For the fourth year running, quagga mussels have turned up in the Snake River near Twin Falls, but state officials say the infestation is showing signs of decline as Idaho prepares a new round of chemical treatment aimed at wiping out the invasive species for good.
As of August 14, state inspectors had counted eight quagga mussel larvae and seven adult mussels in water samples from the area. That represents a 77% drop compared to detections from the previous year, a figure the Idaho State Department of Agriculture views as encouraging but not sufficient reason to back off.
“Our outlook is that not treating this infestation is not an option,” Sydney Kennedy of the Idaho State Department of Agriculture told the Idaho Capital Sun.
The state’s treatment plan is set to begin this week, with an expected wrap date of September 13. The approach involves two rounds of copper-based chemical treatments, and state officials have applied for a special local exemption to use an algaecide called Cutrine Plus. One new element this year is the use of scuba divers to deliver targeted doses of copper chemicals directly into smaller pools and rock fissures where mussels tend to take hold.
The treatment carries real consequences for river life. Copper-based chemicals kill fish and other aquatic species along with the mussels. In 2023, after a treatment using a chemical called Natrix, the state removed six or seven tons of dead fish from the river. State officials have acknowledged that conducting a treatment at this scale on a moving river has never been attempted before.
The stakes for doing nothing are also significant. Quagga mussels reproduce rapidly and are notorious for clogging irrigation infrastructure and drinking water pipes. More than 1,700 users draw water from the Snake River, including municipal systems that supply drinking water to communities throughout the region. Idaho Department of Agriculture first detected quagga mussel larvae in the Snake River in September 2023, and the infested stretch of river has shrunk in the years since, though the mussels have not been eliminated.
Josh Johnson of the Idaho Conservation League framed the cost-benefit calculation plainly, as first reported by the Idaho Capital Sun: “Prevention, however much it costs, is going to cost a whole lot less and be a whole lot less difficult than actually dealing with quagga-infested waters.”
Quagga mussels have devastated water systems in other parts of the western United States, particularly in Lake Mead and the Colorado River system, where removal costs have run into the hundreds of millions of dollars. Idaho has been working aggressively to prevent that outcome on the Snake River, and the 77% reduction in detected mussels suggests the earlier treatments have had some measurable effect.
Water availability along the Snake River has already drawn significant attention this season. Idaho came close to water cutoffs for Snake River trust users earlier this year due to severe drought conditions, and the prospect of quagga mussels compromising water delivery infrastructure adds another layer of concern for the communities and agricultural operations that depend on the river.
The two-step copper treatment process and the use of divers to reach harder-to-access areas represents a more refined strategy than prior years. Whether the combination is enough to bring detections to zero remains to be seen, but the trajectory heading into this year’s treatment window is the most favorable the state has seen since the problem was first identified.
State agriculture officials have not indicated what their plan would be if this year’s treatment fails to eliminate the remaining mussels, but Kennedy’s statement makes clear that ongoing intervention is the only path the department is willing to accept.
Results from this season’s treatment effort are expected by mid-September.
North Idaho Republican Staff