As they travel between coastal wintering ranges and breeding territories in Canada and Alaska, vast numbers of shorebirds pass through the lower Susquehanna region each spring and fall—though few stop here to rest, feed, and provide us with an opportunity to observe them. Prior to the construction of man-made dams and other alterations on our lotic (flowing) waterways, shorebirds took advantage of lateral bars, stream deltas, and other alluvial deposits as places to loaf and re-energize. Before they were drained and filled, some wetlands probably included sparsely vegetated flats where shorebirds could drop in for a brief visit. Prior to their extirpation from our region, North American Beavers were the primary providers of quality habitat for shorebirds and other migrating waders. Their widespread network of dams, pools, and marshes maximized floodplain function by keeping streams thoroughly connected to their wetlands, nurturing plant communities that not only provided food and shelter for the beavers and other wildlife, but provided superb buffering against erosion while protecting against sediment and nutrient imbalances in lower Susquehanna waterways.
Beaver dams need not be large, particularly on low-gradient streams where a structure like this is sufficient to create a pool with depth adequate for building and maintaining a lodge and transporting leafy branches and other food items by water.A beaver lodge assembled in a pool with less than three feet of water, a depth adequate to provide the family with a measure of protection against terrestrial predators.Beaver pool ecosystems provide homes for hundreds of species of plants and animals, including migrating shorebirds and other waders.Mud flats in the margins between emergent shrubs and herbaceous plant growth attract migrating shorebirds like this Solitary Sandpiper to the abundance of invertebrate life. Seasonal movements of migrating shorebirds regularly coincide with the reductions of water levels in beaver pools which typically occur between May and September each year.During its southbound migration in September, a Least Sandpiper searches for arthropods and annelids as it visits a food-rich puddle along the periphery of a beaver pool.A Pectoral Sandpiper feeds in shallow water near a stand of Common Cattails in a beaver pool.During migration, rails including the Sora are attracted to dense emergent vegetation in beaver pools. Some will nest in these rodent-managed refugia.Green Herons visit beaver pools during migration and, due to the reliability of the food supply, will often nest in their vicinity.
So how did this happen? How did the North American Beaver become a keystone species—an animal upon which the majority of other life forms within its ecosystem are so reliant? Well, it’s largely due to the fact that our beavers aren’t particularly fond of a constant stream of noise. More specifically, they don’t like the sound of running water in places where they intend to build and maintain a lodge. And so, as they begin to place sticks, mud, limbs, stones, and other materials within a noisy riffle on a stream, they create a dam, and behind it a pool—a pool that is particularly advantageous for protecting their home and providing a means of conveyance for their construction materials and food supplies.
On a high-gradient segment of stream, beavers will create a cascading series of pools. Because water filters through a beaver dam instead of spilling over it, the work of these meticulous rodents soon silences the sounds of water changing altitude. No more sonorous riffle.Quiet please. High seasonal stream flow and damage from storms may create areas where water begins to erode the structure of the dam. Where this condition persists, an adult beaver will soon mend the breach, just to quiet things down. Why would beavers demand such a hush upon their domain? Well, they have poor eyesight, but their hearing is excellent, and they rely upon it to detect danger.The Louisiana Waterthrush (Parkesia motacilla), one of our earliest-arriving warblers, nests in forests along clear, high-gradient streams. In mid-April, we found this individual and three others squabbling over a breeding territory adjacent to the series of cascading beaver pools shown in the previous images.Native denizens of coldwater streams, neither the Brook Trout nor the Eastern Blacknose Dace has any difficulty finding its way through the voids in beaver dams to ascend and descend the sequence of pools.On a low-gradient segment of stream, a dam just over a foot in height may be sufficient to create a beaver pool of considerable size. Resembling the water-logged muskeg of the far north, well-established beaver pools form boggy habitats familiar to migrating shorebirds.Water levels in the pools usually drop along with the stream’s base flow as soon as the effects of the spring thaw and rain showers subside. Feeding areas in shallow water and on muddy ground are often revealed just in time for northbound shorebirds and other waders to stop by in late April and May. These conditions often persist through the growing season and the fall migration of these birds which begins as early as late June and sometimes extends into October.Where the pool inundates standing timber such as Red Maple and other species not used by beavers, the dead snags provide vital feeding areas for birds and other wildlife. Cavity nesters like this Tree Swallow seldom find suitable natural housing elsewhere.The mosaic of marshlands and braided stream channels within the beaver pool complex supports an abundance of aquatic life including these breeding Eastern Newts, seen here surrounded by the exuvia left behind following a massive hatch of alderflies (Sialinae). Alderflies are a stream inhabitant during their larval stage and are indicator of clean water conditions.A modern-day example of the way fully functional stream floodplains used to look in the Lower Susquehanna River Watershed. Though the pools may appear pond-like during the cooler and wetter months of the year, by summer the water levels behind the beaver dams recede as the base flow of the stream wanes. Now the true nature of the marsh, shallow pool, and braided stream complex is revealed. Unlike most man-made dams that set a fixed pool elevation regardless of flow by discharging water over the top of the structure or through a spillway or gate, beaver dams merely throttle the flow through their porous construction. Unless the beaver begins plugging small leaks as fast as the stream flow ebbs, the water level in the pools will drop. So they’re not left high and dry, lodges are located in the deepest pools, usually in close proximity to the dam and/or one of the stream courses. Though not inundated during the dry season, the soil in the pool complex is almost always damp and plants grow vigorously there, sequestering nutrients and retaining sediments in beneficial deposition patterns that actually inhibit erosion of the riparian landscape. These streams and floodplains retain their hyporheic zone and freely exchange water with the underlying water table and aquifer. It’s the ultimate floodplain management system, and the beavers don’t even know their doing all the work. (Google Maps base image)Where another high-gradient segment of stream enters the main pool complex, beavers have created an additional series of cascading pools. The impoundments created by these dams help diffuse stormwater energy and process more nutrients and sediments within the floodplain’s wetland vegetation system. (Google Maps base image)Beaver pools often refill as stream flows increase following autumn rains. The stockpiles of vegetative foods that grew within the beaver’s domain through the summer then become flooded and are a prime source of nutrition for not only the beavers, but waterfowl during both their autumn and spring migrations. Gleaning and probing Sandhill Cranes often find these habitats to their liking as well.
While North American Beavers have returned to the region, most live as “bank beavers”, residing in the river and larger creeks of the valley where they excavate shelters among the roots of Black Willows and other shoreline trees and shrubs. Floodplain encroachment, legacy sediment deposits, and just plain human intolerance have all conspired to prohibit North American Beavers from performing their magic on smaller local streams. For migrating shorebirds, this continued absence of beaver dam ecosystems has turned much of the lower Susquehanna valley into “fly-over country”. Those travelers that do stop to rest and feed concentrate at the few favorable locations such as the lateral bars and the hydroelectric dam-created delta at the Conejohela Flats on the river in Lancaster County. But centralization has its drawbacks. Migrants spending time at concentration points may have a greater chance of contracting and spreading disease. Protracted heavy foraging can degrade these habitats. And over time, features such the lateral bars and delta deposits, including those on the Conejohela Flats, transition into other habitats—riparian forests. A more widespread selection of favorable stopover points for shorebirds, waders, waterfowl, and other migrants is certainly desirable.
IMITATION IS THE SINCEREST FORM OF FLATTERY
Until public sentiment sways in favor of the North American Beaver, wildlife managers are mimicking some of the attributes of their sound-inspired installations.
Created by excavating a depression in heavy clay soils, this new impoundment at the Pennsylvania Game Commission’s Middle Creek Wildlife Management Area collects water directly from rainfall and from surface runoff. Its depth at no point is greater than about two or three feet. During the driest times of the year, this space will be a mudflat, and a haven for migrating shorebirds.
Conservation impoundments designed, constructed, and managed for migratory waterfowl and waders including shorebirds are not what we typically refer to as ponds—though they are lentic (still) waters. They more closely resemble the prairie potholes created by glacial scour in the north-central United States and adjacent portions of Canada. Many populations of migratory birds are familiar with these ecosystems and, like the beaver pools and marshes, have relied upon them as waypoints along their journeys for centuries.
Impoundments most beneficial to migrating waterfowl and waders including shorebirds are shallow in depth. They lack the deeper waters of a pond or lake and thus have no limnetic (open water) or aphotic zones. Managed throughout as a littoral zone, impoundments grow plants in shallow water or on damp soil through the summer months to chew up nutrients accumulated in waste from visiting birds. This vegetation is then flooded from late fall through early spring as forage habitat for visiting waterfowl. The timing of the fluctuations in water levels approximates those of the beaver pools and marshes on lotic (flowing) waters.A gate assembly is a water control structure installed to provide seasonal management of the water levels in a conservation impoundment. More than once, beavers have heard the sounds of tumbling waters inside these types of devices and tried to dam them up!
You’ve heard the line, “If you build it, they will come.” Well, it’s true. Here is a sample of the activity witnessed during the past two weeks at the new impoundment completed just several months ago at Middle Creek Wildlife Management Area along the tour route just beyond the “Stop 3” overlook.
The ubiquitous Snow Geese were among the first migratory occupants of the new impoundment. A few are currently lingering in the vicinity.Ducks soon followed. These Blue-winged Teal were among the last to pass through earlier this month.Wilson’s Snipe were among the first species of migratory shorebirds to visit the new impoundment at Middle Creek……as were Killdeer, a species which nests nearby.Then, earlier this month, flocks of shorebirds including these Least Sandpipers were arriving to feed and rest.Least Sandpipers search for small invertebrates in shallow water and exposed soil.A Least Sandpiper (left) and a Semipalmated Sandpiper (right).Not all shorebirds seen by themselves are alone, and that includes the Solitary Sandpiper.Here’s a Solitary Sandpiper (right) feeding alongside a Least Sandpiper (center) and a White-rumped Sandpiper (left).A Least Sandpiper (left) and a White-rumped Sandpiper (right).Rare in our area during spring, the White-rumped Sandpiper (Calidris fuscicollis) flies north using the central flyway, then heads south along the Atlantic flyway in the fall, when it tends to be more regular here.Rocking and teetering along as it looks for food, the Spotted Sandpiper may be one of the easiest of shorebirds to identify.Chunky little Dunlin (Calidris alpina) with their conspicuously down-curved bills are another easy-to-identify species, particularly in the spring when their breeding (alternate) plumage includes a black belly.Black-bellied Plovers acquire a handsome set of plumage in the spring as well.Killdeer too are plovers and this pair appears to have taken up residence on barren ground along the periphery of the new impoundment.The Semipalmated Plover doesn’t have nearly the flair for ornament its close relatives the Killdeer do; these little shorebirds wear only one ring around their neck instead of two. Think this plover is cute?,……then check out this newly hatched Killdeer. It starts life with just one necklace too, but acquires a second as it grows. Look at those legs! (If you visit the new impoundment at Middle Creek, drive slowly and please watch where you’re going. Baby Killdeer and other young birds, as well as mammals and turtles, are commonly crossing the paved surfaces right now.)Speaking of legs, here’s one of dozens of Lesser Yellowlegs that visited the new impoundment during their recent northbound travels.Though less numerous than the smaller Lesser Yellowlegs, the Greater Yellowlegs seldom goes unnoticed when dropping by the impoundment.Have trouble telling a Greater Yellowlegs like this one from a Lesser Yellowlegs? Look for the heavier, longer bill on the former, as well as dark barring along its flanks below the wings while the bird is in breeding (alternate) plumage during the spring migration.Everybody likes to see something unusual every now and then, and this impoundment delivers. Just yesterday, we photographed this migrating Wilson’s Phalarope (top center) among two Least Sandpipers (top left and right), a Lesser Yellowlegs (left and slightly forward of the phalarope), and a White-rumped Sandpiper (foreground).Renowned for spinning in circles as they feed in shallow water, Wilson’s Phalaropes (Phalaropus tricolor) passing through the lower Susquehanna basin are headed toward nesting areas in the prairie wetlands, including potholes, of the northern United States and adjacent sections of southern Canada.
Species of wildlife in addition to shorebirds and waterfowl have already found the new impoundment favorable…
Pairs of breeding Spot-winged Gliders (Pantala hymenaea), seen here in tandem while ovipositing, were swarming the impoundment after arriving earlier this week. These ofttimes unpredictable nomads are similar in appearance to the usually more numerous Wandering Gliders. To recolonize seasonal portions of their range, both are famous for hitching a ride en masse on storm systems. They share the behavior of finding ephemeral and new bodies of water favorable for egg laying due to their low density of aquatic predators.We watched this Snapping Turtle arrive to apparently find the newly created waters to its liking. Snapping Turtles are important consumers in a wetland ecosystem. Larger specimens may fill the role of an upper trophic level or apex predator, eliminating vulnerable mid-level consumers including other Snapping Turtles.Because it bounces its tail up and down like a Spotted Sandpiper, you might mistake an American Pipit for a shorebird. Long known as the Water Pipit, these birds have been visiting the impoundment while migrating north and stopping to feed in nearby croplands.Seen here with Least Sandpipers is a visiting Glossy Ibis.Sometimes twenty or more of these mostly coastal waders have made a pit stop at the impoundment, though none chose to remain long. Apparently, food sources sufficient to sustain a bird of their size have yet to develop in its benthos.Western Cattle Egrets visiting Middle Creek this spring have been frequenting the new impoundment.With still little in the way of insects such as grasshoppers available in the surrounding landscape,……cattle egrets are looking to find prey like this Green Frog.Many have been observed hunting the adjacent grasslands..Where small mammals, mostly Meadow Voles, are being taken in abundance.
Managing saturation levels in shallow-water impoundments to resemble the seasonal variations in beaver pool and marsh systems can create lush growth and wildlife-rich environments. Take a look at some images from a project in a headwaters area of a tributary to Conewago Creek (west)…
Seen in late July, this impoundment is dense with emergent and aquatic plants, the latter kept hydrated in deeper pools of the project by the influx of several captured springs that supplement direct rainfall and sheet runoff to supply its water. Through the heat of mid-summer, southbound shorebirds were already using these mudflats to feed and rest.In the deeper pools of the impoundment, water lilies and other aquatic plants grow in dense mats to provide cover and feeding areas for resident populations of reptiles and amphibians.An abundance of foods are available for waders including this Green Heron……and this immature Little Blue Heron, a wanderer typical of more southern latitudes.While walking the road among tall grasses in the supporting landscape surrounding this impoundment, we were at first startled when these Sandhill Cranes strode by going the other direction. We quietly kept moving,……then spotted them again as we looked across the impoundment to realize they weren’t alone, but were escorting a colt.The hatching of this colt is testimony to the vital role wetland ecosystems play in the lives of hundreds of species. Whether they be beaver pools and marshes on lotic waters or man-made shallow lentic waters, each of these habitats is filling a void that left floodplains and other critical lowland biomes faltering. While they can’t replace the full-function floodplain management provided by an active beaver colony, shallow-water impoundments can provide relief for habitat-starved populations of the animals and plants that rely upon them. A constellation of these projects on lands public or private across the lower Susquehanna watershed could help provide refuge for many of our flora and fauna with the most desperately fragmented of ranges.
So that you can relax while observing the comings and goings at a pair of the lower Susquehanna valley’s man-made impoundments, the Pennsylvania Game Commission has erected two viewing pavilions for public use on its lands…
The Haldeman Island pavilion is located on State Game Lands 290 just upstream from the Juniata River’s mouth on the Susquehanna at Clark’s Ferry in Dauphin County.It overlooks not only the island’s man-made shallow-water impoundments and neighboring grasslands, but the tower used in the 1970s to reintroduce Bald Eagles from Saskatchewan to the lower Susquehanna. Interpretive signs explain the conservation stories of habitats and the eagle reintroduction program.The observation pavilion at Middle Creek Wildlife Management Area is of similar construction to the one at Haldeman Island. It is accessed from the parking pull-off along the tour road at its intersection with Chapel Road, just before the right turn and incline that leads to the “Stop 3” grassland overlook.It too includes numerous interpretive signs to help visitors understand impoundment management.
During the next two weeks, the exodus of migrating shorebirds now staged and feeding upon Atlantic Horseshoe Crab eggs on Delaware Bay will commence. During the evening of their departure from the bay, many of these birds cross portions of the eastern Lower Susquehanna River Watershed. Stormy weather and other climatic conditions may force some of them to seek a place to put down temporarily, so keeping a close eye on the new impoundment at Middle Creek may be a prudent move. After that, waders known as “post-breeding wanderers” can show up at any time. Then, beginning as early as late June, shorebirds begin moving south on a migration that can provide us with viewing opportunities into September and beyond. See you out there!
