The United States Bureau of Reclamation constructed the Vaquero Dam and Reservoir in the late 1950s as part of the Santa Maria Project. The Project provides recharge to the groundwater basin underlying the Santa Maria Valley and provides for flood protection. The project was completed in 1959 at a cost of approximately $11 million dollars, which was 30% less than the original estimate. The name was eventually changed to Twitchell Dam and Reservoir to honor Mr. T.A. Twitchell of Santa Maria, who was instrumental in bringing about the project. Twitchell Reservoir is operated and maintained by the Santa Maria Valley Water Conservation District.
Twitchell Reservoir is important to both the water supply and the flood protection of the Santa Maria Valley. The reservoir supplies about 20,000 AF of recharge to the Santa Maria Groundwater Basin annually. The replacement cost of getting this water from other sources would be millions of dollars every year.
Since its completion, Twitchell Reservoir has been trapping sediments from the 1,140 square mile Cuyama River watershed. Original studies estimated that 40,000 AF of sediment would accumulate in the reservoir during the first one hundred years of operation. In 1981, a study found that the rate of sedimentation was about 70% greater than the original estimate. As of 2018, the accumulated sediment had reached an estimated 45,836 AF. Because of this, the SBCWA and the Santa Maria Valley Water Conservation District have prepared a sediment management plan. This plan will help to ensure the continued safe operation of the reservoir's water release works, and also extend the usable life of the reservoir.