Twitchell Reservoir is a reservoir in southern San Luis Obispo County and northern Santa Barbara County in California. The reservoir has a capacity of 197,756 acre·ft (243,928,000 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.