Lower level set for lake behind Mossyrock Dam

CENTRALIA (AP) — Updated earthquake concerns for Mossyrock Dam, the tallest structure of its kind in Washington, have prompted Tacoma Power to commit to a new, reduced water level at Riffe Lake that will likely stretch well into the next decade. The Chronicle reported the lower water level will be employed during the spring and summer months when mountain snow runoff is typically allowed to fill the reservoir.

By keeping less water behind the dam, Tacoma Power hopes to alleviate risks that might exist to downstream communities should an earthquake of at least 7.5 magnitude strike close to the dam.

The fear is that the upstream piers that direct water through the spillway could become damaged, or even break off, during a strong quake. That scenario would disable the typical spillway function and allow the lake to rapidly drain.

Representatives from Tacoma Power said that only the 65-feet tall, 90-feet long spillway piers are believed to be at risk during the projected earthquake event. Tacoma Power said there is no risk to the integrity of the main arched concrete dam structure, and noted that no dam of its kind has ever failed due to earthquake activity.

If a full capacity Riffe Lake reservoir were to drain through a damaged spillway, a flow of 230,000 cubic feet per second would inundate communities downstream.

Tacoma Power representatives said communities from at least Toledo to Longview would be affected. They referred to that worst-case scenario as a regional flooding event. By maintaining a lower lake level, officials believe that even a complete failure of the spillway function would lead to a release of water that would not cause substantial flooding issues along either the Cowlitz or lower Columbia rivers.

Pat McCarty, generation manager for Tacoma Power, said there is no imminent danger and that the summer water level reduction is being done as a precaution for public safety.

He also said Tacoma Power’s announcement has no connection to the situation at California’s Oroville Dam, where authorities ordered a large evacuation last week.

“This is 100 percent about public safety. Frankly, the risk we are talking about is very slim but that was not a risk we were willing to take,” McCarty said.