[1]Mossyrock Dam on the Cowlitz River sits at the west end of Riffe Lake. / Courtesy photo by Tacoma Power
By Sharyn L. Decker
Lewis County Sirens news reporter
CHEHALIS – Recently revised earthquake predictions, which portend failure of the spillway gates of the Mossyrock Dam have prompted the the dam’s owner to begin considering seismic retrofits.
In the meantime, Tacoma Power plans to keep less water in the reservoir which is Riffe Lake to reduce the amount of downstream flooding if a big one should strike.
Tacoma Power announced this week the changes it is implementing.
They want to keep the level of Riffe Lake 30 feet lower than normal, according to Chris Gleason, the community and media services manager for Tacoma Public Utilities.
The United States Geological Survey recently revised its earthquake predictions for the Cowlitz River basin, information that was passed along to the owner of the hydroelectric dam in November, according to Gleason.
Riffe Lake, more than 23 miles long, is 778.5 feet when full.
The probability of a large earthquake is low, and the issue isn’t the dam itself, according to Tacoma Power.
But, preliminary analysis concluded that specific seismic events could render the spillway gates useless, which could cause considerable downstream flooding, according to the utility.
A spillway is the part of the dam that water flows through. The four spillways are located high in the middle of the dam.
According to Gleason, they’ve submitted their plans to the Federal Energy Regulatory Commission which will review and accept or modify the proposal.
They are required keep the lake below 745.5 feet during December and January, so it’s currently close to their lower level proposed, and they will maintain it there for the foreseeable future, according to Gleason.
The new lower level is expected to be in place into the next decade.
Mossyrock Dam, a concrete structure completed in 1968 is the state’s tallest dam. It produces enough electricity for 78,000 homes.
No concrete arch dams have failed due to earthquakes, according to Tacoma Power.
Boating, swimming and fishing will remain possible on Riffe Lake, but access could be limited.
Two Tacoma Power-owned boat launches are anticipated to be usable during the summer, but two others will be closed. They are considering a new swim beach at Mossyrock Park.
The seismic retrofits they are beginning to consider won’t take place for years. The development and implementation of possible solutions involves substantial analysis, planning and federal approval, according to Tacoma Power.
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[2]On the east side of the dam are the piers that hold the spillway gates in place. / Courtesy photo by Tacoma Power