Folsom Lake is a reservoir on the American River in the Sierra Nevada foothills of California, United States.

It is a concrete structure with approximately 9 miles of earth fill wing dams and dikes supporting its surrounding areas.

About 20,000 acres of water and land are within the boundaries of the recreation area; it spans parts of El Dorado, Placer, and Sacramento Counties, as well as the City of Folsom.

The California Office of Environmental Health Hazard Assessment has developed a safe eating advisory for Folsom Lake based on levels of mercury found in fish caught here.

The smaller and more remote Rattlesnake Bar visitor area provides boat-launch facilities and informal access to the shoreline for fishing, swimming, and picnicking.

Facilities at these locations include swim beaches, picnic areas, food and beach equipment concessions, equestrian staging areas, restrooms and drinking water fountains, and trail heads with over 94 miles of trails (used by hikers, runners, mountain bikers, and horseback riders).

A variety of wildflowers thrive in the spring; Indian paintbrush, California poppy, larkspur, lupine, brodiaea, fiddleneck, Dutchman's pipe, and monkeyflower can be seen throughout the SRA.

A number of mammals inhabit the lake area, including coyotes, gray foxes, rabbits, skunks, raccoons, ground squirrels, black-tailed deer, beavers, and opossums, and on occasion, mountain lions, bobcats, and black bears have been sighted.

Several bird species call Folsom Lake home year-round, including bushtits, quails, wrens, scrub jays, blackbirds, and towhees.

The Sacramento Basin is notorious for flooding, and the dam helps relieve winter storm runoff and snow melt from the Sierra.