O'Neill Dam is an earthfill dam on San Luis Creek, 12 miles (19.3 km) west of Los Banos, California

Los Banos, California

Los Banos is a city in Merced County, California, near the junction of State Route 152 and Interstate 5. Los Banos is located southwest of Merced, at an elevation of 118 feet . The population was 35,972 at the 2010 census, up from 25,869 at the 2000 census...

, on the eastern slopes of the Pacific Coast Ranges of Merced County

Merced County, California

Merced County , is a county located in the Central Valley of the U.S. state of California, north of Fresno and southeast of San Jose. As of the 2010 census, the population was 255,793, up from 210,554 at the 2000 census. The county seat is Merced...

. Forming the O'Neill Forebay

O'Neill Forebay

O'Neill Forebay is a forebay to the San Luis Reservoir created by the construction of O'Neill Dam across San Luis Creek approximately west of Los Banos, California, on the eastern slopes of the Pacific Coast Ranges of Merced County.-Background:...

, a forebay to the San Luis Reservoir

San Luis Reservoir

The San Luis Reservoir is an off-stream artificial lake 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. The reservoir stores water taken from the San...

, it is roughly 2.5 miles (4 km) downstream from the San Luis Dam

San Luis Dam

San Luis Dam is a dam that creates San Luis Reservoir, which serves as an off-stream reservoir for the California State Water Project and the Central Valley Project. It is also known as the B.F. Sisk Dam, after Bernie Sisk. The earth-fill gravity embankment dam is 305 feet tall and was completed...

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Background

Built from 1963 to 1967, the dam is an earthfill and rockfill construction stretching over three miles (5 km) across the valley of San Luis Creek. A morning-glory type spillway lies at the left bank of the reservoir. At 87.5 feet (26.7 m) high, with a maximum reservoir depth of 57 feet (17.4 m), the crest of the dam is 14300 feet (4,358.6 m) long, at an elevation of 223 feet (68 m). The spillway is, as mentioned before, a morning-glory (inverted bell) design, capacity 3250 ft3 per second, and with a circumference of 641.5 feet (195.5 m).

Reservoir

The O'Neill Forebay reservoir is fed by releases from the San Luis Dam as well as from the Delta–Mendota Canal. Water from the Delta–Mendota Canal is lifted a vertical distance of 8 feet (2.4 m) into a channel running 2200 feet (670.6 m) into the forebay. The peak inflow to the forebay is 15600 ft3 per second, from both the San Luis Dam and the Delta–Mendota Canal. Drainage area of the reservoir downstream of the San Luis Dam is only 18 acre (0.07284348 km²). The O'Neill Pumping-Generating Plant produces 28 megawatts. Irregular water releases from the San Luis Dam and William R. Gianelli Powerplant are collected in the reservoir of the O'Neill Dam, which has a capacity of 56400 acre.ft.