The U.S. Army Corps of Engineers is proposing to partner with states and pay 70 percent of the costs to implement a rapid response plan to eradicate zebra and quagga mussels if the invasive species are found in Washington, Oregon, Idaho or Montana.

The agency released an environmental assessment and draft Finding of No Significant Impact on its plan, and is accepting public comment through Aug. 19.

According to the Corps' assessment, there is "high potential" for an invasion of the mussels in the four-state area, and it's "highly probable" that the mussels will be introduced into the Columbia Basin at some point from a recreational watercraft unknowingly transporting invasive species from another part of the country.

An infestation "has the potential to cause billions of dollars in damage and increase operation and maintenance costs to water-related infrastructure, and untold damage to the ecosystem and the species dependent upon it," the draft FONSI says.

Along with other underwater structures, the agency says that the mussels pose a significant threat to the Federal Columbia River Power System's hydropower and nuclear generation projects. Pointing to an area with established mussel populations, the analysis says zebra mussels in the Great Lakes cost the power industry $3.1 billion from 1993 to 1999, and had a total economic impact on the region of more than $5 billion. At Hoover Dam, $1 million annually is spent on quagga mussel control. "Similar costs are expected in the west in the event of an invasion," and could even be higher, the analysis says.

Mussels--which reproduce rapidly and adhere themselves to underwater structures in great numbers--could inundate major hydropower components and result in a powerhouse shutdown. Other Columbia Basin structures that would be threatened by a mussel infestation include major navigation lock components, fish ladders, screens and bypass systems, and irrigation and municipal water supply and treatment facilities.

The Corps evaluated three alternatives: no action; independent actions by states and the Corps; and its preferred alternative--to set up cost-share plans with states and to act independently when water-related facilities are owned and managed by the Corps or other federal agencies.

If a cost-share with states goes forward, states would carry out a rapid response under the plan and the Corps would reimburse the state 70 percent of the cost.

Under that plan, the Corps identified 4,174 locations throughout the four states where mussel detection is most likely, including boat ramps and shallow areas close to public access sites. Once presence of a quagga or zebra mussel is detected, the severity of the invasion would be determined and a rapid response initiated. Steps include mobilizing to the site, isolating a site for treatment, salvaging fish within the treatment area and treating the area.

Treatment could include the use of chemicals, ultraviolet radiation and ozone, either individually or in combination depending on the risk of the infestation escaping, the Corps' analysis says.

Sites would be isolated using impervious silt curtains or inflatable bladder dams. Fish would be salvaged with electrofishing and netting and transferred to a holding tank before they are released.Under the proposal, the site would be monitored following treatment, and could be treated again up to four times in one year.

Concern about the potential spread of zebra and quagga mussels in the Pacific Northwest--and especially in the Columbia Basin--grew in 2016, when the presence of the mussels were detected in Canyon Ferry and Tiber reservoirs in Montana.

Montana officials are now hopeful that those populations failed. A July 24 news release from Montana Fish, Wildlife and Parks says that no evidence of the mussels was found in Tiber Reservoir in 2018, or this year, after 45 plankton tow samples were collected and analyzed for presence of the invasive species.

Still, Montana is continuing an aggressive effort to prevent a new introduction. State and tribal officials apprised the Northwest Power and Conservation Council of its new laws and asked for additional support for more mandatory inspection and enforcement to prevent the mussels from traveling into the state attached to a vessel.

Fish, Wildlife and Parks reported that the state and its partner agencies have conducted more than 52,000 watercraft inspections so far this year, and have intercepted 13 boats with zebra or quagga mussels. The state has also gathered more than 600 plankton tow samples for early detection analysis, and found no evidence of mussels after processing 522 of those.

The Corps' analysis acknowledges questions about whether the Columbia River and other rivers and lakes in the Pacific Northwest are suitable for mussels. It found that the mussels inhabit a range of depths and temperatures across the continent, and notes some studies have suggested that the Columbia Basin is at low risk of invasion due to its relatively low levels of calcium.

"Other models suggest otherwise," the agency writes. Calcium concentration--a critical component for shell production--has been widely studied, the analysis states. A calcium rate of more than 12 milligrams per liter is generally recognized as a requirement for adult survival and reproduction, and rates in the Columbia Basin range from 8 to 20 mg/L.

The analysis notes that while one study indicates that the Columbia Basin has a relatively low risk of invasion due to calcium levels, lab studies have shown the mussels can be successful with calcium concentrations of between 4 and 8 mg/L, with success rates varying from 10 to 90 percent, suggesting that success rates are highly variable.

Ultimately, the analysis says, invasive species are often able to adapt and invade quickly. Some research has shown that mussels in North American waters thrive in water with much lower calcium levels than in European countries.

The analysis says that a 2004 model "predicted moderate to high invasion risk for the [Columbia Basin] based on a combination of average annual temperature, frost frequency, solar radiation, minimum temperature, bedrock geology, elevation, flow accumulation, slope, and surface geology."

K.C. Mehaffey covers fish issues for Clearing Up, and is editor of the NW Fishletter. She joined the NewsData writing team in February 2018. From lawsuits to scientific studies, she is enjoying the deep dive into the Columbia Basin's many fish topics.