Sylvan Lake, known as the "crown jewel" of Custer State Park, is located in the Black Hills of South Dakota. Created in 1881 when Theodore Reder built a dam (the Sylvan Lake Water Dam) across Sunday Gulch Creek. The lake area offers picnic places, rock climbing, small rental boats, swimming, and hiking trails. It is also popular as a starting point for excursions to Harney Peak and The Needles. A hotel was operated on the shore of the lake in the early 20th century (Postcard).
The lake was featured in Disney's 2007 film National Treasure: Book of Secrets. The film made the lake appear to be located directly behind Mount Rushmore when in reality it is actually five miles southwest of Mount Rushmore.