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Lower Monumental Dam
USACE Lower Monumental Dam.jpg
Location Franklin and Walla Walla counties, Washington, U.S.
Coordinates 46°33′47″N 118°32′20″W / 46.563°N 118.539°W / 46.563; -118.539Coordinates: 46°33′47″N 118°32′20″W / 46.563°N 118.539°W / 46.563; -118.539
Construction began June 1961
Opening date 1969
Operator(s) U.S. Army Corps of Engineers
Dam and spillways
Type of dam Concrete gravity,
run-of-the-river
Impounds Snake River
Height 100 feet (30 m)
Length 3,791 feet (1,155 m)
Spillway type Service, gate-controlled
Reservoir
Creates Lake Herbert G. West
Total capacity 432,000 acre⋅ft (0.53 km3)
Surface area 6,590 acres (26.7 km2)
Normal elevation 540 ft (165 m)
Power station
Turbines 6 x 135-153 MW
Installed capacity 810 MW
932 MW (max)

Lower Monumental Lock and Dam is a hydroelectric, concrete, run-of-the-river dam in the northwest United States. Located on the Snake River, it bridges Franklin and Walla Walla counties in southeast Washington. It is six miles (10 km) south of Kahlotus and 43 miles (70 km) north of Walla Walla.

Construction began in June 1961, and 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 (174 m) in length.

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 miles (45 km) east to the base of Little Goose Dam. Lake Sacajawea, formed from Ice Harbor Dam, runs 22 miles (35 km) southwest, downstream from the base of the dam.

LwrMnDam2
Lower Monumental Dam with the lock in the center of the river (far side in photo), the power generation on the northwest side of the river, and spillway in the middle of the dam, between the powerhouse and the lock.
Pacific Northwest River System
Columbia River Basin
Navigation lock
  • Single-lift
  • Width: 86 ft (26 m)
  • Length: 666 ft (203 m)
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