Riffe Lake Restoration
Why Riffe Lake remains at a lower level
Tacoma Power lowered Riffe Lake in 2017 to prioritize public safety while evaluating the seismic performance of Mossyrock Dam.
Keeping the lake lower is an interim safety measure while we complete the engineering, analysis and improvements needed to safely restore the lake. This work is part of our ongoing responsibility to safely operate and maintain Mossyrock Dam and meet federal dam safety and regulatory requirements.
We acknowledge the impacts of lower lake levels on recreation, access and hydropower generation. This page will provide clear, current information about the work underway and the path to restoration.
Riffe Lake, a reservoir on the Cowlitz River in Lewis County, Washington.
Public safety remains our top priority
The Riffe Lake Restoration Program supports the long-term safe operation of Mossyrock Dam and creates a path to restoring the lake. Restoration will improve recreation and access and restore the Cowlitz River Project’s ability to generate hydropower at levels available before the lake was lowered.
Our path to restoration
Tacoma Power is working with the Federal Energy Regulatory Commission (FERC) on a coordinated program of dam safety improvements, engineering studies and environmental work needed to safely restore Riffe Lake to its original maximum elevation of 778.5 feet.
The current target date for restoration is 2031, pending completion of required work and FERC review and approval.
Estimated Restoration Timeline
Project schedules are estimates and may change as engineering, construction and regulatory reviews progress. Many milestones depend on FERC review and approval.
Work Underway
-
We are evaluating and improving the four spillway gates at Mossyrock Dam to support their safe, reliable operation and meet current requirements.
-
Removable barriers, called bulkheads, will be installed in the spillway downstream of the spillway gates. The bulkheads will help limit the amount of water released from the lake if the spillway gates or supporting piers are damaged during an earthquake.
Once the design is complete, federal regulators must review and accept the bulkheads as an alternative to maintaining the current lower lake elevation.
-
Advanced 3D models of the dam, spillway, and spillway piers are in development. These models will allow us to better understand how each element would perform during an earthquake and determine whether additional work is needed.
The results will also provide important information to support the other dam safety projects at Mossyrock Dam.
-
The left embankment at Mossyrock Dam is under evaluation to determine whether improvements are needed.
-
Vegetation has grown along the exposed shoreline since the lake was lowered in 2017. Before the lake is restored to higher levels, vegetation will be removed from select areas to support safe public access and recreation. Vegetation in other areas will remain and will be inundated as the lake refills, similar to what has occurred during past changes in lake levels.
-
We continue to identify, monitor and protect historic and culturally significant resources exposed by the lower lake level in coordination with the Yakama Nation, Cowlitz Indian Tribe and Washington State Department of Archaeology and Historic Preservation.
-
We are continuing environmental investigation and cleanup work at the historic Kosmos Mill site to prepare the area for restoration of the reservoir and protect Riffe Lake.
Questions and past meeting information
For questions about Mossyrock Dam and Riffe Lake, or to request summaries from past public meetings, contact cowlitz@tacoma.gov.