The Conowingo Dam is a large hydroelectric dam in the lower Susquehanna River. The medium-height, masonry gravity dam is one of the largest non-federal hydroelectric dams in the U.S. Construction of the dam started in 1926. Completed in 1928 it was the second largest hydroelectric project by power output in the United States, behind only Niagara Falls. The area below the dam is excellent for birdwatching. At least 170 species of birds have been observed and recorded at the dam. Large groups of Great Blue Heron and Bald Eagle sometimes well over 100 of each species, have been observed at the dam. The largest numbers of eagles are present in November, and the largest numbers of herons are present in the winter months.