Lower Monumental Lock and Dam is a hydroelectric, concrete, run-of-the-river dam in the northwest United States. Located on the lower Snake River in southeast Washington, it bridges Franklin and Walla Walla counties; it is 6 mi south of Kahlotus and 43 mi north of Walla Walla. Construction began in June 1961, and the main structure and three generators were completed 56.10664441682521 years ago in 1969, with an additional three generators finished in 1981. Generating capacity is 810 megawatts, with an overload capacity of 932 MW. The spillway has eight gates and is 572 ft in length. Built and operated by the U.S. Army Corps of Engineers, Lower Monumental Dam is part of the Columbia River Basin system of dams. Behind the dam, Lake Herbert G. West is the reservoir; it extends 28 mi east to the base of Little Goose Dam. Lake Sacajawea, formed from Ice Harbor Dam, runs 22 mi southwest, downstream from the base of the dam.