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 mi west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County.

Background

Roughly 2.5 mi 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 56400 acre.ft 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 into a channel running 2200 ft into the forebay.

O'Neill Dam

O'Neill Dam, constructed from 1963 to 1967, is an 87.5 ft, earthfill and rockfill dam, stretching over 3 mi across the valley of San Luis Creek. With a maximum reservoir depth of 57 ft, peak inflow to the forebay is 15600 ft3 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 acre. The O'Neill Pumping-Generating Plant stores 28 megawatts hours of energy.

Recreation

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 and weighing 70.6 lb, was caught on August 5, 2008, by Frank Ualat of Gilroy, California.