas of greater than 20 inches of precipitation above normal (pink).  Severe flooding occurred on many streams during numerous events throughout the second half of 2018.  Note the closer to normal totals in central New York in the upper Susquehanna watershed.  The lesser amounts of rain there and the localized pattern of the flooding events in Pennsylvania prevented the main stem of the lower Susquehanna from experiencing catastrophic high water in 2018.  (NOAA/National Weather Service image)   
Though there has been no severe flooding, frequent rain events in the Susquehanna watershed have maintained persistently high river levels in Conewago Falls.  Pothole Rocks seen here on December 9 during an ebb in the flow were soon inundated again as rains fell in the Susquehanna basin upstream. 
Of course, each time the river receded it left behind a fresh pile of plastic garbage.  What didn’t end up on the shoreline found its way to Chesapeake Bay…then on to the Atlantic.  Is that your cooler? 

They Call Me the Wanderer

It’s been an atypical summer.  The lower Susquehanna River valley has been in a cycle of heavy rains for over a month and stream flooding has been a recurring event.  At Conewago Falls, the Pothole Rocks have been inundated for weeks.  The location used as a lookout for the Autumn Migration Count last fall is at the moment submerged in ten feet of roaring water.  Any attempt to tally the migrants which are passing thru in 2018 will thus be delayed indefinitely.  Of greater import, the flooding at Conewago Falls is impacting many of the animals and plants there at a critical time in their annual life cycle.  Having been displaced from its usual breeding sites on the river, one insect species in particular seems to be omnipresent in upland areas right now, and few people have ever heard of it.

So, you take a cruise in the motorcar to your favorite store and arrive at the sprawling parking lot.  Not wishing to have your doors dented or paint chipped because you settled for a space tightly packed among other shopper’s conveyances, you park out there in the “boondocks”.  You know the place, the lightly-used portion of the lot where sometimes brush grows from cracks in the asphalt and you must be on alert for impatient consumers who throttle-up to high speeds and dash diagonally across the carefully painted grids on the pavement to reach their favorite parking destination in the front row.  Coming to a stop, you take the car out of gear, set the brake, disengage the safety belt, and gather your shopping list.  You grasp the door handle and, not wanting to be flattened  by one of the aforementioned motorists, you have a look around before exiting.

It was then that you saw the thing, hovering above your shiny bright hood.  For a brief moment, it seemed to be peering right through the windshield at you with big reddish-brown eyes.  In just a second or two, it turned its whole bronze body ninety degrees to the left and darted away on its cellophane wings.  Maybe you didn’t really get a good look at it.  It was so fast.  But it certainly was odd.  Oh well, time to walk inside a grab a few provisions.  Away you go.

Upon completion of your shopping, you’re taking the long stroll back to your car and you notice more of these peculiar creatures.  Two are coupled together and are hovering above someone’s automobile hood, then they drop down, and the lower of the two taps its abdomen on the paint.  You ask yourself, “What are these bizarre things?”

Meet the Wandering Glider (Pantala flavescens), also known as the Globe Wanderer or Globe Skimmer, a wide-ranging dragonfly known to occur on every continent with the exception of Antarctica.

Wandering Gliders sometimes arrive in the lower Susquehanna River valley in large numbers after catching a ride on sustained winds from southerly directions and will often fly and migrate in storm systems.  Conditions for such movements have been optimal in our region since mid-July.  These dragonflies will often hover above motor vehicle hoods and, after mating, females will deposit eggs upon them, apparently mistaking their glossy surface for small pools of water.

Wandering Gliders travel the globe, and as such are accomplished fliers.  Adults spend most of the day on the wing, feeding upon a variety of flying insects.  Days ago, I watched several intercepting a swarm of flying ants.  As fast as ants left the ground they were grabbed and devoured by the gliders.  Wandering Gliders are adept at taking day-flying mosquitos, often zipping stealthily past a person’s head or shoulders to grab one of the little pests—the would-be skeeter victim usually unaware of the whole affair.

Due to their nomadic life history, Wandering Gliders are opportunists when breeding and will lay eggs in most any body of freshwater.  Their larvae do not overwinter prior to maturity; adults can be expected in a little more than one to two months.  Repetitive flooding in the Lower Susquehanna River Watershed this summer may be reducing the availability of the best local breeding sites for this species—riverine, stream, and floodplain pools of standing water with prey.  This may explain why thousands of Wandering Gliders are patrolling parking lots, farmlands, and urban areas this summer.  And it’s the likely reason for their use of puddles on asphalt pavement, on rubber roofs, and in fields as places to try to deposit eggs.  Unfortunately, they may be as likely to succeed there as they are on your motor vehicle hood.

At this time a year ago, the airspace above the Diabase Pothole Rocks at Conewago Falls was jammed with territorial male Wandering Gliders.  Each male hovered at various locations around his breeding territory consisting of pools and water-filled potholes.  Intruders would quickly be dispatched from the area, then the male would resume his patrols from a set of repetitively-used hovering positions about six feet above the rocks.  Mating and egg-laying continued into late September.  The larvae, also called nymphs or naiads, were readily observed in many pools and potholes in early October and the emergence of juveniles was noted in mid-October.  The absence of flooding, the mild autumn weather, and the moderation of water temperatures in the pools and potholes courtesy of the sun-drenched diabase boulders helped to extend the 2017 breeding season for Wandering Gliders in Conewago Falls.  They aren’t likely to experience the same favor this year, but their great ability to travel and adapt should overcome this momentary misfortune.

A male Wandering Glider aggressively patrols his territory in the Diabase Pothole Rocks Microhabitat at Conewago Falls.  August 20, 2017.

A mating pair of Wandering Gliders continue flying non-stop above one of thousands of suitable breeding pools among the Diabase Pothole Rocks at Conewago Falls.  September 23, 2017.

A female (bottom)Wandering Glider has deposited eggs in a pool while flying in tandem with a male (top).  They’ll do the same thing on your automobile hood!  Conewago Falls Diabase Pothole Rocks Microhabitat.  September 23, 2017.

Wandering Glider larvae are at the top of the food chain in flooded potholes.  As they grew, these dragonfly larvae decimated the mosquito larvae which were abundant there earlier in the summer.  October 7, 2017.

A juvenile male Wandering Glider emerges from the pool where it fed and grew as a larva.  It remained at water’s edge on the surface of a sun-warmed diabase rock for several hours to dry its wings.  It soon flew away to parts unknown, possibly traveling hundreds or thousands of miles.  Look carefully at the wings for the beige dash marks on the forward edge near the terminal end.  Females lack this marking.  Conewago Falls Diabase Pothole Rocks Microhabitat.  October 14, 2017.

A Wandering Glider exuviae, the shed exoskeleton of a creature gone, but not forgotten.  October 14, 2017.

 

Essential Ice

Two days ago, widespread rain fell intermittently through the day and steadily into the night in the Susquehanna drainage basin.  The temperature was sixty degrees, climbing out of a three-week-long spell of sub-freezing cold in a dramatic way.  Above the ice-covered river, a very localized fog swirled in the southerly breezes.

By yesterday, the rain had ended as light snow and a stiff wind from the northwest brought sub-freezing air back to the region.  Though less than an inch of rain fell during this event, much of it drained to waterways from frozen or saturated ground.  Streams throughout the watershed are being pushed clear of ice as minor flooding lifts and breaks the solid sheets into floating chunks.

Today, as their high flows recede, the smaller creeks and runs are beginning to freeze once again.  On larger streams, ice is still exiting with the cresting flows and entering the rising river.

Ice chunks on Swatara Creek merge into a dense flow of ice on the river in the distance.  Swatara Creek is the largest tributary to enter the Susquehanna in the Gettysburg Basin.  The risk of an ice jam impounding the Swatara here at its mouth is lessened because rising water on the river has lifted and broken the ice pack to keep it moving without serious impingement by submerged obstacles.  Immovable ice jams on the river can easily block the outflow from tributaries, resulting in catastrophic flooding along these streams.

Fast-moving flows of jagged ice race toward Three Mile Island and Conewago Falls.  The rising water began relieving the compression of ice along the shoreline during the mid-morning.  Here on the river just downstream of the mouth of Swatara Creek, ice-free openings allowed near-shore piles to separate and begin floating away after 10:30 A.M. E.S.T.  Moving masses of ice created loud rumbles, sounding like a distant thunderstorm.

Ice being pushed and heaved over the crest of the York Haven Dam at Conewago Falls due to compression and rising water levels.

Enormous chunks of ice being forced up and over the York Haven Dam into Conewago Falls and the Pothole Rocks below.

Ice scours Conewago Falls, as it has for thousands of years.

The action of ice and suspended abrasives has carved the York Haven Diabase boulders and bedrock of Conewago Falls into the amazing Pothole Rocks.

The roaring torrents of ice-choked water will clear some of the woody growth from the Riverine Grasslands of Conewago Falls.

To the right of center in this image, a motorcar-sized chunk of ice tumbles over the dam and crashes into the Pothole Rocks.  It was one of thousands of similar tree-and-shrub-clearing projectiles to go through the falls today.

The events of today provide a superb snapshot of how Conewago Falls, particularly the Diabase Pothole Rocks, became such a unique place, thousands of years in the making.  Ice and flood events of varying intensity, duration, and composition have sculpted these geomorphologic features and contributed to the creation of the specialized plant and animal communities we find there.  Their periodic occurrence is essential to maintaining the uncommon habitats in which these communities thrive.

Fish Crows (Corvus ossifragus) gather along the flooding river shoreline.  Soon there’ll be plenty of rubbish to pick through, some carrion maybe, or even a displaced aquatic creature or two to snack upon.

Eighteen, and I Like It

Is this the same Conewago Falls I visited a week ago?  Could it really be?  Where are all the gulls, the herons, the tiny critters swimming in the potholes, and the leaping fish?  Except for a Bald Eagle on a nearby perch, the falls seems inanimate.

Yes, a week of deep freeze has stifled the Susquehanna and much of Conewago Falls.  A hike up into the area where the falls churns with great turbulence provided a view of some open water.  And a flow of open water is found downstream of the York Haven Dam powerhouse discharge.  All else is icing over and freezing solid.  The flow of the river pinned beneath is already beginning to heave the flat sheets into piles of jagged ice which accumulate behind obstacles and shallows.

Ice and snow surround a small zone of open water in a high-gradient area of Conewago Falls.

Ice chunks and sheets accumulate atop the York Haven Dam.  The weight of miles of ice backed up behind the dam eventually forces the accumulation over the top and into the Pothole Rocks below.  The popping and cracking sounds of ice both above and below the dam could be heard throughout the day as hydraulic forces continuously break and move ice sheets.

Steam from the Unit 1 cooling towers at the Three Mile Island Nuclear Generating Station rises above the frozen Riverine Grasslands at Conewago Falls.  The scouring action of winter ice keeps the grasslands clear of substantial woody growth and prevents succession into forest.

Despite a lack of activity on the river, mixed flocks of resident and wintering birds, including this White-breasted Nuthatch (Sitta carolinensis), were busy feeding in the Riparian Woodlands.  The White-breasted Nuthatch is a cavity nester and year-round denizen of hardwoods, often finding shelter during harsh winter nights in small tree holes.

The White-breasted Nuthatch is often seen working its way head-first down a tree trunk as it probes with its well-adapted bill for insects among the bark.

Jackpot!

Looking upstream from the river’s east shore at ice and snow cover on the Susquehanna above Conewago Falls and the York Haven Dam.  The impoundment, known as Lake Frederic, and its numerous islands of the Gettysburg Basin Archipelago were locked in winter’s frosty grip today.  Hill Island (Left) and Poplar Island (Center) consist of erosion-resistant York Haven Diabase, as does the ridge on the far shoreline seen rising in the distance between them.  To the right of Poplar Island in this image, the river passes by the Harrisburg International Airport.  At the weather station there, the high temperature was eighteen degrees Fahrenheit on this first day of 2018.

Migrating North?

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A steady stream of birds was on the move this morning over Conewago Falls.  There were hundreds of Ring-billed Gulls, scores of Herring Gulls, and a few Great Black-backed Gulls to dominate the flight.  Then too there were thirteen Mallards, Turkey Vultures and a Black Vulture, twenty or more American Robins, a half a dozen Bald Eagles (juvenile and immature birds), a couple of Red-winged Blackbirds, and, perhaps most unusual of all, a flock of a dozen Scoters (Melanitta species), a waterfowl typical of the Mid-Atlantic surf in winter.  All of these birds were diligently following the river, and into a headwind no less.

“Hold on just a minute there, buster,” you may say, “I’ve looked at the migration count by dutifully clicking on the logo above and there is nothing but zeroes on the count sheet for today.  The season totals have not changed since the previous count day!”

Ah-ha, my dedicated friend, correct you are.  It seems that today’s bird flight was solely in one direction.  And that direction was upriver, moving north into a north breeze, on a heading which conflicts with all logic for creatures that should still be headed south for winter.  As a result, none of the birds observed today were counted on the “Autumn Migration Count”.

You might say, “Don’t you know that Winter Solstice was three days ago, so autumn and autumn migration is over.”

Okay, point well taken.  I should therefore clarify that what we title as “Autumn Migration Count” is more accurately a census of birds, insects, and other creatures transiting from northerly latitudes to more favorable latitudes to the south for winter.  This transit can begin as early as late June and extend into the first weeks of winter.  While most of this movement is motivated by the reduced hours of daylight during the period, late season migrants are often responding to ice, bad weather, or lack of food to prompt a journey further south.  Migration south in late December and January occurs even while the amount of daylight is increasing slightly in the days following the Winter Solstice.

So what of the birds seen flying north today?  There was some snow cover that has melted away, and the ice that formed on the river a week ago is gone due to the milder than normal temperatures this week.

One may ask, “Were the birds seen today migrating north?”

Let’s look at the species seen moving upriver today a try to determine their motivation.

First, and perhaps most straight-forward, is the huge flight of gulls.  Wintering gulls on the Susquehanna River near Conewago Falls tend to spend their nights in flocks on the water or on treeless islands and rocky outcrops in the river.  Many hundreds, sometimes thousands, find such favorable sites along the fifteen mile stretch of river from Conewago Falls downstream to Lake Clarke and the Conejohela Flats at Washington Boro.  Each morning most of these gulls venture out to suburbia, farmland, landfill, hydroelectric dams, and other sections of river in search of food.  Gulls are very able fliers and easily cover dozens of miles outbound and inbound each day in search of food.  Many of the gulls seen this morning were probably on their way to the Harrisburg metropolitan area to eat trash.  Barring any extraordinary buildups of ice on this section of river, one would expect these gulls to remain and make these daily excursions to food sources through early spring.

Ring-billed Gulls fly upriver through the Pothole Rocks at Conewago Falls.

Herring Gulls stream upriver through Conewago Falls on their way to fine dining.

Second, throughout the season Bald Eagles have been tallied on the migration count with caution.  Flight altitude, behavior, plumage, and the reaction of the “local” eagles to these transients was carefully considered before counting an eagle as a migrant.  They roam a lot, particularly when young, and range widely to feed.  The movement of eagles up the river today was probably food related.  A gathering of adult, juvenile, and immature Bald Eagles could be seen more than a half mile upstream from the migration count lookout.  Those moving up the river seemed to assemble with the “locals” there throughout the morning.  White-tailed Deities occasionally drown, particularly when there is thin or unstable ice on the river (as there was last week) and they attempt to tread upon it.  Then, their bodies are often stranded among rocks, in trees, or on the crown of the dam.  After such a mishap, their carcasses become meals for carrion-eaters in the falls.  Such an unfortunate deity, or another source of food, may have been attracting the eagles in numbers today.

A distant gathering of Bald Eagles at the south end of Three Mile Island in upper Conewago Falls.

Next, Black and Turkey Vultures often roam widely in search of food.  The small numbers seen headed up-river today would tend to mean very little when trying to determine if there is a trend or population shift.  Again, food may have been luring them upriver from nearby roosts.

And finally, the scoters, Mallards, American Robins, and Red-winged Blackbirds may have been wandering as well.  Toward mid-day, the wind speed picked up and the direction changed to the east.  This raises the possibility that these and others of the birds seen today may sense a change in weather, and may seek to take flight from the inclement conditions.  Prompted by the ocean breeze and in an attempt to avoid a storm, was there some movement away from the Atlantic Coastal Plain to the upper Piedmont today?  Many species may make these types of reactive movements.  Is it possible that some birds flee or avoid ever-changing storm tracks and alter there wintering locations based on jet streams, water currents, and other climatic conditions?  Probably.  These are interesting dynamics and something worthy of study outside the simpler methods of a migration count.

A Ring-billed Gull begins feeding as storm clouds approach Conewago Falls at mid-day.  This and other gull species travel widely in their winter range to find food and safe roosting sites.  For them, northward spring migration usually begins no earlier than late February.

Conewago Lineman

…And if it snows that stretch down south won’t ever stand the strain…                                                                  –Jimmy Webb

The lower Susquehanna valley’s first snowfall of the season arrived yesterday.  By this morning it measured just an inch in depth at Conewago Falls, more to the south and east, less to the west and north.  By mid-morning a cold fresh to moderate breeze from the northwest was blowing through the falls and stirring up ripples on the river.

Light snow on the Conewago Falls Pothole Rocks this morning.

Gulls sailed high overhead on the wind, taking a speedy ride downriver toward Chesapeake Bay, the Atlantic coast, and countless fast-food restaurant parking lots where surviving winter weather is more of a sure thing.  Nearly a thousand Ring-billed Gulls soared past the migration count lookout today.  Thirteen Herring Gulls and four Great Black-backed Gulls were among them.

Other migrants today included a Mallard, twenty-nine American Black Ducks, two Bald Eagles, eleven Black Vultures, fifteen Turkey Vultures, five American Goldfinches, and fifteen Red-winged Blackbirds.  The wintery weather seems to be prompting these late-season travelers to be on their way.

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You know, today was like many other days at the falls.  As I arrive, I have the habit of checking all the power line towers on both river shorelines to see what may be there awaiting discovery.  More often than not, something interesting is perched on one or more of the structures…

…sometimes there are large flocks of European Starlings…

…other times there might be one or more Turkey Vultures…

…or possibly a Bald Eagle or two…

…or maybe the fastest-flying bird on the planet…

…or perhaps, wait; what’s he doing up there?

Yes friends, while the birds migrated through high above, down below a coordinated effort was underway to replace some of the electric transmission cable that stretches across the Susquehanna River at Conewago Falls.  As you’ll see, this project requires precise planning, preparation, and skill.  And it was fascinating to watch!

A helicopter is used to raise/lower men and equipment to/from the top of the towers.

A crew doing preparation work is lifted from a tower on the west shore of Conewago Falls.

A crew member is raised to a tower on the east shore of Conewago Falls to begin the next phase of the project.

Crew members are positioned on the two towers on the west shore.

The helicopter hovers in a stand-by position above the Pothole Rocks.  By keeping the chopper downwind from and below the wires being replaced, the pilot avoids putting rotor wash into the work area.   Note the linemen on the upper left side of each tower.  These men monitored the pulleys as the old cable, followed by the new, was pulled from the west shore to the east.

At the ready, the pilot skillfully hovers his craft, nose into the gusty wind, just 100 feet to the east of the migration count site on the Pothole Rocks.

Even as the chopper maintained position near and immediately over the count lookout, migrating birds continued to be seen streaming in a downriver direction high above.

A migrating immature Bald Eagle passes overhead, apparently undaunted by the commotion created by the use of a helicopter to tend the crew advancing replacement wire across the river below.

With the new cable in place, workers are lifted from the towers and lowered to the ground where they can get out of the cold wind after a job well done.

Anthropoavians

Temperatures plummeted to well below freezing during the past two nights, but there was little sign of it in Conewago Falls this morning.  The fast current in the rapids and swirling waters in flooded Pothole Rocks did not freeze.  Ice coated the standing water in potholes only in those rocks lacking a favorable orientation to the sun for collecting solar heat during the day to conduct into the water during the cold nights.

On the shoreline, the cold snap has left its mark.  Ice covers the still waters of the wetlands.  Frost on exposed vegetation lasted until nearly noontime in shady areas.  Insect activity is now grounded and out of sight.  The leaves of the trees tumble and fall to cover the evidence of a lively summer.

The nocturnal bird flight is narrowing down to just a few species.  White-throated Sparrows, a Swamp Sparrow (Melospiza georgiana), and Song Sparrows are still on the move.  Though their numbers are not included in the migration count, hundreds of the latter are along the shoreline and in edge habitat around the falls right now.  Song Sparrows are present year-round, migrate at night, and are not seen far from cover in daylight, so migratory movements are difficult to detect.  It is certain that many, if not all of the Song Sparrows here today have migrated and arrived here recently.  The breeding population from spring and summer has probably moved further south.  And many of the birds here now may remain for the winter.  Defining the moment of this dynamic, yet discrete, population change and logging it in a count would certainly require different methods.

Song Sparrows are now abundant in the brushy edges of fields and woodlands.  They may even break into song on sunny days.

Diurnal migration was foiled today by winds from southerly directions and moderating temperatures.  The only highlight was an American Robin flight that extended into the morning for a couple of hours after daybreak and totaled over 800 birds.  This flight was peppered with an occasional flock of blackbirds.  Then too, there were the villains.

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They’re dastardly, devious, selfish, opportunistic, and abundant.  Today, they were the most numerous diurnal migrant.  Their numbers made this one of the biggest migration days of the season, but they are not recorded on the count sheet.  It’s no landmark day.  They excite no one.  For the most part, they are not recognized as migrants because of their nearly complete occupation of North America south of the taiga.  If people build on it or alter it, these birds will be there.  They’re everywhere people are.  If the rotten attributes of man were wrapped up into one bird, an “anthropoavian”, this would be it.

Meet the European Starling (Sturnus vulgaris).  Introduced into North America in 1890, the species has spread across the entire continent.  It nests in cavities in buildings and in trees.  Starlings are aggressive, particularly when nesting, and have had detrimental impacts on the populations of native cavity nesting birds, particularly Red-headed Woodpeckers, Purple Martins (Progne subis), and Eastern Bluebirds.  They commonly terrorize these and other native species to evict them from their nest sites.  European Starlings are one of the earlier of the scores of introduced plants and animals we have come to call invasive species.

Noisy flocks of European Starlings are right at home on man-made structures in city and country.

Today, thousands of European Starlings were on the move, working their way down the river shoreline and raiding berries from the vines and trees of the Riparian Woodlands.  My estimate is between three and five thousand migrated through during the morning.  But don’t worry, thousands more will be around for the winter.

European Starlings mob a Sharp-shinned Hawk from above, a common behavior.

An Eastern Bluebird feeds on the few berries left untouched by passing European Starlings.

Feathered Fallout

The NOAA National Weather Service radar images from last evening provided an indication that there may be a good fallout of birds at daybreak in the lower Susquehanna valley.  The moon was bright, nearly full, and there was a gentle breeze from the north to move the nocturnal migrants along.  The conditions were ideal.

Rising from daytime roosts in New York and Pennsylvania, then streaming south in moonlit skies, migrating birds are recorded as echoes on this post-sunset composite NEXRAD loop from last evening.  (NOAA/National Weather Service image)

The Riparian Woodlands at Conewago Falls were alive with migrants this morning.  American Robins and White-throated Sparrows were joined by new arrivals for the season: Brown Creeper (Certhia americana), Ruby-crowned Kinglets (Regulus calendula), Golden-crowned Kinglets (Regulus satrapa), Dark-eyed Junco (Junco hyemalis), and Yellow-rumped Warbler (Setophaga coronata).  These are the perching birds one would expect to have comprised the overnight flight.  While the individuals that will remain may not yet be among them, these are the species we will see wintering in the Mid-Atlantic states.  No trip to the tropics for these hardy passerines.

American Robins continued migratory flight into the first hour of daylight this morning.  Their calls are commonly heard at night as migrating individuals pass overhead.

White-throated Sparrows are nocturnal migrants, and are a familiar find on woodland edges and at suburban feeding stations through the winter.

Dark-eyed Juncos, also nocturnal migrants, are common winter residents in the Lower Susquehanna River Watershed, frequently visiting bird feeders.

Heavy rain earlier this week in the Susquehanna River drainage basin has flooded most of the Pothole Rocks; the rapids of Conewago Falls have returned.

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A Quick Getaway

It was a placid morning on Conewago Falls with blue skies dotted every now and then by a small flock of migrating robins or blackbirds.  The jumbled notes of a singing Winter Wren (Troglodytes hiemalis) in the Riparian Woodland softly mixed with the sounds of water spilling over the dam.  The season’s first Wood Ducks (Aix sponsa), Blue-winged Teal (Spatula discors), Herring Gull (Larus argentatus), Horned Larks (Eremophila alpestris), and White-throated Sparrows (Zonotrichia albicollis) were seen.

There was a small ruckus when one of the adult Bald Eagles from a local pair spotted an Osprey passing through carrying a fish.  This eagle’s effort to steal the Osprey’s catch was soon interrupted when an adult eagle from a second pair that has been lingering in the area joined the pursuit.  Two eagles are certainly better than one when it’s time to hustle a skinny little Osprey, don’t you think?

But you see, this just won’t do.  It’s a breach of eagle etiquette, don’t you know?  Soon both pairs of adult eagles were engaged in a noisy dogfight.  It was fussing and cackling and the four eagles going in every direction overhead.  Things calmed down after about five minutes, then a staring match commenced on the crest of the dam with the two pairs of eagles, the “home team” and the “visiting team”, perched about 100 feet from each other.  Soon the pair which seems to be visiting gave up and moved out of the falls for the remainder of the day.  The Osprey, in the meantime, was able to slip away.

In recent weeks, the “home team” pair of Bald Eagles, seen regularly defending territory at Conewago Falls, has been hanging sticks and branched tree limbs on the cross members of the power line tower where they often perch.  They seem only to collect and display these would-be nest materials when the “visiting team” pair is perched in the nearby tower just several hundred yards away…an attempt to intimidate by homesteading.  It appears that with winter and breeding time approaching, territorial behavior is on the increase.

The second migrating Osprey of the day ran the gauntlet of marauding eagles without incident.

In the afternoon, a fresh breeze from the south sent ripples across the waters among the Pothole Rocks.  The updraft on the south face of the diabase ridge on the east shore was like a highway for some migrating hawks, falcons, and vultures.  Black Vultures (Coragyps atratus) and Turkey Vultures streamed off to the south headlong into the wind after leaving the ridge and crossing the river.  A male and female Northern Harrier (Circus hudsonius), ten Red-tailed Hawks, two Red-shouldered Hawks (Buteo lineatus), six Sharp-shinned Hawks, and two Merlins crossed the river and continued along the diabase ridge on the west shore, accessing a strong updraft along its slope to propel their journey further to the southwest.  Four high-flying Bald Eagles migrated through, each following the east river shore downstream and making little use of the ridge except to gain a little altitude while passing by.

(Top and Middle) Turkey Vultures riding the fresh breeze and teetering to-and-fro on up-tilted wings.  This wing posture is known as a dihedral.  (Bottom) More than 100 migrating Black Vultures climbed high on the afternoon breeze to make an oblique crossing of the river and maintain a southbound course.

Late in the afternoon, the local Bald Eagles were again airborne and cackling up a storm.  This time they intercepted an eagle coming down the ridge toward the river and immediately forced the bird to climb if it intended to pass.  It turned out to be the best sighting of the day, and these “home team” eagles found it first.  It was a Golden Eagle (Aquila chrysaetos) in crisp juvenile plumage.  On its first southward voyage, it seemed to linger after climbing high enough for the Bald Eagles to loose concern, then finally selected the ridge route and crossed the river to head off to the southwest.

Ring-billed Gulls began feeding during the afternoon as clouds preceding stormy weather approached.

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Summer Breeze

A moderate breeze from the south placed a headwind into the face of migrants trying to wing their way to winter quarters.  The urge to reach their destination overwhelmed any inclination a bird or insect may have had to stay put and try again another day.

Blue Jays were joined by increasing numbers of American Robins crossing the river in small groups to continue their migratory voyages.  Killdeer (Charadrius vociferous) and a handful of sandpipers headed down the river route.  Other migrants today included a Cooper’s Hawk (Accipiter cooperii), Eastern Bluebirds (Sialia sialis), and a few Common Mergansers (Mergus merganser), House Finches (Haemorhous mexicanus), and Common Grackles (Quiscalus quiscula).

The afternoon belonged to the insects.  The warm wind blew scores of Monarchs toward the north as they persistently flapped on a southwest heading.  Many may have actually lost ground today.  Painted Lady (Vanessa cardui) and Cloudless Sulphur butterflies were observed battling their way south as well.  All three of the common migrating dragonflies were seen: Common Green Darner (Anax junius), Wandering Glider (Pantala flavescens), and Black Saddlebags (Tramea lacerata).

The warm weather and summer breeze are expected to continue as the rain and wind from Hurricane Nate, today striking coastal Alabama and Mississippi, progresses toward the Susquehanna River watershed during the coming forty-eight hours.

This Great Blue Heron was joined by numerous other fishermen and a good number of sightseers in the falls today.

A colorful young Northern Water Snake (Nerodia sipedon) takes advantage of the sun-heated surface of a Pothole Rock to remain nimble and active.  Cooler weather will soon compel this and other reptiles to find shelter for winter hibernation.

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Swallows by the Thousands

A fresh breeze from the north brought cooler air and a reminder that summer is gone and autumn has arrived.

Fast-moving dark clouds provided a perfect backdrop for viewing passing diurnal migrants.  Bald Eagles utilized the tail wind to cruise down the Susquehanna toward Chesapeake Bay and points further south.  A migrating Merlin began a chase from which a Northern Flicker narrowly escaped by finding shelter among Pothole Rocks and a few small trees.  The season’s first American Black Duck (Anas rubripes), Common Loon (Gavia immer), Yellow-bellied Sapsucker (Sphyrapicus varia), and American Pipits (Anthus rubescens) moved through.

Blue Jays continued their hesitant crossings of the river at Conewago Falls.  The majority completed the journey by forming groups of a dozen or more birds and following the lead of a lone American Robin, a Northern Flicker, or, odd as it appeared, a small warbler.

By far the most numerous migrants today were swallows.  Thousands of Northern Rough-winged Swallows and hundreds of Tree Swallows were on the wing in search of what was suddenly a sparse flying insect supply.  To get out of the brisk wind, some of the more resourceful birds landed on the warm rocks.  To satisfy their appetite, many were able to pick crawling arthropods from the surface of the boulders.  They swallow them whole.

A few of the thousands of swallows seen at Conewago Falls today.

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Piles of Green Tape

A couple of inches of rain this week caused a small increase in the flow of the river, just a burp, nothing major.  This higher water coincided with some breezy days that kicked up some chop on the open waters of the Susquehanna upstream of Conewago Falls.  Apparently it was just enough turbulence to uproot some aquatic plants and send them floating into the falls.

Piled against and upon the upstream side of many of the Pothole Rocks were thousands of two to three feet-long flat ribbon-like opaque green leaves of Tapegrass, also called Wild Celery, but better known as American Eelgrass (Vallisneria americana).  Some leaves were still attached to a short set of clustered roots.  It appears that most of the plants broke free from creeping rootstock along the edge of one of this species’ spreading masses which happened to thrive during the second half of the summer.  You’ll recall that persistent high water through much of the growing season kept aquatic plants beneath a blanket of muddy current.  The American Eelgrass colonies from which these specimens originated must have grown vigorously during the favorable conditions in the month of August.  A few plants bore the long thread-like pistillate flower stems with a fruit cluster still intact.  During the recent few weeks, there have been mats of American Eelgrass visible, the tops of their leaves floating on the shallow river surface, near the east and west shorelines of the Susquehanna where it begins its pass through the Gettysburg Basin near the Pennsylvania Turnpike bridge at Highspire.  This location is a probable source of the plants found in the falls today.

Uprooted American Eelgrass floating into the Pothole Rocks under the power of a north wind.  Note the white thread-like pistillate flower stem to the left and the small rooted specimen to the upper right.  The latter is likely a plant from the creeping rootstock on the edge of a colony.  As a native aquatic species, American Eelgrass is a critical link in the Susquehanna River and Chesapeake Bay food chain.  Its decimation by pollution during the twentieth century led to migration pattern alterations and severe population losses for the Canvasback (Aythya valisineria) duck.

American Eelgrass, a very small specimen, found growing in a low-lying Pothole Rock alongside the accumulations of freshly arriving material from upstream.  Note that the creeping rootstock has leaves growing from at least three nodes on this plant.  Eelgrass dislocations are regular occurrences which sometimes begin new colonies, like the small one seen here in this Diabase Pothole Rock Microhabitat.

The cool breeze from the north was a perfect fit for today’s migration count.  Nocturnal migrants settling down for the day in the Riparian Woodlands at sunrise included more than a dozen warblers and some Gray Catbirds (Dumetella carolinensis).  Diurnal migration was underway shortly thereafter.

A moderate flight of nocturnal migrants is indicated around NEXRAD sites in the northeastern states at 3:18 AM EDT.  The outer rain bands of Hurricane Irma can be seen approaching the Florida Keys as the storm closes in on the peninsula.  (NOAA/National Weather Service image)

Four Bald Eagles were counted as migrants this morning.  Based on plumage, two were first-year eagles (Juvenile) seen up high and flying the river downstream, one was a second-year bird (Basic I) with a jagged-looking wing molt, and a third was probably a fourth year (Basic III) eagle looking much like an adult with the exception of a black terminal band on the tail.  These birds were the only ones which could safely be differentiated from the seven or more Bald Eagles of varying ages found within the past few weeks to be lingering at Conewago Falls.  There were as many as a dozen eagles which appeared to be moving through the falls area that may have been migrating, but the four counted were the only ones readily separable from the locals.

Red-tailed Hawks (Buteo jamaicensis) were observed riding the wind to journey not on a course following the river, but flying across it and riding the updraft on the York Haven Diabase ridge from northeast to southwest.

Bank Swallows (Riparia riparia) seem to have moved on.  None were discovered among the swarms of other species today.

Ruby-throated Hummingbirds, Caspian Terns, Cedar Waxwings (Bombycilla cedrorum), and Chimney Swifts (Chaetura pelagica) were migrating today, as were Monarch butterflies.

Not migrating, but always fun to have around, all four wise guys were here today.  I’m referring to the four members of the Corvid family regularly found in the Mid-Atlantic states: Blue Jay (Cyanocitta cristata), American Crow (Corvus brachyrhynchos), Fish Crow (Corvus ossifragus), and Common Raven (Corvus corax).

It looks like a big Blue Jay, but it’s not.  This Belted Kingfisher (Megaceryle alcyon) takes a break after flying around the falls trying to shake a marauding Ruby-throated Hummingbird off its tail.

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SOURCES

Klots, Elsie B.  1966.  The New Field Book of Freshwater Life.  G. P. Putnam’s Sons.  New York, NY.

Suggestive Selling

A Baltimore Oriole (Icterus galbula) glowed in the first sunlight of the day as it began illuminating the treetops.  I’m not certain of the cause, but I often have the urge to dig into a bowl of orange sherbet after seeing one these magnificent blackbirds.  That’s right, in the Americas, orioles and blackbirds are members of the same family, Icteridae.  Look at blackbirds more carefully, you might see the resemblance.

Sunshine at dawn and migrating warblers were again active in the foliage.  Eight species were identified today.  Off to the tropics they go.  To the land of palm and citrus, yes citrus…limes, lemons, grapefruits, and oranges.

The Ruby-throated Hummingbirds (Archilochus colubris) are on the way toward the gulf states, then on to Central and South America.  Five dashed by the rocky lookout in the falls this morning.  Remember, keep your feeders clean, wash and rinse all the parts, and refill them with a fresh batch of “nectar”, four or five parts water to one part sugar.  Repeating this process daily during hot weather should keep contamination from overtaking your feeder.  It’s not a bad idea to rotate two feeders.  Have one cleaned, rinsed, and air drying while the second is filled and in use at your feeding station, then just swap them around.  Your equipment will be just as clean as it is at the sanitary dairy…you know, where they make sherbet.

The first of the season Caspian Terns (Hydroprogne caspia), giant freshwater versions of the terns you see at the seashore, passed through the falls late this morning.  Their bills are blood-red, not orange like the more familiar terns on the coast.  They’re stunning.

Ring-billed Gulls (Larus delawarensis) have been at the falls for several weeks.  Total numbers and the composition of the age groups in the flock change over the days, so birds appear to be trickling through and are then replaced by others coming south.  The big push of southbound migrants for this and many other species that winter locally in the Mid-Atlantic region and in the southern United States is still more than a month away.  There are still plenty more birds to come after the hours of daylight are reduced and the temperatures take a dip.

A Ring-billed Gull on the lookout for a morning snack.  They’ll eat almost anything and do a good job of keeping the river picked clean of the remains of animals that have met misfortune.  They’ll linger around landfills, hydroelectric dams, and fast-food restaurant parking lots through the winter.

Turkey Vultures (Cathartes aura) are common around the falls due to the abundance of carrion in the vicinity and because of the strong thermal updrafts of air over the sun-heated Pothole Rocks.  These rising currents provide lift for circling vultures.  We would expect migrating birds of a number of species will also take advantage of these thermals to gain altitude and extend the distance of their glides.

Some migrating butterflies were counted today.  Cloudless Sulphurs, more of a vagrant than a migrant, and, of course, Monarchs.  I’ll bet you know the Monarch, it’s black and orange.  How can you miss them, colored orange.

That’s it, that’s all for now, I bid you adieu…I’m going to have a dip of orange sherbet, or two.

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Harvey Passes By

Rain from the remnants of Hurricane/Tropical Storm Harvey ended just after daybreak this morning.  Locally, the precipitation was mostly absorbed into the soil.   There was little runoff and no flooding.  The river level at Conewago Falls is presently as low as it has been all summer.  Among the pools and rapids of the Pothole Rocks, numbers of migrating birds are building.

Mist and a low cloud ceiling created poor visibility while trying to see early morning birds, but they’re here.  The warblers are moving south and a small wave of them was filtering through the foliage on the edge of the Riparian Woodlands.  One must bend backwards to have a look, and most could not be identified due to the poor lighting in the crowns of the trees where they were zipping about.  Five species of warblers and two species of vireos were discerned.

There are increasing concentrations of swallows feeding on insects over the falls.  Hundreds were here today, mostly Tree Swallows (Tachycineta bicolor).  Bank Swallows (Stelgidopteryx riparia) numbered in the hundreds, far below the thousands, often 10,000, which staged here for migration and peaked during the first week of September annually during the 1980s and 1990s.  Their numbers have been falling steadily.  Loss of nesting locations in embankments near water may be impacting the entire population.  A reduction in the abundance of late-summer flying insects here on the lower Susquehanna River may be cause for them to abandon this area as a migration staging point.

Bank and Tree Swallows by the hundreds were feeding upon flying insects above the waters of Conewago Falls today.  Lesser numbers of Northern Rough-winged Swallows (Stelgidopteryx serripennis) and Barn Swallows (Hirundo rustica) joined the swarm.

Clear weather in the coming nights and days may get the migrants up and flying in large numbers.  For those species headed to the tropics for winter, the time to get moving has arrived.

CLICK ON THE LOGO FOR TODAY’S MIGRATION COUNT TOTALS

A Little Black Spot on the Sun Today

Was there a better place to have a look at the dark side of the moon easing across the summer sun than from the Pothole Rocks at Conewago Falls?  O.K., alright, so there must have been a venue or two with bigger crowds, grand emotions, prepared foods, and near darkness, but the pseudolunar landscape of the falls seemed like an ideal observation point for the great North American solar eclipse of 2017.

The craters of the moon right here on earth, the Pothole Rocks of Conewago Falls.

Being the only person on the entire falls had its advantages, not the least of which was the luxury of pointing the camera directly at the sun and clicking off a few shots without getting funny looks and scolding comments.  Priceless solitude.

Point that camera right at that eclipse for a nice little photograph of the big event.

If you think it looks like the above photograph was taken in a house of mirrors, then you’re pretty sharp.  You’ve got it figured out.  After getting a bad case of welder’s burns on the first day of a job at a metal fabricating shop during my teen years, I learned the value of a four dollar piece of glass.

A number 12 welder’s lens in action while viewing and photographing today’s solar eclipse.

The eclipse at 2:19 P.M. Eastern Daylight Time (18:19 U.T.C), nearly as good as it was going to get at Conewago Falls.  The lunar disc would continue to the left, leaving the top fifth of the sun “uneclipsed”.

For those of you who prefer not to look at the sun, even with protection (I heard those S.P.F. 30 sunblock eye drops were a fraud…I hope you didn’t buy any.), here is the indirect viewing method as it happened today.

The eclipse is projected into the bottom of a cavernous hole in a Pothole Rock. (Three or four people can sit inside this hole.)  The tube, lenses, and mirrors of one side of a pair of binoculars were used to focus the thin sliver of sunlight onto the diabase stone “floor”.  The optics have inverted the image.

If you were to our south in the path of totality for this eclipse, you probably noted reactions by flora and fauna.  Here, there was really not much to report.  The leaves of Partridge Pea didn’t fold for the night, birds didn’t fly away to roost, and the chorus of evening and nighttime singing insects didn’t get cranked up.  The only sensation was the reduced brightness of the sun, as if a really dark cloud was filtering the light without changing its color or eliminating shadows.  And that was the great solar eclipse of 2017.

S’more

The tall seed-topped stems swaying in a summer breeze are a pleasant scene.  And the colorful autumn shades of blue, orange, purple, red, and, of course, green leaves on these clumping plants are nice.  But of the multitude of flowering plants, Big Bluestem, Freshwater Cordgrass, and Switchgrass aren’t much of a draw.  No self-respecting bloom addict is going out of their way to have a gander at any grass that hasn’t been subjugated and tamed by a hideous set of spinning steel blades.  Grass flowers…are you kidding?

Big Bluestem in flower in the Riverine Grasslands at Conewago Falls.

O.K., so you need something more.  Here’s more.

Meet the Partridge Pea (Chamaecrista fasciculata).  It’s an annual plant growing in the Riverine Grasslands at Conewago Falls as a companion to Big Bluestem.  It has a special niche growing in the sandy and, in summertime, dry soils left behind by earlier flooding and ice scour.  The divided leaves close upon contact and also at nightfall.  Bees and other pollinators are drawn to the abundance of butter-yellow blossoms.  Like the familiar pea of the vegetable garden, the flowers are followed by flat seed pods.

The Partridge Pea can tolerate dry sandy soils.

But wait, here’s more.

In addition to its abundance in Conewago Falls, the Halberd-leaved Rose Mallow (Hibiscus laevis) is the ubiquitous water’s edge plant along the free-flowing Susquehanna River for miles downstream.  It grows in large clumps, often defining the border between the emergent zone and shore-rooted plants.  It is particularly successful in accumulations of alluvium interspersed with heavier pebbles and stone into which the roots will anchor to endure flooding and scour.  Such substrate buildup around the falls, along mid-river islands, and along the shores of the low-lying Riparian Woodlands immediately below the falls are often quite hospitable to the species.

Halberd-leaved Rose Mallow is a durable inhabitant of the falls.  Regular flooding keeps competing species at bay.  A taproot helps to safeguard against dislocation, allowing plants to grow in places subjected to turbulent currents.

Halberd-leaved Rose Mallow in bloom.  The similarity to cultivated members of the Hibiscus genus can readily be seen.  It is one of the showiest of perennial wildflowers in the floodplain.  Note the lobed, halberd-shaped leaves, source of its former species name militaris.

The seeds of Halberd-leaved Rose Mallow are contained in bladders which can float to assist in their distribution.  Some of these bladders cling to the dead leafless stems in winter, making it an easy plant to identify in nearly any season.

A second native wildflower species in the genus Hibiscus is found in the Conewago Falls floodplain, this one in wetlands.  The Swamp Rose Mallow (H. moscheutos) is similar to Halberd-leaved Rose Mallow, but sports more variable and colorful blooms.  The leaves are toothed without the deep halberd-style lobes and, like the stems, are downy.  As the common name implies, it requires swampy habitat with ample water and sunlight.

Swamp Rose Mallow in a sunny wetland.  This variety with solid-colored flowers (without dark centers) and pale green leaves and stems was formerly known as a separate species of  Swamp Rose Mallow, H. palustris.  Note that the flowers are terminal on the stems.

A few scattered specimens of a more typical variety of Swamp Rose Mallow are found on the shoreline and in the Riverine Grasslands of Conewago Falls.  The blooms are bright pink with darker centers and the leaf stems are robust and reddish.  This one is seen growing among Halberd-leaved Rose Mallow, with which it shares the characteristic of having flower stems growing from some of the upper leaf axils.  A variety with red-centered white flowers is often found throughout the plant’s range.

In summary, we find Partridge Pea in the Riverine Grasslands growing in sandy deposits left by flood and ice scour.  We find Halberd-leaved Rose Mallow rooted at the border between shore and the emergent zone.  We find Swamp Rose Mallow as an emergent in the wetlands of the floodplain.  And finally, we find marshmallows in only one location in the area of Conewago Falls.  Bon ap’.

Here’s S’more

SOURCES

Newcomb, Lawrence.  1977.  Newcomb’s Wildflower Guide.  Little, Brown and Company.  Boston, Massachusetts.

Summer Grasses

It has not been a good summer if you happen to be a submerged plant species in the lower Susquehanna River.  Regularly occurring showers and thunderstorms have produced torrents of rain and higher than usual river stages.  The high water alone wouldn’t prevent you from growing, colonizing a wider area, and floating several small flowers on the surface, however, the turbidity, the suspended sediment, would.  The muddy current casts a dirty shadow on the benthic zone preventing bottom-rooted plants from getting much headway.  There will be smaller floating mats of the uppermost leaves of these species.  Fish and invertebrates which rely upon this habitat for food and shelter will find sparse accommodation…better luck next year.

Due to the dirty water, fish-eating birds are having a challenging season as they try to catch sufficient quantities of prey to feed themselves and their offspring.  A family of Ospreys (Pandion haliaetus) at Conewago Falls, including recently fledged young, were observed throughout this morning and had no successful catches.  Of the hundred or more individual piscivores of various species present, none were seen retrieving fish from the river.  The visibility in the water column needs to improve before fishing is a viable enterprise again.

Ospreys competing for a suitable fishing perch.  Improving water conditions in the coming week should increase their success as predators.

Versatile at finding food, adult Bald Eagles are experienced and know to be on the lookout for Ospreys with fish, a meal they can steal through intimidation.

While the submerged plant communities may be stunted by 2017’s extraordinary water levels, there is a very unique habitat in Conewago Falls which endures summer flooding and, in addition, requires the scouring effects of river ice to maintain its mosaic of unique plants.  It is known as a Riverine Grassland or scour grassland.

The predominant plants of the Riverine Grasslands are perennial warm-season grasses.  The deep root systems of these hardy species have evolved to survive events which prevent the grassland from reverting to woodland through succession.  Fire, intense grazing by wild herd animals, poor soils, drought, and other hardships, including flooding and ice scour, will eliminate intolerant plant species and prevent an area from reforesting.  In winter and early spring, scraping and grinding by flood-driven chunk ice mechanically removes large woody and poorly rooted herbaceous growth from susceptible portions of the falls.  These adverse conditions clear the way for populations of species more often associated with North America’s tall grass prairies to take root.  Let’s have a look at some of the common species found in the “Conewago Falls Pothole Rocks Prairie”.

Big Bluestem (Andropogon gerardi), seen here growing in the cracks of a pothole rock. High water nourishes the plant by filling the crevices with nutrient-loaded sediment. This species evolved with roots over three feet deep to survive fires, trampling by bison, and drought.

Freshwater Cordgrass (Spartina pectinata) does well with its roots in water.  It creates exceptional bird habitat and grows in the falls and on ice-scoured small islands in free-flowing segments of the Susquehanna River downstream.

Switchgrass (Panicum virgatum), like Big Bluestem, is one of the tall grass prairie species and, like Freshwater Cordgrass, grows in near pure stands on ice-scoured islands.  It takes flooding well and its extensive root system prevents erosion.

Though not a grass, Water Willow (Justicia americana) is familiar as a flood-enduring emergent plant of river islands, gravel bars, and shorelines where its creeping rhizome root system spreads the plant into large masses.  These stands are often known locally as “grass beds”.  This member of the acanthus family provides habitat for fish and invertebrates among its flooded leaves and stems.  Its presence is critical to aquatic life in a year such as this.

The Conewago Falls Riverine Grassland is home to numerous other very interesting plants.  We’ll look at more of them next time.

SOURCES

Brown, Lauren.  1979.  Grasses, An Identification Guide.  Houghton Mifflin Company.  New York, NY.

Fussy Eaters

She ate only toaster pastries…that’s it…nothing else.  Every now and then, on special occasions, when a big dinner was served, she’d have a small helping of mashed potatoes, no gravy, just plain, thank you.  She received all her nutrition from several meals a week of macaroni and cheese assembled from processed ingredients found in a cardboard box.  It contains eight essential vitamins and minerals, don’t you know?  You remember her, don’t you?

Adult female butterflies must lay their eggs where the hatched larvae will promptly find the precise food needed to fuel their growth.  These caterpillars are fussy eaters, with some able to feed upon only one particular species or genus of plant to grow through the five stages, the instars, of larval life.  The energy for their fifth molt into a pupa, known as a chrysalis, and metamorphosis into an adult butterfly requires mass consumption of the required plant matter.  Their life cycle causes most butterflies to be very habitat specific.  These splendid insects may visit the urban or suburban garden as adults to feed on nectar plants, however, successful reproduction relies upon environs which include suitable, thriving, pesticide-free host plants for the caterpillars.  Their survival depends upon more than the vegetation surrounding the typical lawn will provide.

The Monarch (Danaus plexippus), a butterfly familiar in North America for its conspicuous autumn migrations to forests in Mexico, uses the milkweeds (Asclepias) almost exclusively as a host plant.  Here at  Conewago Falls, wetlands with Swamp Milkweed (Asclepias incarnata) and unsprayed clearings with Common Milkweed (A. syriaca) are essential to the successful reproduction of the species.  Human disturbance, including liberal use of herbicides, and invasive plant species can diminish the biomass of the Monarch’s favored nourishment, thus reducing significantly the abundance of the migratory late-season generation.

Monarch caterpillar after a fourth molt.  The fifth instar feeding on Swamp Milkweed.

A fifth molt begets the Monarch pupa, the chrysalis, from which the showy adult butterfly will emerge.

Adult Monarch feeding on Goldenrod (Solidago) nectar.

Butterflies are good indicators of the ecological health of a given environment.  A diversity of butterfly species in a given area requires a wide array of mostly indigenous plants to provide food for reproduction.  Let’s have a look at some of the species seen around Conewago Falls this week…

An adult Silvery Checkerspot (Chlosyne nycteis) visiting a nectar plant, Partridge Pea (Chamaecrista fasciculata).  Wingstem (Verbesina alternifolia), a plant of the Riparian Woodlands, is among the probable hosts for the caterpillars.

A Gray Hairstreak (Strymon melinus) visits Crown Vetch, a possible host plant.  Other potential larval food in the area includes Partridge Pea, Halberd-leaved Rose Mallow (Hibiscus laevis) of the river shoreline, and Swamp Rose Mallow (Hibiscus moscheutos), a plant of wetlands.

The Eastern Tailed Blue (Cupido comyntas) may use Partridge Pea , a native wildflower species, and the introduced Crown Vetch (Securigera/Coronilla varia) as host and nectar plants at Conewago Falls.

The Least Skipper (Ancyloxypha numitor) is at home among tall grasses in woodland openings, at riverside, and in the scoured grassland habitat of the Pothole Rocks in the falls.  Host plants available include Switchgrass (Panicum vigatum), Freshwater Cordgrass (Spartina pectinata), and Foxtails (Setaria).

The Zabulon Skipper (Poanes zabulon) is an inhabitant of moist clearings where the caterpillars may feed upon Lovegrasses (Eragrostis) and Purpletop (Tridens flavus).

The Eastern Tiger Swallowtail (Papilio glaucus), a female seen here gathering nectar from Joe-Pye Weed (Eutrochium), relies upon several forest trees as hosts. Black Cherry (Prunus serotina), Willow (Salix), Yellow Poplar (Liriodendron tulipifera), also known as Tuliptree, and Green Ash (Fraxinus pennsylvanica) are among the local species known to be used.  The future of the latter food species at Conewago Falls is doubtful.  Fortunately for the Eastern Tiger Swallowtail, the “generalist” feeding requirements of this butterfly’s larvae enable the species to survive the loss of a host plant.

A female Eastern Tiger Swallowtail, black morph, gathering nectar from Joe-Pye Weed.

The Zebra Swallowtail (Protographium marcellus), an adult seen here on Joe-Pye Weed, feeds exclusively upon Pawpaw (Asimina) trees as a caterpillar.  This butterfly species may wander, but its breeding range is limited to the moist Riparian Woodlands where colonial groves of Pawpaw may be found.  The Common Pawpaw (Asimina triloba), our native species in Pennsylvania, and the Zebra Swallowtail occur at the northern edge of their geographic ranges in the Lower Susquehanna River Watershed.  Planting Pawpaw trees as an element of streamside reforestation projects certainly benefits this marvelous butterfly.

The spectacularly colorful butterflies are a real treat on a hot summer day.  Their affinity for showy plants doubles the pleasure.

By the way, I’m certain by now you’ve recalled that fussy eater…and how beautiful she grew up to be.

SOURCES

Brock, Jim P., and Kaufman, Kenn.  2003.  Butterflies of North America.  Houghton Mifflin Company.  New York, NY.

The Wall

It was one of the very first of my memories.  From the lawn of our home I could look across the road and down the hill through a gap in the woodlands.  There I could see water, sometimes still with numerous boulders exposed, other times rushing, muddy, and roaring.  Behind these waters was a great stone wall and beyond that a wooded hillside.  I recall my dad asking me if I could see the dam down there.  I couldn’t see a dam, just fascinating water and the gray wall behind it.  I looked and searched but not a trace of a structure spanning the near to far shore was to be seen.  Finally, at some point, I answered in the affirmative to his query; I could see the dam…but I couldn’t.

We lived in a small house in the village of Falmouth along the Susquehanna River in the northwest corner of Lancaster County over fifty years ago.  A few years after we had left our riverside domicile and moved to a larger town, the little house was relocated to make way for an electric distribution sub-station and a second set of electric transmission wires in the gap in the woodlands.  The Brunner Island coal-fired electric generating station was being upgraded downstream and, just upstream, a new nuclear-powered generating station was being constructed on Three Mile Island.  To make way for the expanding energy grid, our former residence was trucked to a nearby boat landing where there were numerous other river shacks and cabins.  Because it was placed in the floodplain, the building was raised onto a set of wooden stilts to escape high water.  It didn’t help.  The record-breaking floods of Hurricane Agnes in June of 1972 swept the house away.

The view through the cut in the woodland, a little wider than in the early 1960s with the addition of the newer electric transmission wire towers. The “Wall” is the same.

During the time we lived along the Susquehanna, the river experienced record-low flow rates, particularly in the autumn of 1963 and again in 1964.  My dad was a dedicated 8mm home-movie photographer.  Among his reels was film of buses parked haphazardly along the road (PA Route 441 today) near our home.  Sightseers were coming to explore the widely publicized dry riverbed and a curious moon-like landscape of cratered rocks and boulders.  It’s hard to fathom, but people did things like that during their weekends before Sunday-afternoon football was invented.  Scores of visitors climbed through the rocks and truck-size boulders inspecting this peculiar scene.  My dad, his friends, and so many others with camera in hand were experiencing the amazing geological feature known as the Pothole Rocks of Conewago Falls.

Conewago Falls on the Susquehanna River and several exposed York Haven Diabase Pothole Rocks.  Lancaster (foreground) and Dauphin (center) Counties meet along a southwest to northeast borderline through the rapids.  Lands on the west shoreline in the background are in York County.  Three Mile Island is seen in the upper right.

The river here meets serious resistance as it pushes its way through the complex geology of south-central Pennsylvania.  These hard dark-gray rocks, York Haven Diabase, are igneous in origin.  Diabase sheets and sills intruded the Triassic sediments of the Gettysburg Formation here over 190 million years ago.  It may be difficult to visualize, but these sediments were eroded from surrounding mountains into the opening rift valley we call the Gettysburg Basin.  This rift and others in a line from Nova Scotia to Georgia formed as the supercontinent Pangaea began dividing into the continents we know today.  Eventually the Atlantic Ocean rift would dominate as the active dynamic force and open to separate Africa from North America.  The inactive Gettysburg Basin, filled with sediments and intruded by igneous diabase, would henceforth, like the mountainous highlands surrounding it, be subjected to millions of years of erosion.  Of the regional rocks, the formations of Triassic redbeds, sandstones, and particularly diabase in the Gettysburg Basin are among the more resistant to the forces of erosion.  Many less resistant older rocks, particularly those of surrounding mountains, are gone.  Today, the remains of the Gettysburg Basin’s rock formations stand as rolling highlands in the Piedmont Province.

Flooded from the heavy rains of Tropical Storm Lee, the sediment-laden Susquehanna River flows through the Gettysburg Basin just south of Harrisburg, PA, September 10, 2011.  The “Wall” as seen from space.  (NASA Earth Observatory Image)

The weekend visitors in 1963 and 1964 marveled at evidence of the river’s fight to break down the hard York Haven Diabase.   Scoured bedrock traced the water’s turbulent flow patterns through the topography of the falls.  Meltwater from the receding glaciers of the Pleistocene Ice Ages thousands to tens of thousands of years ago raged in high-volume, abrasive-loaded torrents to sculpt the Pothole Rocks into the forms we see today.  Our modern floodwaters with ice and fine suspended sediments continue to wear at the smooth rocks and boulders, yet few are broken or crumbled to be swept away.  It’s a very slow process.  The river elevation here drops approximately 19 feet in a quarter of a mile, a testament to the bedrock’s persisting resistance to erosion.  Conewago Falls stands as a natural anomaly on a predominantly uniform gradient along the lower Susquehanna’s downhill path from the Appalachian Mountains to the Chesapeake Bay.

Normally the scene of dangerous tumbling rapids, the drought and low water of 1963 and 1964 had left the falls to resemble a placid scene—a moonscape during a time when people were obsessed with mankind’s effort to visit earth’s satellite.  Visitors saw the falls as few others had during the twentieth century.  Dr. Herbert Beck of Franklin and Marshall College described an earlier period of exposure, “…pot holes…were uncovered during the third week in October, 1947, for the first time in the memory of man, when the drought parched Susquehanna River retreated far below its normal low stage”.  Then, as in 1963 and on occasions more recent, much of it was due to the presence of the wall.  I had to be a bit older than four years old to grasp it.  You see the wall and the dam are one and the same.  The wall is the York Haven Dam.  And it is responsible for channeling away the low flow of the Susquehanna during periods of drought so that we might have the opportunity to visit and explore the Pothole Rocks of Conewago Falls along the river’s east shore.

The initial segment of the dam, a crib structure built in 1885 by the York Haven Paper Company to supply water power to their mill, took advantage of the geomorphic features of the diabase bedrock of Conewago Falls to divert additional river flow into the abandoned Conewago Canal.  The former canal, opened in 1797 to allow passage around the rapids along the west shore, was being used as a headrace to channel water into the grinding mill’s turbines.  Strategic placement of this first wall directed as much water as possible toward the mill with the smallest dam practicable.  The York Haven Power Company incorporated the paper mill’s crib dam into the “run-of-the-river” dam built through the falls from the electric turbine powerhouse they constructed on the west shore to the southern portion of Three Mile Island more than a mile away.   The facility began electric generation in 1904.  The construction of the “Red Hill Dam” from the east shore of Three Mile Island to the river’s east shore made York Haven Dam a complete impoundment on the Susquehanna.  The pool, “Lake Frederic”, thus floods that portion of the Pothole Rocks of Conewago Falls located behind the dam.   On the downstream side, water spilling over or through the dam often inundates the rocks or renders them inaccessible.

During the droughts of the early 1960s, diversion of nearly all river flow to the York Haven Dam powerhouse cleared the way for weekend explorers to see the Pothole Rocks in detail.  Void of water, the intriguing bedrock of Conewago Falls below the dam greeted the curious with its ripples, cavities, and oddity.  It was an opportunity nature alone would not provide.  It was all because of the wall.

York Haven Dam and powerhouse. The “Wall” traverses Conewago Falls upstream to Three Mile Island to direct water to the powerhouse on the west shore of the Susquehanna River.

SOURCES

Beck, Herbert H.  1948.  “The Pot Holes of Conewago Falls”.  Proceedings of the Pennsylvania Academy of Science.  Penn State University Press.  22: pp. 127-130.

Smith, Stephen H.  2015.  #6 York Haven Paper Company; on the Site of One of the Earliest Canals in America.  York Past website www.yorkblog.com/yorkpast/2015/02/17/6-york-haven-paper-company-on-the-site-of-one-of-the-earliest-canals-in-america/  as accessed July 17, 2017.

Stranahan, Susan Q.  1993.  Susquehanna, River of Dreams.  The Johns Hopkins University Press.  Baltimore, Maryland.

Van Diver, Bradford B.  1990.  Roadside Geology of Pennsylvania.  Mountain Press Publishing Company.  Missoula, Montana.