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Mossyrock Dam Precautions

Posted on 16th February 2017 at 08:56

Tacoma Power is announcing that Riffe Lake behind Mossyrock Dam will be drawn down, as the utility announces precautions in the event of seismic damage to the dam. Tacoma Power announced the drawdown on Tuesday, saying that they have received new information from the U. S. Geological Survey. They say that the probability of an earthquake that could damage the 606-foot dam is “very low,” but these revised predictions show that an earthquake could damage the spillways on Mossyrock Dam. As a precaution, Tacoma Power plans to keep the level of Riffe Lake at 30 feet lower than full, a level they plan to maintain at least into the next decade. Utility officials say that it’s the shape of the dam that is the biggest issue; because of the dam design, specific seismic events could render the spillways useless, which could lead to major flooding downstream. They are exploring some seismic retrofits, but that’s a long and arduous process, which would also involve several federal agencies. Boating, swimming and fishing will still be available on Riffe Lake, but boat launches and swimming areas at parks along the lake will be affected. Additional information is available on the Tacoma Public Utilities web page.