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O'Neill Forebay facts for kids

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O'Neill Forebay
O’Neill Forebay and Dam as seen from left bank
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Location Merced County, California
Coordinates 37°05′00″N 121°02′33″W / 37.0832°N 121.0426°W / 37.0832; -121.0426
Type Reservoir
Primary inflows California Aqueduct
Delta–Mendota Canal
San Luis Reservoir
Primary outflows California Aqueduct
Catchment area 18 acres (7.3 ha)
Basin countries United States
Max. length 3 km (1.9 mi)
Max. width 3 km (1.9 mi)
Surface area 2,250 acres (910 ha)
Max. depth 17 m (56 ft)
Water volume 56,400 acre feet (69,600 dam3)
Shore length1 19 km (12 mi)
Surface elevation 71 m (233 ft)
1 Shore length is not a well-defined measure.

O'Neill Forebay is a forebay to the San Luis Reservoir created by the construction of O'Neill Dam across San Luis Creek approximately 12 miles (19 km) west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County.


Roughly 2.5 miles (4.0 km) downstream from the San Luis Dam, O'Neill Forebay collects irregular water releases from the San Luis Dam and William R. Gianelli Powerplant in its 56,400 acre⋅ft (69,600 dam3) basin. A morning-glory type spillway lies at the left bank of the reservoir. The reservoir is fed by the California Aqueduct and the Delta–Mendota Canal. Water from the canal is lifted a vertical distance of 8 ft (2.4 m) into a channel running 2,200 ft (670 m) into the forebay.

O'Neill Dam

O'Neill Dam, constructed from 1963 to 1967, is an 87.5 ft (26.7 m), earthfill and rockfill dam, stretching over 3 mi (4.8 km) across the valley of San Luis Creek. With a maximum reservoir depth of 57 ft (17 m), peak inflow to the forebay is 15,600 cu ft (0.44 dam3) per second, from both the San Luis Dam and the Delta–Mendota Canal. The drainage area of the reservoir downstream of the San Luis Dam is only 18 acres (730 dam2). The O'Neill Pumping-Generating Plant stores 28 megawatts hours of energy.


The California Office of Environmental Health Hazard Assessment (OEHHA) has developed an advisory for O'Neill Forebay because of mercury and PCBs found in fish caught here. The advisory provides safe eating advice for multiple fish species.

The largest striped bass caught in California was caught in the O’Neill Forebay. The fish, 52.5 in (1,330 mm) and weighing 70.6 lb (32.0 kg), was caught on August 5, 2008, by Frank Ualat of Gilroy, California.

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