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Mossyrock Dam
Mossyrock Dam
Location Lewis County, Washington, USA
Coordinates 46°32′4″N 122°25′43″W / 46.53444°N 122.42861°W / 46.53444; -122.42861Coordinates: 46°32′4″N 122°25′43″W / 46.53444°N 122.42861°W / 46.53444; -122.42861
Construction began 1965
Opening date 1968
Construction cost $117,769,400
Operator(s) City of Tacoma
Dam and spillways
Type of dam Concrete, arch-gravity
Impounds Cowlitz River
Height 606 feet (185 m)
Length 1,648 feet (502 m)
Spillway type Service, gate-controlled
Creates Riffe Lake
Total capacity 1,685,000 acre feet (2.08 km3)
Catchment area 1,042 square miles (2,700 km2)
Surface area 11,830 acres (4,790 ha)

Mossyrock Dam is a concrete arch-gravity dam on the Cowlitz River near Mossyrock in Lewis County, Washington. The reservoir created by the dam is called Riffe Lake and the primary purpose of the dam is hydroelectric production while flood control is secondary. The dam is the tallest in Washington State and its hydroelectric power station supplies 40% of Tacoma Power's electricity.


Planning for the Mossyrock Dam began in the 1940s but opposition from local fishers and Washington State's Fish and Wildlife Department delayed construction. During World War II, the city of Tacoma, Washington purchased its electricity from the Bonneville Power Administration and from Seattle, with costs of up to $1 million/year. To generate its own electricity, Tacoma City Light (now Tacoma Power) built several dams, including the Mossyrock Dam. Plans for the Mossyrock Dam were announced in 1948 but met stiff opposition. The Washington State Legislature enacted a fish sanctuary on the Cowlitz River that initially blocked the project. Tacoma City sued, with its suit being raised to the US Supreme Court a total of three times. After that, construction of the dam was approved. Construction began in 1965 and ended in 1968. On October 13, 1968, the dam's power plant generated its first electricity. Several towns were forced to evacuate ahead of the rising dam waters including Riffe, Nesika, and Kosmos.

Power plant

The Mossyrock Dam's power plant contains two Francis turbine hydroelectric generators, each rated at 150 MW, or 300 MW total capacity. They are fed by three penstocks ranging from 248 – 285 feet in overall length. One of the three penstocks is presently unused, awaiting possible future installation of a third turbine.

Power plant upgrade

In 2006, GE Energy was selected to upgrade the two turbines. The $50 million project included replacing the turbines, stators, wicket gates and transformers. Installation of the first generator began in 2008; it began operation in 2009. Replacement of the second generator was completed in late 2010. The power plant upgrade increased the total capacity by 70 MW, with an expected overall energy-conversion efficiency of near 95 percent.

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