Work to prevent catastrophic Cowlitz dam failure due to start in 2027

Mossyrock Dam spillway, vulnerable to a large earthquake, will be shored up as part of $76 million seismic safety, environmental restoration project at state's highest dam

Water surges out of the spillway of Mossyrock Dam on the upper Cowlitz River in this undated photo from Tacoma Public Utilities, which owns the dam. Arrows point to areas that are part of a seismic safety retrofit of the 60-year-old structure.

Editor’s note: Mari Kanagy is a Seattle-based freelance journalist. She graduated from the University of Washington in 2024 with a bachelor's degrees in journalism and socio-legal studies. She originally is from Vashon Island, Washington, and is a regular contributor to the island’s local paper, The Beachcomber. This is her first assignment for Lower Columbia Currents.

By Mari Kanagy

Construction to prevent an earthquake from damaging Mossyrock Dam and causing a catastrophic flood along the lower Cowlitz River will begin in early 2027, a decade after the dam’s spillway structure was found vulnerable to a 7.5 magnitude temblor.

However, Riffe Lake will continue to be drawn down at least 30 feet below its capacity until 2031 while a $76 million safety and environmental restoration program takes place, according to Tacoma Power, the dam’s owner.

The drawdown has had a noticeable impact on recreation. Shorelines at places receded into steep, muddy banks, causing frustration for fishers. Several boat launches, including those at Kosmos and Taidnapam, are inaccessible in winter and early spring when the water is at its lowest.

Emergency Management departments in Cowlitz and Lewis counties said they didn’t know enough about the restoration project to comment on the process or Tacoma’s timeline.

Built in the early 1960s about 20 miles east of Toledo, Mossyrock is the state’s tallest dam — an arching concrete structure standing 600 feet tall and stretching 1,650 feet across a narrow chasm of the Cowlitz River.

Behind it sits Riffe Lake, a 23-mile-long reservoir that can hold more than 1.6 million acre-feet of water (enough to fill nearly 800,000 Olympic swimming pools) when full.

Tacoma Power lowered Riffe Lake in 2017 after the U.S. Geological Survey found that a 7.5 earthquake on a nearby fault could damage Mossyrock’s spillway, leading to an uncontrolled release of water.

Although Tacoma Power says the probability of such a large quake is small, the Federal Energy Regulatory Commission (FERC) directed Tacoma Power to lower Riffe Lake roughly 30 feet below its capacity of 778.5 feet above sea level.

The lake has been capped at about 750 feet for the past seven years, and it is drawn down further during winter months for flood control purposes.

Tacoma Power must ensure the lake has enough water for fish to spawn and to meet minimum flows downstream in the Cowlitz River, especially in the fall, according to the Washington Department of Fish and Wildlife.

“That balancing act means that water levels may not always look how people expect it to at certain times of the year,” said Britton Ransford, communications contact for the Southwest Region.

“However, while low water levels can limit recreation like boating or fishing, it can provide shoreline that campers and other visitors can enjoy.”

Tacoma Power does not anticipate that the spillway work will require further significant Riffe Lake drawdowns. However, some may be necessary during other parts of the retrofit work.

The Riffe Lake restoration program includes 11 projects addressing both seismic and environmental concerns. These include ongoing contamination cleanup at the former Kosmos Mill site — which was inundated after the dam was built — and management of vegetation growth on the exposed lakebed.

Tacoma Power is also working with tribal and state partners to protect archaeological sites revealed by the lowered water levels. All of these efforts must be completed before federal regulators will approve refilling the lake.

The expansive nature of the restoration project is just one reason the project is taking so long.

The design of Mossyrock — a concrete arch dam, like Hoover Dam — makes spillway retrofitting particularly difficult, said Chris Mattson, generation manager at Tacoma Power hydroelectric project. The spillway is built right on the two faces of the dam and is not a separate structure, like those at U.S. Army Corps of Engineers dams on the Columbia River.

“Every dam is unique, especially a more complex, double-curvature, arched dam like Mossyrock,” Mattson said.

In addition, the federal process for reviewing Tacoma’s retrofit plans is cumbersome and time consuming. Tacoma Power is in a constant back-and-forth with regulators. Every step forward must move through layers of federal oversight.

For each phase of the project, FERC requires approval not only from its own engineers but also from a board of independent consultants. That board, composed of experts in geology, seismology and structural engineering, was convened specifically for the Mossyrock project.

Tacoma Power says that much of the timeline is dependent on review times from regulators. Just scheduling and securing the board’s approval can take between three and six months. FERC’s final response can take up to a year.

Furthermore, seismologists from both FERC and Tacoma Power recommended waiting for updated earthquake hazard models before planning the restoration program. In 2018, the Pacific Earthquake Engineering Research Center began research into the fault lines near Mossyrock.

“I don’t think anybody anticipated it to take three years for those studies, but it did,” Mattson said.

The delay was likely exacerbated by the 2020-21 COVID pandemic.

Tacoma Power says the long timeline reflects technical complexity of the project and the high stakes involved.

“Public safety is our top priority, above all else,” Mattson said. “We will continue to make decisions and take actions related to Riffe Lake that first and foremost ensure the safety of the public downstream.”