Twitchell Reservoir is a reservoir in southern San Luis Obispo County, California. The reservoir has a capacity of 224,300 acre-feet (277,000,000 m³) and is formed by Twitchell Dam on the Cuyama River about 66 miles (106 km) from its headwaters in the Chumash Wilderness Area and about 6 miles (9.7 km) from its confluence with the Sisquoc River, where they form the Santa Maria River. Twitchell dam was built by the United States Bureau of Reclamation between 1956 and 1958. The original names were Vacquero Dam and Vacquero Reservoir, but they were changed to honor T. A. Twitchell of Santa Maria, a proponent of the project.
The dam and reservoir provide flood control and water conservation. The Central Coast of California only receives significant amounts of rainfall during the winter, this area averaging 14 inches (360 mm) per year.
The water is stored in the reservoir during big winter storms and released as quickly as possible while still allowing it to percolate into the soil and recharge the groundwater. This means that the reservoir is usually far from full. It is estimated that the project increases reacharge by 20,000 acre feet (25,000,000 m3) per year.
Sedimentation is a problem for the reservoir, as the reservoir is being filled 70% faster than expected. This reduces its capacity and blocks the water inlet to the control gates. Some sediment has been removed by flushing it out during releases, but much of it is simply deposited immediately downstream, interfering with flows.
There is no public access to the dam or reservoir.