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Riffe Lake to be lower as hedge against earthquakes

Riffe Lake to be lower as hedge against earthquakes

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Mossyrock Dam

The piers holding the spillway gates on the back side of Mossyrock Dam may be vulnerable to earthquake damage, so Tacoma Power plans to lower the lake 30 feet for the next several years while it figures out how to strengthen the structures. Riffe Lake backs up behind the dam, which is located on the upper Cowlitz River.

The operator of Mossyrock Dam, the tallest dam in Washington state, has proposed lowering the lake behind it by 30 feet into the next decade as a hedge against damage from a large earthquake.

The action by Tacoma City Light has nothing to do with the current emergency situation at Oroville Dam in Northern California. There is no impending emergency at Mossyrock Dam and there is no structural damage, Tacoma City Light said in a press release issued Tuesday.

Mossyrock Dam, built on the upper Cowlitz River in 1968, stands 606 feet above bedrock. The dam forms 23.5-mile-long Riffe Lake, which is used as a major storage reservoir to prevent floods on the lower Cowlitz River.

Tacoma announced its action is based on a new review of earthquake hazards assessment by the U.S. Geological Survey, which recently revised its earthquake predictions for the Cowlitz River basin.

“While the probability of a large earthquake is very low, the revisions showed an increase to the potential impact just on the spillways of Mossyrock Dam (not to the dam structure itself). To protect public safety, Tacoma Power has proposed to hold Riffe Lake’s elevation down approximately 30 feet lower than full (778 feet) at least into the next decade. Approval by the federal agency that regulates the utility is pending,” the utility said in a press release.

Mossyrock is a curving “arch” dam made of concrete. No concrete arch dam has failed due to an earthquake, Tacoma said. However, it’s possible a strong earthquake could damage the piers that hold the spillway gates in place, making it them useless. Failure oft the gates “could cause considerable downstream flooding. Lowering the lake level will limit the impact,” according to Tacoma Power.

“We are looking at seismic retrofits. The development and implementation of possible solutions involves substantial analysis, planning and federal approval. The process required for making changes to a federally licensed hydroelectric dam is long and arduous,” Tacoma’s statement says.

Lowering the lake 30 feet is nothing unusual. In fact, Tacoma’s federal operating license requires it to lower the lake more than that each winter so Riffe Lake can absorb high runoff and reduce flooding downstream. But the utility typically allows the dam to refill in the spring for recreation purposes.

Tacoma Power operates two campgrounds on the shoreline of Riffe Lake: Mossyrock Park is located on the southwest shore near the dam; Taidnapam Park sits on the east end where the Cowlitz River flows into the lake.

Boating, swimming and fishing will remain possible on Riffe Lake, although access could be limited. Two Tacoma Power-owned boat launches are anticipated to be usable during the summer at the lower lake elevations. The Taidnapam Park north boat launch has a dock servicing these lower elevations. Both the Kosmos and Taidnapam Park south boat launches will remain closed.

Tacoma Power said it is considering other modifications to enable use at lower lake levels, including updating the existing boat launch at Mossyrock Park to accommodate a new dock and a new swim beach at Mossyrock Park.

Contact City Editor Andre Stepankowsky at 360-577-2520.

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