Riffe Lake drawdown expected to last years

A proposal by Tacoma Power to draw down the level of Riffe Lake by at least 30 feet from maximum capacity for the foreseeable future will create 52 linear miles of freshly exposed shoreline. With the controversial plan seeming inevitable, locals and visitors are wondering what Tacoma Power plans to do to control erosion and the spread of nonnative and noxious weeds along those three-story reservoir flanks.

According to Tacoma Power wildlife and lands manager Bret Forrester, the grand plan is quite simple — let nature take its course and hope for the best.

“The grass that dominates the shoreline naturally reseeds exposed areas each year during reservoir drawdowns,” Forrester said in an email. “Our normal refilling of the reservoir has suppressed the establishment of grass in these areas in the past. Now that the reservoir will not likely be refilled to the previous levels for several years, this natural expansion will be allowed to take hold. This has already begun in many areas because of the lower-than-normal reservoir levels that have occurred during the past two years.”

According to Tacoma Power, the proposed drawdown of the reservoir behind Mossyrock Dam, known as Riffe Lake, was necessitated by concerns about the stability of the spillway piers on the upriver side of the dam in the event of a large earthquake. Because of uncertainties caused by a lengthy permitting process, Tacoma Power has been unable to provide a timeframe for the project, other than to say it will more than likely stretch into the next decade.

Pat McCarty, generation manager for Tacoma Power, has said that if an earthquake strikes during the summer, when the reservoir is typically kept at full capacity, the spillway piers could fail. The failure would cause the 23.5-mile-long reservoir to rapidly fall by about 60 feet, resulting in catastrophic flooding downstream all the way to Longview. By reducing the level of Riffe Lake by some 30 feet from its normal summer level, McCarty said, a flood event caused by the spillway’s failure would be minimal.

“Because we anticipate that we will be able to correct the spillway pier concern in the future and return to refilling the reservoir to its capacity, we do not want to promote the development of trees and shrubs within the exposed areas,” said Forrester, who said new patches of vegetation could wind up becoming boating, fishing or swimming hazards when the reservoir’s level is raised again.

At a town meeting in Mossyrock this month, McCarty said he did not believe Tacoma Power made a specific commitment about reseeding or controlling the vegetation on the banks of the lake in south-central Lewis County. He noted that because the newly exposed areas are adjacent to a waterway, weed management actions are limited in scope.

McCarty said that it would likely not be possible to spray the banks with herbicide or other treatments, or to set about cutting trees or other vegetation in direct proximity to the watershed.

“We will monitor the natural vegetation establishment. And if we identify undesirable conditions developing, we will actively manage those areas, which could include seeding or weed control,” Forrester said.

For additional information from Tacoma Power on the proposal, go online to mytpu.org/RiffeLake. Specific questions and comments can be routed through email to cowlitz@cityoftacoma.org.