Mossyrock Dam

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Mossyrock Dam
Mossyrock Dam
Location Lewis County, Washington, USA
Coordinates 46°32′4″N 122°25′34″W / 46.53444°N 122.42611°W / 46.53444; -122.42611Coordinates: 46°32′4″N 122°25′34″W / 46.53444°N 122.42611°W / 46.53444; -122.42611
Construction began 1965
Opening date 1968[1]
Construction cost $117,769,400[2]
Operator(s) City of Tacoma[1]
Dam and spillways
Type of dam Concrete, arch-gravity
Impounds Cowlitz River
Height 606 feet (185 m)[1]
Length 1,648 feet (502 m)[1]
Spillway type Service, gate-controlled
Creates Riffe Lake
Total capacity 1,685,000 acre feet (2.08 km3)[1]
Catchment area 1,042 square miles (2,700 km2)[1]
Surface area 11,830 acres (4,790 ha)[1]

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.[1] The dam is the tallest in Washington State and its hydroelectric power station supplies 40% of Tacoma Power's electricity.[3][4]


The Mossyrock Dam was originally planned in the 1940s but opposition from local fishers and Washington State's Game Department delayed construction. During World War II, the city of Tacoma purchased its electricity from the Bonneville Power Administration and Seattle which would cost as much as $1 million a year. To generate its own electricity, Tacoma City Light, now known as Tacoma Power would build 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. The City of Tacoma then went to court and after the case was before the U.S. Supreme Court three times, 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.[5] Several towns were forced to evacuate ahead of the rising dam waters including Riffe, Nesika, and Kosmos.[6][7]

Power plant[edit]

The Mossyrock Dam's power plant contains 2 x 150 MW Francis turbine hydroelectric generators totaling a 300 MW capacity. The generators are fed by three penstocks ranging from 248 – 285 ft. feet long.[3] One of the three penstocks is unused as the plant has an additional space for one more generator.[2]

Power plant upgrade[edit]

In 2006, GE Energy was selected to upgrade both of the power plant's generators.[3] The $50 million project included replacing the turbines, stators, wicket gates and transformers among other improvements.[8] Installation of the first generator began in 2008 and was completed and commissioned in 2009.[9] The second generator was completed in late 2010. The power plant upgrade increased the total capacity by 70 MW and it should operate around 95% efficiency.[4]

See also[edit]

External links[edit]