Mossyrock Dam Earthquake Fears -- Tacoma Power Plans Prolonged Low Levels at Riffe Lake

In this courtesy photo provided by Tacoma Power, Mossyrock Dam is seen in August 2013.

Posted Thursday, February 16, 2017 10:42 am

By Jordan Nailon / jnailon@chronline.com

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 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 jut out like fingers from the dam and direct water through the spillway could become damaged, or even break off, during a high intensity seismic event. That scenario would disable the typical spillway function and allow the lake to rapidly drain.

In a set of interviews with The Chronicle on Tuesday, representatives from Tacoma Power made sure to note that only the 65 foot tall, 90 foot long spillway piers are believed to be at risk during the projected earthquake event. Tacoma Power maintains that there is no risk to the integrity of the main arched concrete dam structure, and noted that no dam of its kind has ever failed due to earthquake activity.

If a full capacity Riffe Lake reservoir were to drain through a damaged spillway, a flow of 230,000 cubic feet per second would inundate communities downstream. Tacoma Power representatives said communities from at least Toledo to Longview would be affected. They referred to that worst case scenario as a regional flooding event. By maintaining a lower lake level, officials believe that even a complete failure of the spillway function would lead to a release of water that would not cause substantial flooding issues along either the Cowlitz or lower Columbia rivers.

Pat McCarty, generation manager for Tacoma Power, insists that there is no imminent danger and that the summer water level reduction is being done as a precaution for public safety. He dismissed the likelihood of the size and type of earthquake required to cause such extensive damage to the function of Mossyrock dam as unlikely.

He also stressed that Tacoma Power’s announcement has no connection to the ongoing crisis at California’s Oroville Dam, which is currently at risk of failing and putting homes and property downstream at risk.

“This is 100 percent about public safety. Frankly, the risk we are talking about is very slim but that was not a risk we were willing to take,” said McCarty.

During winter months, the Riffe Lake reservoir level is kept around 745 feet. When big rain and snowmelt comes through the system during those inclement months, the reservoir is allowed to temporarily hold back additional water so that it can be gradually released downstream in order to prevent flooding. In past summers, the reservoir is allowed to fill up to about 778 feet, but going forward that water level will be increased to just 749 feet, nearly 30 feet below historic levels. McCarty said there is no timetable for a permanent fix to the pier instability issue at the dam and called the lower water levels “the new normal” for Riffe Lake.

“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,” read a press release delivered by McCarty on Tuesday.

The study that led Tacoma Power to commit to a lower water level at Riffe Lake was conducted by the U.S. Geological Survey. A similar study at Mayfield Dam found that the bridge deck does not meet safety standards for a similar worst case scenario seismic event. McCarty noted that no action has yet been taken in regard to the Mayfield Dam issue, but he insisted that a plan is currently being developed in order to update the suspect infrastructure.

Mossyrock Dam, which holds the water that creates Riffe Lake, rises 606 feet above bedrock and was constructed in 1968. Riffe Lake stretches 23.5 miles east from the dam and is used for both power generation and flood mitigation. The lower water levels will mean a loss in revenue for Tacoma Power since Mossyrock Dam will not be able to generate as much electricity.

Recreation Opportunities at Riffe

While the water level of Riffe Lake will not be altered in the winter, it will drastically alter the summer recreation landscape for fishing, boating, swimming and camping.

Tacoma Power currently operates two campgrounds along the manmade shoreline of Riffe Lake. Mossyrock Campground is located on the southwest shore of Riffe Lake while Taidnapam Park is located at the far east end of the lake.

Low summer water levels were experienced at Riffe Lake last year, although McCarty said they were not related to any issues at Mossyrock Dam. Instead, McCarty blamed a warm June and hot July for quickly evaporating snowpack that would have otherwise trickled downhill to fill the Riffe Lake reservoir. No matter the cause, the low lake level presented problems for anglers, boaters and other outdoor recreators. Condensed water channels and an increase in water temperature put the squeeze on fish in the lake while simultaneously limiting access to, or the existence of, historic fishing holes. Likewise, boaters were faced with a fresh barrage of exposed hazards and debris such as tree stumps and rocks. Access to boat launches was also compromised. Likewise, camping and swimming opportunities were also impacted as what were once prime lakeside spots like Kosmos Flats, were left high and dry. Going forward, those conditions will be a part of the “new normal” Riffe Lake summers, Tacoma Power officials said.

With the new summer water level in place the Taidnapam and Mossyrock swimming areas, as well as the Mossyrock dock, will be put out of commission year round. McCarty said that preliminary discussions have taken place about the possibility of creating a new swimming area for the Mossyrock Campground and making modifications to the Mossyrock Park boat launch that would make it functional in the lower water. The Taidnapam Park north boat launch already has an adjustable dock that can accommodate shifting lake levels, however, both the Kosmos and Taidnapam Park south boat launches are slated to remain closed at least until modifications to the spillway piers have been completed.

As for the negative impact on fishing conditions at Riffe Lake for species like smallmouth bass, McCarty said, “There’s not a lot we can do about that.”

Those water condition issues include altered access and functionality of the popular Taidnapam fishing bridge.

McCarty said that there is a chance Tacoma Power may begin a trout stocking program at Riffe Lake in order to offset some of the lost opportunity for other fish species. However, that stocking effort would not represent an increase in area angling prospects since, as McCarty noted, any trout stocked at Riffe would simply be carved out of the stock currently reserved for Mayfield Lake.

In addition to the amenity and access changes in store at Riffe Lake, McCarty noted that he will soon begin reviewing park staffing and operations in order to determine if Tacoma Power would be better off contracting the day-to-day responsibilities of Mossyrock and Taidnapam parks to an outside entity.

McCarty said it is simply a coincidence that the evaluation of park operations is taking place at the same time as the beginning of the prolonged lake drawdown and the accompanying reduction of recreation access. He noted that longtime Taidnapam Park manager Arnie Lund is set to retire in April and another manager is set to retire within the next year.

“I’ve got two managers retiring,” said McCarty, who added general budget issues to his two tiered explanation for possible changes in Riffe Lake parks management.

McCarty said that budget issues are forcing him to consider multiple options for future management of the parks including hiring replacement managers, reducing days of operation, contracting out daily operations, or consolidating duties between park managers at Mossyrock, Taidnapam and Mayfield parks. McCarty said he plans to hire a six-month temporary replacement for Lund’s position and expects that a final decision on a management plan for the parks will not be made until this fall at the earliest.