Sylvan Lake is a lake located in Custer State Park, in the Black Hills of South Dakota, United States. It was created in 1891 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 Black Elk Peak and The Needles. A hotel was operated on the shore of the lake in the early 20th century .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.

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