Harlan County Reservoir was created to store water for flood control and irrigation, holding water flowing down the Republican River in fall, winter, and spring for summer use on croplands. From spring to the end of irrigation season, the lake level can drop several feet, changing its value for wildlife as the shoreline shifts. In spring, when the lake is full and water may reach the bank, it provides habitat for migrating birds adapted to deeper water, including diving ducks, mergansers, cormorants, osprey, and white pelicans. During the summer, water drawn for irrigation creates shallow water used by wading birds such as great blue herons and great and snowy egrets, and by August receding water exposes mudflats used by gulls and shorebirds including killdeer, dowitchers, avocets, sandpipers, Franklin’s and ring-billed gulls, and Forster’s and black terns. In the moist soil left by the receding water, annual plants such as smartweed germinate. By fall, shallow water and smartweed may extend hundreds of feet from the bank, attracting hundreds of migrating waterfowl, and if the soil stays somewhat wet, the smartweed produces abundant seeds that provide high-energy food for ducks and a seed bank for the following year. As irrigation season ends, the lake begins storing water again for the next growing season, flooding the smartweed and making plant food available to migrating waterfowl, while opportunities for gulls and late migrating shorebirds can continue well into fall and, for some gulls such as ring-billed gulls, even into winter. In most winters, hardy mallards and Canada geese can be found on open water and in nearby flowing streams.