Folsom Dam was built in 1955 as a concrete dam flanked by earth wing dams and dikes, with a total length of about nine miles. The shoreline extends about 15 miles up the forks of the American River. Lake levels at Folsom Lake State Recreation Area normally vary from 460 feet in early spring to less than 400 feet by summer as the rainy weather passes and snow in the Sierras melts. Downstream, behind Nimbus Dam, smaller Lake Natoma has about 500 surface acres of water. Folsom and Nimbus Dams were built by the Bureau of Reclamation as part of California’s Central Valley Project to control the waters of the American River. Other functions of the dams include flood protection, household water supply, power and irrigation.