Tacoma Power’s Mossyrock Dam, completed in 1968, stands 606 feet from bedrock and is Washington’s tallest dam. It produces enough electricity to serve more than 78,000 Northwest homes. This double curvature concrete arch dam has three penstocks between 248 to 285 feet in length that extend down to the powerhouse, which is adjacent to the base of the dam.
The powerhouse contains two turbine generators and has room for a third. Tacoma Power has tackled the huge job of replacing both of the existing turbine generators.
Riffe Lake
Mossyrock Dam forms Riffe Lake. Riffe (rhymes with "life") Lake, when it’s full, is 23 miles long, covers 11,830 acres and has 52 miles of shoreline. The lake was named after a small pioneering community that was covered when the lake was formed.
The reservoir serves many important purposes besides power generation. It is a popular recreation destination, especially in the summer and fall. During winter and spring the level of the reservoir is lowered to provide space to help manage down-river flooding. During the summer months, water is gradually released to provide supplemental river flows for the benefit of fish habitat.
Tacoma Power operates two very popular campgrounds by Riffe Lake. Mossyrock Park is located on the southwest shore near the dam. Taidnapam Park sits on the east end where the Cowlitz River flows into Riffe Lake.
Tacoma Power provides four boat launches on Riffe Lake. At the southwest end of the lake is the multiple-lane Mossyrock Park boat launch, with one lane useable during lake levels as low as 720 feet. At the northeast end of the lake, near Glenoma and State Highway 12, is the Kosmos boat launch, which is useable in summer months. At the southeast end of the lake are the Taidnapam Park and the North Taidnapam boat launches. While the park boat launch is useable mostly in the summer, the Taidnapam North boat launch is useable to a lake level of 720 feet. Tacoma Power owns all of the land on Riffe Lake's shoreline, most of which is designated for wildlife habitat.