Tacoma Power to reduce water level at Riffe Lake due to quake concerns for Mossyrock Dam

CENTRALIA, Wash. (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 reports ( http://bit.ly/2m60zMl ) 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.