Anderson Ranch Dam is an earth rockfill type dam on the South Fork of the Boise River, in Elmore County, Idaho, U.S.A. Its reservoir is plainly called "Anderson Ranch Reservoir" and has a spillway elevation of 4196 feet (1279 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. Its secondary purpose is for the production of hydroelectric power. The South Fork of the Boise River originates in the Smoky Mountains north of Fairfield. Below the dam the South Fork flows northwestward into the reservoir behind the concrete Arrowrock Dam.