The San Luis Reservoir is the largest off-stream reservoir in the United States, serving as a key water facility for both the State Water Project (SWP) and the federal Central Valley Project (CVP).

Located west of Los Banos in California’s San Joaquin Valley, the reservoir impounded behind the 380-foot-high B.F. Sisk Dam can hold 2 million acre-feet of water. The reservoir complex was built by the Bureau of Reclamation and is operated and maintained by the California Department of Water Resources. The state’s share of the San Luis Reservoir water is about 55 percent.

Authorized as part of the federal San Luis Unit project in 1960, the reservoir’s construction began in 1963 and was completed four years later. The reservoir, filled in 1969, also provides a variety of recreational activities, as well as fish and wildlife benefits.

The San Luis Reservoir stores water that has traveled south from the Sacramento-San Joaquin Delta to the O’Neill Forebay via the California Aqueduct and CVP’s Delta-Mendota Canal. The San Luis Canal, the federally-built section of the California Aqueduct, starts at the O’Neill Forebay and travels 102 miles south to Kettleman City in Kings County. There, it connects with the state-built southern portion of the California Aqueduct, continuing to Southern California.

San Luis Reservoir provides additional flexibility to the state and federal water delivery systems by allowing for storage of excess winter and spring flows diverted from the 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 the 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 provides flexibility for both water projects. 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 Northern California) in higher volumes than normal at certain times to make space for anticipated flood flows. Likewise, Reclamation stores excess water in San Luis Reservoir for later use for agricultural irrigation, urban and industrial users and wildlife refuges.

In 2020, Reclamation submitted to Congress the final feasibility report to raise Sisk Dam by 10 feet to create 130,000 acre-feet of additional water storage and provide dam safety modifications. The additional space created in San Luis Reservoir would be used to store water that could be delivered to south-of-Delta water contractors and wildlife refuges. The water would be used to meet existing contractual obligations and not serve any new demands.