San Luis Reservoir is designed to capture excess runoff flowing out of the Sacramento–San Joaquin River Delta, the largest watershed of California, during the winter and spring rainy season so it can be used during the dry season. This water includes both natural river flows, and water that is evacuated from Shasta Lake, Lake Oroville, Folsom Lake and other reservoirs for flood control purposes. This is accomplished via a system of canals and pumping stations which divert water from the Delta and deliver it south to water users. . .
[link to en.m.wikipedia.org (secure)]