Sylvan Lake, Mountain lake in Custer State Park, South Dakota.

Sylvan Lake is a lake in Custer State Park in the Black Hills of South Dakota, created in 1891 when Theodore Reder dammed Sunday Gulch Creek. The roughly 10-metre-high Sylvan Lake Water Dam stands at the narrowest point of the surrounding mountains on the lake's northwest side. The shoreline offers picnic areas, rock climbing, small rental boats, swimming and hiking trails.

Theodore Reder built a dam across Sunday Gulch Creek in 1891, creating this mountain lake. That early engineering project transformed a mountain stream into a lasting outdoor recreation site for the region.

The lake serves as a recreation hub where visitors swim, climb rocks, and explore the surrounding trails on foot. The granite cliffs shape how people experience and use this water-centered gathering place.

Swimming is allowed and small boats can be rented, but motorized watercraft are prohibited to preserve the natural setting. Several hiking trails start near the shoreline and lead to nearby destinations including Black Elk Peak and rock formations called The Needles.

Scenes from Disney's 2007 film National Treasure: Book of Secrets were shot at Sylvan Lake, where it was made to appear directly behind Mount Rushmore. In reality the lake lies about five miles southwest of the monument, a gap the film editing hides.