Los Vaqueros was designed first as a water quality reservoir. The Contra Costa Water District pumps all of its water from the Sacramento/San Joaquin Delta. During the late winter and spring months, when the Sierra Nevada snow pack melts and flows into the Delta, the quality of the water in the Delta is very high.
But in the late summer and early fall, high levels of salt often creep into the Delta from the San Francisco Bay and cannot be treated. With Los Vaqueros, the District pumps high quality water into the reservoir and saves it. When water in the Delta gets salty, the District blends water from the Delta with the quality water from Los Vaqueros, lowering the salt content dramatically.
The project cost $450 million when it was completed in 1997 with a capacity of 100,000 acre-feet. It was the first major dam completed in California in more than 15 years and won numerous awards. In 2012, the District made the dam 34-feet taller at a cost of $120 million and expanded capacity to 160,000 acre-feet.