Mossyrock Dam

Completed in 1968, Mossyrock Dam is Washington state’s tallest dam at 606 feet high above bedrock (or 365 feet above the riverbed). It produces enough clean, renewable electricity to serve more than 78,000 homes annually. The dam’s double curvature concrete arch has three penstocks between 248 and 285 feet in length that extend down to the powerhouse.

We replaced the 40-year-old turbine generators in the powerhouse in 2010 and 2011 with modern, efficient turbines that produce more power using the same amount of water. Watch a video detailing how we rebuilt the two Mossyrock powerhouse generators.

The dam created 23.5-mile-long Riffe Lake. The lake offers 52 miles of shoreline to explore by watercraft, and two parks offering a variety of outdoor recreation: Mossyrock Park, located on the southwest shore near the dam, and Taidnapam Park on the east end where the Cowlitz River flows into the lake. These parks provide access to boat launches, swimming (Mossyrock Park only), fishing, camping, a kitchen shelter and day-use facilities.

Riffe Lake Levels

In 2017, the United States Geological Survey (USGS) revised its earthquake predictions for the Cowlitz River basin. Although the probability of a large earthquake is very low, the updated predictions showed an increase to the potential impact a quake could have on just the spillways of Mossyrock Dam — not to the dam structure itself. So, to reduce this risk and protect public safety, we will hold Riffe Lake’s elevation to about 30 feet lower than full (778 feet) at least into the next decade.

In the event of significant rainfall or flooding, we may need to use the storage capacity of Riffe Lake to minimize downstream flooding by temporarily allowing the lake level to rise. After the risk of downstream flooding passes, we would then gradually lower the reservoir to previous levels.

Seismic Information

No concrete arch dams, such as Mossyrock Dam, have failed due to earthquakes. However, analysis shows that specific seismic events could potentially cause the spillway piers and spillway gates to fail, resulting in an uncontrolled release of water and considerable downstream flooding. Lowering the lake level to 30 feet lower than full helps limit this impact.

We are currently considering seismic retrofits, but making changes to a federally licensed hydroelectric dam is a long and complicated process. The development of possible solutions involves substantial analysis, planning and federal approval. We will keep you updated on this process.

For more information about Mossyrock Dam, contact us at cowlitz@cityoftacoma.org or see the information below:

Statistics
  • Year completed – 1968
  • River system – Cowlitz
  • Original investment – $117.8 million
  • Type of dam – concrete arch/gravity and embankment
  • Height – above riverbed, 365 feet; above bedrock, 606 feet
  • Length – 1,648 feet
  • Width – top, 27 feet, base, 115 feet
  • Volume of concrete in dam – 1.27 million cubic yards
  • Average flow – 5,140 cubic feet/second
  • Drainage area – 1,042 square miles
  • Reservoir length – 23.5 miles
  • Miles of shoreline – 52 miles
  • Reservoir area at maximum elevation – 11,830 acres
  • Reservoir elevation – full, 778.5 feet; lowest allowable for generation, 600 feet
  • Average annual generation – 1,100,000,000 kilowatt-hours
  • Equivalent number of NW homes served – 78,500
  • Number of generators – two 150,000 kilowatt generators
  • Installed capacity (nameplate rating) – 382,000 kilowatts
Recreation

Our new Mossyrock Park boat launch has two long lanes usable at lower elevations: one lane is usable to approximately 720 feet; the second lane is usable to approximately 708 feet. The longer lane should be available all summer, fall and early winter (in years with normal inflows).

The Taidnapam North boat launch is open most of the year (when water levels permit) with one usable lane and a dock servicing elevations down to 720 feet.

The Taidnapam Park boat launch is closed. It reaches elevation 760 feet; there is no way to lengthen it due to a drop-off at the end.

The Kosmos boat launch is closed. It is not usable below 747 feet and cannot be extended.

Get up-to-date info about boat launches and lake levels here or call (888) 502-8690.

Swim Areas

The new Mossyrock Park swim area is usable at approximately 728 feet, so it should be available all summer unless there is a drought. We added an ADA-accessible parking lot, trail to the swim area and portable toilet to help make the area more accessible for our visitors who have disabilities.

Fishing

The North Shore Fishing Access area will be usable to 735 feet.

Latest News

Riffe Lake Update meeting materials

Tacoma Power hosted a Riffe Lake Update public meeting on Thursday, May 2, from 7 to 8 p.m. at the Mossyrock Community Center.

Questions? Please email cowlitz@cityoftacoma.org.

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