Folsom Dam is a multi-purpose dam on the American River about 23 miles east of Sacramento. Originally authorized in 1944 solely as a flood control facility, Folsom Dam was reauthorized in 1949, a year after construction began, as a multi-purpose facility for flood control, water storage and hydroelectric generation.

The reauthorization also nearly tripled the size of the reservoir behind the dam, to 977,000 acre-feet. The reservoir capacity was later increased to 1,010,000 acre-feet. A powerplant was constructed at the base of the dam, capable of producing nearly 198,000 kilowatt-hours of electricity. About seven miles downstream is Nimbus Dam, used for regulating American River flows.

Folsom’s 340-foot-high concrete gravity dam was built by the U.S. Army Corps of Engineers and then transferred to the Bureau of Reclamation as part of the federal Central Valley Project. The dam is flanked by two earth-fill wing dams and saddle dams around the reservoir’s rim help to contain the water. Construction was completed in 1956, but the first water was stored in 1955. That occurred when a warm tropical storm in December 1955 swept across the Sacramento region and rapidly melted snow in the American River watershed, quickly filling the reservoir and illustrating its value for flood protection.

Since its commissioning, subsequent storms have demonstrated the need for improvements to Folsom Dam to secure additional flood protection for the Sacramento region.

Improvements to the dam completed in 2017 included a new spillway and submerged gates at a lower elevation to allow earlier water releases for flood management. An additional project to increase temporary flood storage is expected to be completed by 2028.

Beyond physical improvements, the Army Corps and Reclamation have adopted a weather-based system of reservoir management called Forecast-Informed Reservoir Operations (FIRO). Under FIRO, the agencies use weather forecasts to guide them on whether to retain or release water, rather than adhering to more rigid calendar-based rules. The goal is to allow Folsom Reservoir to retain more water for dry times without detracting from Folsom’s flood control mission.

Updated February 2026