The lake was formed about 20,000 years ago when rising magma-encountered groundwater resulting in violent steam explosions. The resulting explosions formed a broad shallow crater, which later filled with water. The lake covers an area of 99 acres with average depth of 33 feet with a maximum depth of 58 feet. South Twin Lake is a lure's toss from the Deschutes arm of Wickiup Reservoir and less than a mile south of North Twin lake.

Motorized boats are prohibited on this body of water. 

For boat launch information see South Twin Day Use.