Mossyrock Dam is a

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

arch-gravity dam on the

Cowlitz River The Cowlitz River is a river in the state of Washington in the United States, a tributary of the Columbia River. Its tributaries drain a large region including the slopes of Mount Rainier, Mount Adams, and Mount St. Helens. The Cowlitz has a ...

near Mossyrock in Lewis County,

Washington Washington most commonly refers to: * George Washington (1732–1799), the first president of the United States * Washington (state), a state in the Pacific Northwest of the United States * Washington, D.C., the capital of the United States ** A ...

, United States. The reservoir created by the dam is called Riffe Lake. The primary purpose of the dam is hydroelectric production while

flood control Flood management or flood control are methods used to reduce or prevent the detrimental effects of flood waters. Flooding can be caused by a mix of both natural processes, such as extreme weather upstream, and human changes to waterbodies and ru ...

is a secondary function.Mossyrock Dam
, NPDP Dam Directory The dam is the tallest in Washington state and its

hydroelectric Hydroelectricity, or hydroelectric power, is Electricity generation, electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other Renewable energ ...

power station supplies 40% of

Tacoma Power Tacoma Power is a public utility providing electrical power to Tacoma, Washington and the surrounding areas. Tacoma Power serves the cities of Tacoma, University Place, Fircrest, and Fife, and also provides service to parts of Steilacoom, L ...

's electricity.

History

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 World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, the city of

Tacoma, Washington Tacoma ( ) is the county seat of Pierce County, Washington, United States. A port city, it is situated along Washington's Puget Sound, southwest of Seattle, southwest of Bellevue, Washington, Bellevue, northeast of the state capital, Olympia ...

, purchased its electricity from the

Bonneville Power Administration The Bonneville Power Administration (BPA) is an American federal agency operating in the Pacific Northwest. BPA was created by an act of United States Congress, Congress in 1937 to market electric power from the Bonneville Dam located on the Col ...

and from

Seattle Seattle ( ) is the most populous city in the U.S. state of Washington and in the Pacific Northwest region of North America. With a population of 780,995 in 2024, it is the 18th-most populous city in the United States. The city is the cou ...

, 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 The Washington State Legislature is the state legislature of the State of Washington. It is a bicameral body, composed of the lower Washington House of Representatives, composed of 98 representatives, and the upper Washington State Senate, w ...

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 The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all Federal tribunals in the United States, U.S. federal court cases, and over Stat ...

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 Kosmos, Nesika, and Riffe. Tacoma City Light originally proposed naming the facility the "Homer T. Bone Dam", in honor of Senator

Homer Bone Homer Truett Bone (January 25, 1883 – March 11, 1970) was an American attorney and politician in Washington state, where he settled in Tacoma, Washington, Tacoma as a youth with his family from Indiana. He ran as a candidate for a variety of par ...

of Tacoma.

Power plant

The Mossyrock Dam's power plant contains two

Francis turbine The Francis turbine is a type of water turbine. It is an inward-flow reaction turbine that combines radial and axial flow concepts. Francis turbines are the most common water turbine in use today, and can achieve over 95% efficiency. The pro ...

generators, Unit #51 is rated at 210MW and Unit #52 at 185mw. MW, or 395 MW total capacity. They are fed by two of the three installed

penstock A penstock is a sluice or gate or intake structure that controls water flow, or an enclosed pipe that delivers water to hydro turbines and sewerage systems. The term is of Scots origin, and was inherited from the earlier technology of mill pond ...

s ranging from 248 – 285 feet in overall length. The third penstock is presently unused, awaiting possible future installation of a third turbine.

Power plant upgrade

In 2006,

GE Energy GE Power (formerly known as GE Energy) was an American energy technology company owned by General Electric (GE). In April 2024, GE completed the spin-off of GE Power into a separate company, GE Vernova. Following this, General Electric ceased t ...

was selected to upgrade the two units. The $50 million project included replacing the

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical ...

stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors, or biological rotors (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

wicket gate A wicket gate, or simply a wicket, is a pedestrian door or gate, particularly one built into a larger door or into a wall or fence. Use in fortifications Wickets are typically small, narrow doors either alongside or within a larger castle or ...

s and

transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...

s. GE sold off their rebuild contract to Andritz AG removing themselves from their contract. Hydro 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 Energy conversion efficiency (''η'') is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be chemical, electric power, mechanical work, light (radi ...

of near 95 percent.

References

External links

{{stack, {{Portal, United States, Water, Renewable energy
''Tacoma News Tribune'' video of the Mossyrock Dam's power plant rebuild
Dams in Washington (state) United States power company dams Hydroelectric power plants in Washington (state) Buildings and structures in Lewis County, Washington Arch–gravity dams Dams completed in 1968 Energy infrastructure completed in 1968 Tacoma Public Utilities

History

Power plant

Power plant upgrade

See also

References

External links