Cowlitz River Project

The Cowlitz River Project is Tacoma Power’s largest hydroelectric facility. It produces enough renewable electricity to serve more than 140,000 homes each year. This hydropower comes from the water stored behind the Mayfield and Mossyrock dams.

Built in 1963, Mayfield Dam forms 13-mile-long Mayfield Lake. Mossyrock Dam, built in 1968, is the tallest dam in Washington state at 606 feet above bedrock, and forms the 23.5-mile-long Riffe Lake. Watch a video detailing how we rebuilt the two Mossyrock powerhouse generators.

Project Licensing

All four of our hydroelectric projects are licensed separately by the Federal Energy Regulatory Commission (FERC), which was created by the Federal Power Act to ensure safe and efficient operation of power plants across the U.S. Many other agencies, tribes, organizations and stakeholders are also an important part of the licensing process. In 2003, we received a 35-year federal license to operate the Cowlitz River Project.

Fish and Wildlife Programs and Public Recreation

Along with generating clean, renewable electricity, our FERC license includes mandates for:

Tour the Facilities​

Free group tours of the Cowlitz River Project facilities are available by appointment for school, civic, business and recreational groups. To arrange a tour, please complete the request form at the bottom of our hydropower page.

Latest News

Tacoma Power will lower Mayfield Lake to 410 feet of elevation Sept. 15-30, 2019

Tacoma Power will lower Mayfield Lake to 410 feet of elevation Sept. 15-30, 2019, in order to repair and extend the Mayfield Lake Park boat launch. This is approximately 10 feet lower than average low water levels of the reservoir.

The planned timeline is:

  • Sept. 15: Begin lowering the lake
  • Sept. 18-24: Perform work on the boat launch
  • Sept. 24-30: Refill the lake

All boat launches will be inoperable during this time of extremely low water. If you have a boat or other watercraft on Mayfield Lake, please remove it before Sept. 15. Failure to do so could result in your boat or watercraft becoming stranded and/or damaged.

If you have permitted shoreline structures (docks, ramps or floats), we recommend you determine if those structures can withstand the lower lake levels. If you identify a need to modify or prepare your structures to prevent potential damage, please contact the following Tacoma Power officer for assistance with the necessary approvals, which may include local, state, federal, and Tacoma Power permits:

Kyrra Wilson, Tacoma Power’s lands officer
(253) 502-8824

This lowering of the lake will expose much more of the lakebed than usual. Hazards will include deep mud, a shallow bottom, and exposed stumps and debris on the water. Boating, swimming, water skiing, diving, and other similar activities will be hazardous in shallow waters or areas where these dangerous obstacles are near the water surface.

To ensure public safety, during the two-week period of low water, Tacoma Power strongly encourages you to:

  • not access the lake or walk along the exposed lakeshore,
  • take special care to keep children and pets away from the shoreline.

If you would like to receive reminders by email, please send your contact information to Please submit any questions to

Public Meetings

Cowlitz River Annual Project Review (APR) Public Meeting – Aug. 14, 2019
Veterans Memorial Museum | 6 to 8 p.m.

  • Agenda
  • Presentation (coming soon)
  • Survey results (coming soon)

Please email to RSVP or with questions.

Cowlitz River Annual Project Review and Fisheries Science Conference – July 10, 2019
Veterans Memorial Museum | 9 a.m. to 3 p.m.
51 attendees