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Cowlitz River Project

Overview of the Cowlitz River Project

The Cowlitz River Project is Tacoma Power’s largest hydroelectric facility. It produces enough renewable electricity to serve more than 151,000 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

Barrier Dam Spillway Upgrade Project

Tacoma Power is performing construction work at the Barrier Dam through Oct. 1. Visitors may see heavy equipment, fencing, trailers, and other related items. These activities will occupy some of the parking areas near the Barrier Dam spillway. The majority of access, and parking spaces downstream of the spillway, will remain available to visitors. Visitors are asked to follow instructions on any safety signs posted at or near the site. The boat launch and fishing access (including the ADA-accessible area) will remain open.

This work is one phase of a multi-year project to upgrade the Barrier Dam’s spillway. The next phase is scheduled to start in the summer of 2022.

Update as of 7/30/21: Although we had to keep our Cowlitz River outflows below Mayfield Dam at around 2,500 cfs for the last few weeks due to the construction, we are now at a phase where we can increase flows again and move back to our typical operations. Please note that the construction work has changed the flow path near the “Cowlitz River Below Mayfield Dam” USGS gage, so the real-time gage readings are not currently available. We will provide further updates on this page and at

Questions? Please email

Kosmos Flats Oil Seep Capped
As part of our commitment to ensuring good stewardship of our environment, Tacoma Power capped the Kosmos Flats oil seep in December 2019. Read more about the project.

Public Meetings

Cowlitz Annual Program Review Virtual Public Meeting – June 23, 2021
Online/conference call | 2 to 4 p.m.
14 attendees

Please email with questions.

Cowlitz Annual Program Review and Science Conference – May 19, 2021
Online/conference call | 10 a.m. to 4 p.m.
37 attendees


Summarize Five-Year Trends

  1. Introduction and Meeting Logistics
  2. Intro to Current Status
  3. Project Operations
  4. Hatchery Operations
  5. Habitat
  6. Passage
  7. Pre-Terminal/Terminal Harvest
  8. SARs
  9. Lower Cowlitz Adult Estimates
  10. Separator Returns
  11. What We Know/Don’t Know in Our Current State

Annual Program Review

  1. Summary of Recent Trends and Current Status, Fisheries and Hatchery Management Plan – Transition Plans Status and Next Steps, 2022 Strategies

Please email with questions.

Cowlitz River Fisheries and Hatchery Management Plan (FHMP) and Annual Project Review (APR) Virtual Public Meeting – Aug. 12, 2020
Online/conference call | 4 to 6 p.m.
21 attendees

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

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