Lower Monumental Lock and Dam is a run-of-the-river hydroelectric dam on the Snake River, and bridges Franklin County and Walla Walla County, in the state of Washington. The dam is located six miles (10 km) south of the town of Kahlotus, and 43 miles (69 km) north of the town of Walla Walla.

Construction of Lower Monumental Lock and Dam began in June 1961. The main structure and three generators were completed 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 feet (176 m) long.

Lower Monumental Dam is part of the Columbia River Basin system of dams.

Lake Herbert G. West, which extends 28 miles (45 km) east to the base of Little Goose Dam, is formed behind the dam. Lake Sacajawea, formed from Ice Harbor Dam, runs 35 miles (56 km) southwest, downstream from the base of the dam.

Lower Monumental Dam
Locale Franklin / Walla Walla counties, Washington, USA
Construction began June 1961
Opening date 1969
Dam and spillways
Type of dam Concrete gravity, run-of-the-river
Height 100 feet (30 m)
Length 3,791 feet (1,155 m)
Impounds Snake River
Type of spillway Service, gate-controlled
Creates Lake Herbert G. West
Capacity 32,266,666 cubic yards (24,669,636 m3)
Surface area 6,590 acres (26.7 km2)
Power station
Type Yes
Turbines 6 x 135-153 MW
Installed capacity 810 MW
Maximum capacity 932 MW