The Schuylkill Pond
Completed in 1822, Fairmount Dam cut diagonally across the river, channeled water into the Waterworks, and acted as a spillway. The Schuylkill is a tidal river, so it also prevented brackish water in the Delaware River from mixing with the fresh water. Above it, the dam created the "Schuylkill Pond", several miles of tranquil water used for recreation. The original dam was replaced in 1928. (via Wikipedia)
Also located directly next to the Philadelphia Museum of Art.