Lower Monumental Dam and Lake West

Lower Monumental Dam and Lake Herbert G. West - Quick Facts

Lower Monumental Lock and Dam was the second of four dams constructed as part of the Lower Snake River Project, authorized in the Rivers and Harbors Act of 1945. Construction began in 1961, and three turbine units were operational in 1970. Three more units were operational in 1978.
Lake Herbert G. West extends upstream of the dam for 28 miles to Little Goose Dam.

Project Description

• Stream: Snake River (RM 41.6)
• Location: Kahlotus, Washington
• Owner: U.S. Army Corps of Engineers, Walla Walla District
• Type of Project: Run-of-river
• Authorized Purposes: Hydropower, Navigation (1945 Rivers and Harbors Act)
• Other Purposes: Recreation, Fish and Wildlife, Water Quality, Irrigation

Hydrologic Data

• Drainage area = 103,200 sq mi
• Maximum historical peak discharge:
• Unregulated = 409,000 cfs (1894)
• Regulated = 295,000 cfs (1894)
• Lake Elevation (NGVD29)
• Maximum pool = 548.3 ft
• Normal pool = 540.0 ft
• Minimum pool = 537.0 ft
• Maximum rate of change per hour = 70,000 cfs

Powerhouse

• Number of units = 6
• Nameplate capacity = 810 MW
• Overload capacity = 930 MW
• Hydraulic capacity = 130,000 cfs

Dam

• Lower Monumental
• Completed: 1970
• Height: 152 feet
• Length: 3,791 feet
• Features: powerhouse, spillway, navigation lock, fish passage facilities
• Forebay Elevation Normal Operating Range: 537-540 feet msl
• Spillway Capacity (max): 850,000 cfs

Authorized Purposes

Hydropower

Lower Monumental Dam has six 135-megawatt turbines, for a total generating capacity of 810 megawatts.

Navigation

Lower Monumental Dam navigation lock is the sixth of eight locks encountered in the Columbia-Snake Inland Waterway, a 465-mile river highway that allows barge transport of commodities between the Pacific Ocean and Lewiston, ID. In 2015, more than 2 million tons of cargo passed through the Lower Monumental lock.

Water Quality

Water quality is monitored and managed consistent with Clean Water Act and state standards for the health of aquatic species. During spill for juvenile fish passage at the four Lower Columbia and four Lower Snake River projects, the USACE  implements a Water Quality Program to manage total dissolved gas.

Recreation

Popular recreation activities around Lower Monumental Dam and Lake West include fishing, swimming, picnicking, boating, hunting, hiking, and camping. There are more than 7,000 acres surrounding Lake West utilized for public recreation, wildlife habitat, wildlife mitigation, and water-connected industry.
Currently, there are 7 day-use areas, 5 campgrounds, 5 boat launch facilities, and 1 designated swimming beach. Lake West is known for the scenic confluence of the Snake and Palouse rivers, the historic Mullan Road and Lyons Ferry crossing, and the Joso Railroad Bridge.

Fish & Wildlife

Lower Monumental Dam has two fish ladders—one on each shore—to provide a passage route for upstream-migrating fish, including adult salmon and steelhead, lamprey, shad, and others. Passage routes operated for downstream migrating fish are the spillway, a spillway weir, and a juvenile bypass system.
In 2015, about 1.2 million juvenile salmon and steelhead were collected in the bypass system—of those, 98,000 were returned to the river and over 1 million were transported downstream by barge or truck and released below Bonneville Dam.