The Quarry Lakes Recreation Unit includes Horseshoe Lake and Rainbow Lake and the turf, beach, swimming and picnic areas around those lakes. Visitors can swim in the designated beach, picnic, play volleyball, fish, and enjoy non-gasoline powered watercraft. The Natural Unit includes Willow Slough and Lago Los Osos and the surrounding areas, where visitors can hike, observe nature, walk dogs and ride bicycles. No water contact of any type is allowed in these two lakes. Dogs must always be kept on leash throughout the entire park. Operated by the East Bay Regional Park District.