Anderson Ranch Dam is an earth rockfill type dam on the South Fork of the Boise River, in Elmore County, Idaho, United States. Its reservoir, Anderson Ranch Reservoir, has a spillway elevation of 4,196 feet (1,279 m) above sea level. The dam is northeast of Mountain Home and several miles north of US 20.
When completed in 1950, Anderson Ranch was the tallest dam of its type in the world. Its primary purpose is to provide irrigation water for agriculture, with a secondary purpose of hydroelectric power. Its generating capacity was increased from 27MW to 40 MW in 1986.
The construction of the dam began in 1941 and experienced numerous challenges with materials, fuel, and labor shortages during World War II. Work was halted for over nine months beginning in late December 1942. The Reclamation Act of 1902 had racial exclusions on labor which were strictly adhered to until Congress changed the law in 1943. This allowed Japanese American internees to work on Reclamation projects; Anderson Ranch utilized internees from the Minidoka War Relocation Center, northeast of Twin Falls.
The South Fork of the Boise River originates in the Smoky Mountains north of Fairfield. Its watershed includes portions of the Smoky Mountains, Soldier Mountains, Boise National Forest, and Sawtooth National Forest. Below the dam, the South Fork flows northwestward into the reservoir behind the concrete Arrowrock Dam, completed in 1915.