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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 City Light 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.

Tacoma Power is rebuilding a boat launch and adjusting swim areas to maintain access to Riffe Lake this summer while the massive Lewis County reservoir is drawn down for safety purposes, the utility announced this week.

The lake is being held down 30 feet in response to a U.S. Geological Survey report last year that found a large earthquake could damage the dam’s spillway gates. The dam itself is considered safe, but a breach of spillway gates would cause a catastrophic flood on the lower Cowlitz and Columbia rivers, according to the report.

Tacoma is developing plans to strengthen the spillway gates, but they are not complete.

At lower levels, the boat launch at Mossyrock Park is not useable. So the utility announced this week that it will rebuild the launch to have two long lanes to reach the lower water levels. The new launch should be ready by Memorial Day weekend, the utility said on its web page.

The Taidnapam Park and the Kosmos boat launches will remain closed, though the Taidnapam North boat launch will be open.

Tacoma Power is finishing the permit process to build a new swimming area at Mossyrock Park and plans to have the area open by the beginning of June. It will still be in the day-use area but further east. It will also include an ADA-accessible parking lot, trail to the swim area and port-a-potty.

The current Taidnapam Park swim area is not usable at the lower lake level but Tacoma Power is searching for an alternative location.

The North Shore Fishing Access area will be usable but the Taidnapam Park fishing bridge will have little access.

Kosmos Flats may not be accessible to vehicles.

Tacoma Power said it does not expect levels at Riffe Lake to impact Mayfield Lake operations or downstream flows on the Cowlitz River.

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