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 12 miles (19 km) west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County.

O'Neill Forebay
O’Neill Forebay and Dam as seen from left bank
Location of lake in California, USA
Location of lake in California, USA
O'Neill Forebay
LocationMerced County, California
Coordinates37°05′00″N 121°02′33″W / 37.0832°N 121.0426°W / 37.0832; -121.0426Coordinates: 37°05′00″N 121°02′33″W / 37.0832°N 121.0426°W / 37.0832; -121.0426
Primary inflowsCalifornia Aqueduct
Delta–Mendota Canal
San Luis Reservoir
Primary outflowsCalifornia Aqueduct
Catchment area18 acres (7.3 ha)
Basin countriesUnited States
Max. length3 km (1.9 mi)
Max. width3 km (1.9 mi)
Surface area2,250 acres (910 ha)
Max. depth17 m (56 ft)
Water volume56,400 acre-feet (69,600 dam3)
Shore length119 km (12 mi)
Surface elevation71 m (233 ft)
ReferencesU.S. Geological Survey Geographic Names Information System: O'Neill Forebay
1 Shore length is not a well-defined measure.


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 the 424 MW pumped-storage hydroelectric 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.[1]

O'Neill DamEdit

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.[1]


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[2] 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.[3]

See alsoEdit


  1. ^ a b Bureau of Reclamation. "O'Neill Dam". U.S. Department of the Interior. Retrieved 2009-04-28.
  2. ^ Pham, Huyen Tran (2017-09-27). "O'Neill Forebay". OEHHA. Retrieved 2018-03-21.
  3. ^ "Angler Catches 71.60 Lb. Striped Bass At O'Neill Forebay". The Fish Sniffer. Retrieved 2009-04-28.
  • Department of Water Resources. "Station Meta Data: O'Neill Forebay (ONF)". Retrieved 2019-05-27.

External linksEdit