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Cowlitz River Project

Overview of the Cowlitz River Project

The Cowlitz River Project is Tacoma Power’s largest hydrpower facility. It produces enough clean, renewable hydropower energy 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.

Cowlitz River Project map

Project Licensing

All four of our hydro 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 hydropower, 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

Would you like to receive updates about the Cowlitz River Project? Click here to sign up for our Cowlitz River Project email list.

The Cowlitz Restoration and Recovery 2024 grant funding round is closed. Year-round engagement is encouraged to prepare for future grant rounds. Learn more about the program here.

Why is Riffe Lake so low?

We typically draw Riffe Lake down to 745 feet every winter to ensure there is room to absorb large storms. This helps protect public safety by limiting potential flooding impacts to the downstream Cowlitz River communities. We provide updates on river flows, lake levels, and boat launch availability at MyTPU.org/LakeLevels.

In 2017, we reduced the maximum lake level from 778.5 to 749 feet due to updated seismic loading concerns on the Mossyrock Dam spillway piers (not to the dam itself).We received the updated seismic hazard analysis at the end of 2022. We assembled a team to determine what projects need to be done to mitigate risk and allow us to bring the lake back up. Once we have a proposal, we will present it to regulators for input.

Other factors impact the lake level: 

  • As a steward of this resource, besides recreational opportunity, we must balance predetermined downstream flows for fish, plus power production and grid reliability, regardless of the amount of water flowing into the reservoir.
  • When our region experiences extreme cold (or heat), we are required to help meet the higher energy demand.
  • We constantly monitor weather, including inflow forecasts, precipitation, and snowpack levels, which are becoming harder to predict with climate change.

Mossyrock Dam seismicity studies update
Revised seismic hazard data from the United States Geological Survey indicated that the Mossyrock Dam spillway piers and gates (not the dam itself) could potentially be damaged in the event of a large earthquake. If the gate structures are damaged when the lake is at a full level, this could cause an extreme downstream flooding event. Although the earthquake probability is very low, we decided to reduce this risk by keeping Riffe Lake’s elevation to about 30 feet lower than full (778.5 feet). The drawdown began in 2017. We worked with our regulators to make this decision, and they approved our plan to voluntarily lower the level.

Our objective is to bring Riffe Lake back to full pool. We are working hard to make progress on identifying potential seismic damage to any other parts of the dam, as well as possible seismic retrofits, but this is a long and complex process. Read more about our progress here.

Satellite fish rearing facilities update
We will build three satellite fish rearing facilities as indicated in our federal license. With the partnership of the Cowlitz Fisheries Technical Committee, Tacoma Power has evaluated many potential satellite rearing facility locations and programs. We have worked for years on this project and are excited to have selected our first site; it is in the upper Cowlitz basin and is owned by Energy Northwest. The Energy Northwest location offers multiple benefits, including a cool and stable water supply, good site security, and excellent access. Read more about our selection process here. We look forward to your feedback on the proposed Energy Northwest location and would also like to know if you have ideas for locations of the other two future facilities. Please send your comments to Cowlitz Fish Facilities manager Eric Shoblom at eshoblom@cityoftacoma.org.

Public Meetings

Past

Cowlitz Fisheries Science Conference and Annual Program Review Hybrid Meeting – April 16, 2025, from 10 a.m. to 5 p.m.  

Click here to view the agenda.

Presentations:

Cowlitz Fisheries Science Conference and Annual Program Review Hybrid Meeting – Sept. 4, 2024

40 in-person attendees; 20 online attendees,

Agenda

Presentations:

Cowlitz Fisheries Science Conference and Annual Program Review Hybrid Meeting – Aug. 22, 2023
In person at the Veterans Memorial Museum (100 SW Veterans Way, Chehalis, 98532)/Microsoft Teams
10 a.m. to 4 p.m.
38 in-person attendees; 26 online attendees

Hosted by Tacoma Power and the Washington Department of Fish and Wildlife.

Agenda

Presentations:

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