Starting in the 1850s during the California Gold Rush , the city of Sacramento rapidly grew around the confluence of the Sacramento River and its tributary the American River , near the middle of the Central Valley of California . The city 's increasing population necessitated the construction of an extensive system of levees on the two rivers to prevent flooding . These early flood control works were insufficient ; in 1862 , the city was inundated so completely that the state government was temporarily moved to San Francisco . In 1956 , the U.S. Army Corps of Engineers built the Folsom Dam at the confluence of the North and South Forks of the American River to provide flood control for the Sacramento metropolitan area . However , the Folsom Dam with a capacity of just 1 million acre feet ( 1.2 km3 ) compared to the annual American River flow of 2.7 million acre feet ( 3.3 km3 ) proved inadequate . A flood in 1955 filled the Folsom Reservoir to capacity , before the dam was even completed ; it has also overflowed many times since
inundated so completely
Great Flood of 1862
Auburn Dam