During the late 1800s, engineers began looking to the lower Susquehanna River as a source of electric power, and from 1895 to 1931 four hydroelectric dams were built at York Haven, Safe Harbor, Holtwood, and Conowingo. These dams created the large reservoirs Lake Frederic, Lake Clarke, Lake Aldred, and Conowingo Pond. They brought electricity, drinking water, and recreational opportunities to the area, but they also damaged natural habitats, disrupted the river's flow, and contributed to the virtual disappearance of certain fish species from parts of the Susquehanna. In recent years, utility companies, governmental agencies, and citizen groups have worked to improve the constancy of the river's flow and restore migratory fish to upstream locations. In 1935, the National Audubon Society leased Mount Johnson Island from the Philadelphia Electric Company and established the world's first Bald Eagle sanctuary there.