The largest offstream reservoir in the United States, the San Luis Reservoir is a key water facility serving both the State Water Project and the federal Central Valley Project.

Located west of Los Banos in California’s Central Valley, the reservoir can hold 2 million acre-feet of water and is jointly owned by the federal Bureau of Reclamation and California’s Department of Water Resources.  The state’s share of the San Luis Reservoir water is 55 percent.

The San Luis Unit conveys water south from the Sacramento-San Joaquin Delta to the unit’s O’Neill Forebay via the state’s California Aqueduct and CVP’s Delta-Mendota Canal. The San Luis Canal starts at the forebay and travels 102 miles south until it becomes the southern portion of the California Aqueduct, continuing to Los Angeles.

San Luis provides additional flexibility to the system by allowing for storage of excess winter and spring flows diverted from the Sacramento-San Joaquin Delta until the water is needed later in the year by SWP and CVP contractors.

About 30 percent of SWP water is delivered to San Joaquin Valley farms and cities via the California Aqueduct. The largest single customer in this area is Kern County Water Agency, whose contract allows it to receive as much as 1 million acre-feet of water per year. Just south of Kettleman City, the Coastal Branch diverges from the main aqueduct to deliver water to Kern, Santa Barbara and San Luis Obispo counties.

San Luis Reservoir also aids Calif. Dept. of Water Resources in increasing or decreasing upstream water releases, changing Delta pumping rates and storing river flows.

For instance, during periods of high runoff, usually in winter and spring, SWP operators pump water into storage at San Luis Reservoir for use during drier periods. Or operators may release water from Lake Oroville in higher volumes than normal at some times to make space for anticipated flood flows.

As SWP operations, especially Delta export pumping, are closely coordinated with the CVP, the San Luis Reservoir supports both systems.