Idaho has ended its 2026 copper treatment for quagga mussels in the Snake River and lifted all related access restrictions after monitoring showed a 77% reduction in detections near Twin Falls. The Idaho State Department of Agriculture said the operation ran from Aug. 27 through Sept. 16, with the final public avoidance advisory ending Sept. 18.
That means there are no active river closures tied to the treatment. The state’s precautionary request to keep people, livestock and pets out of the stretch between Hansen Bridge and the Highway 46 Bridge has expired. Temporary closures at the Twin Falls and Shoshone Falls boat launches also ended, according to the state’s 2026 treatment plan and access guidance.
What the treatment involved
The operation used three copper-based products—Natrix, EarthTec QZ and Cutrine Plus Granular—at two target locations. Crews placed totes on barges and used deep-hole injection equipment to reach potential mussel refuges. Granular product was also deployed in mesh bags at targeted fissures for 96 hours before removal. Idaho officials said the application covered less surface area than earlier operations because this year’s approach was more precisely targeted.
The Agriculture Department said all three products are labeled and approved by the Environmental Protection Agency for this type of aquatic use. Application rates were designed to kill mussels while remaining below the human drinking-water standard, and the state said there were no public drinking-water intakes near the treatment area. Agriculture and Idaho Department of Environmental Quality staff monitored copper levels and dissipation. Officials reported minimal mortality among fish, aquatic plants and algae from Sage View downstream to the Highway 46 Bridge.
Why the 77% decline matters
The decline is measured against 2025 detections of microscopic quagga mussel larvae, or veligers; it does not mean the infestation has been declared eradicated. In an Aug. 18 sampling update, the department said it had collected more than 420 samples in the infested area and more than 3,000 statewide during 2026. The state plans continued monitoring as it evaluates whether surviving mussels remain in deep holes, freshwater inputs or other hard-to-treat areas.
The stakes extend well beyond recreation around Twin Falls. More than 1,700 users depend on Snake River water from Twin Falls to Lewiston, including farms, irrigation districts, cities and hydropower facilities. State officials estimate that allowing the mussels to establish could require about $69 million a year in treatment product for downstream systems alone. Quagga mussels reproduce rapidly and can clog intake pipes and other water-delivery infrastructure.
Boaters and anglers still play a role even though the treatment closure has ended. Idaho’s invasive-species program says quagga mussels can hitchhike in watercraft, ballast tanks and equipment that contacts infested water. Cleaning, draining and drying boats and gear—and complying with inspection requirements—remains important to prevent movement from the known Mid-Snake River area to other Idaho waters.
For residents, the immediate change is straightforward: normal access has resumed, but surveillance and eradication work continue. A 77% year-over-year drop is evidence of progress, not an all-clear on the invasive species itself.
