The San Luis Reservoir is an artificial lake on San Luis Creek in the eastern slopes of the Diablo Range of Merced County, California, approximately west of Los Banos on State Route 152, which crosses Pacheco Pass and runs along its north shore. It is the fifth largest reservoir in California. The reservoir stores water taken from the San Joaquin-Sacramento River Delta. Water is pumped uphill into the reservoir from the O'Neill Forebay which is fed by the California Aqueduct and is released back into the forebay to continue downstream along the aqueduct as needed for farm irrigation and other uses. Depending on water levels, the reservoir is approximately nine miles long from north to south at its longest point, and five miles wide. At the eastern end of the reservoir is the San Luis Dam, or the B.F. Sisk Dam, the fourth largest embankment dam in the United States, which allows for a total capacity of. Completed in 1967 on land formerly part of Rancho San Luis Gonzaga, the reservoir is a joint use facility, being a part of both the California State Water Project and Central Valley Project, which together form a network of reservoirs, dams, pumping stations, and of canals and major conduits to move water across California. The San Luis Reservoir is located in Merced County, and has a visitor center located at the Romero Outlook where visitors can learn more about the dam and reservoir. The surface of the reservoir lies at an elevation of approximately, with the O'Neill Forebay below the dam at above sea level. This elevation difference allows for a hydroelectricplant to be constructed - the Gianelli Hydroelectric Plant. Power from this plant is sent to a Path 15substation, Los Banos via a short power line. Those 500 kV wires, carrying both the power generated here and elsewhere, leave the area and cross the O'Neill Forebay on several man-made islands. San Luis Reservoir also supplies water to of land in the Santa Clara Valley west of the Coast Ranges. San Justo Dam stores water diverted from San Luis Reservoir through the Pacheco Tunnel and Hollister Conduit, which travel through the Diablo Range. A separate canal, the Santa Clara Tunnel and Conduit, carries water to the Coyote Pumping Station in the Santa Clara Valley.
San Luis Reservoir State Recreation Area
San Luis Reservoir is part of the larger San Luis Reservoir State Recreation Area and therefore offers many recreational opportunities for fishermen, boaters, and campers. The park is patrolled by California State Park Peace Officers by vehicle, vessel, and off-highway vehicle. In addition to camping and boating, day use picnic areas are available at San Luis Creek, and an off-highway vehicle area is available east of the main area at the intersection of Gonzaga Road and Jaspar-Sears Road. Camping is available at four campgrounds.
The Basalt Campground on the south-eastern edge of the lake with 79 developed family campsites. Water faucets are available nearby, and some sites can handle RV's to a length of.
San Luis Creek Campground on O'Neill Forebay with 53 sites with water and electric hook-ups.
Medeiros Campground has primitive campsites along the southern shoreline of O'Neill Forebay. This campground has drinking water at three locations and chemical toilets.
Los Banos Creek Campground has primitive campsites and limited turn-around space. It is not suitable for trailers or motor homes. Drinking water and chemical toilets are available.
The National Weather Service has maintained a cooperative weather station at San Luis Dam since 1963. Based on those records, average January temperatures are a maximum of and a minimum of and average July temperatures are a maximum of and a minimum of 64.0 °F. There are an average of 69.3 days with highs of or higher and an average of 14.1 days with lows of or lower. The record high temperature was on July 24, 2006, and the record low temperature was on December 22, 1990. Average annual precipitation is. There are an average of 57 days annually with measurable precipitation. The wettest year was 1998 with and the driest year was 1989 with. The most precipitation in one month was in February 1998. The most precipitation in 24 hours was on May 6, 1998. Snow rarely falls at the reservoir, but of snow fell on January 9, 2001.
