O'Neill Dam is an earthfill dam on San Luis Creek, 12 mi west of Los Banos, California, United States, on the eastern slopes of the Pacific Coast Ranges of Merced County. Forming the O'Neill Forebay, a forebay to the San Luis Reservoir, it is roughly 2.5 mi downstream from the San Luis Dam.

Background

Built from 1963 to 1967, the dam is an earthfill and rockfill construction stretching over 3 mi across the valley of San Luis Creek. A morning-glory type spillway lies at the left bank of the reservoir. At 87.5 ft high, with a maximum reservoir depth of 57 ft, the crest of the dam is 14300 ft long, at an elevation of 223 ft. The spillway is, as mentioned before, a morning-glory (inverted bell) design, capacity 3250 ft3 per second, and with a circumference of 641.5 ft.

O'Neill Forebay 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 ft into a channel running 2200 ft 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.

O'Neill Pumping-Generating Plant

The O'Neill Pumping-Generating Plant produces 28 MW. 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.