Quaggas found again in the Snake River - now what?
Blog updated on September 19, 2025 to reflect new informationOn September 12, 2025, the Idaho State Department of Agriculture (ISDA) confirmed that quagga mussel veligers — the microscopic larval form of this invasive species — were once again detected in the Mid-Snake River near Shoshone Falls. This is the latest chapter in the saga that began with the initial detection of quaggas in September 2023, which subsequently led ISDA to undertake the most ambitious and complex invasive mussel eradication effort ever attempted in North America.
As we have written about many times, this is a no-win situation. Quagga mussels may be tiny, but their impacts are massive. If quaggas are allowed to take hold in the Mid-Snake, there is no doubt that they would significantly disrupt the aquatic ecosystem as well as trigger widespread economic impacts by clogging irrigation systems, disrupting hydropower facilities, and other infrastructure related impacts. Quaggas disrupt food webs, crowd out native species, and can bioaccumulate pollutants up the food chain. All told, the financial toll of quagga mussels is estimated in the hundreds of millions of dollars of direct and indirect costs, and the long-term toll on the ecosystem would be equally as significant. And if quaggas were then to spread to the entire Snake and Columbia River systems, it would be truly catastrophic. However, the eradication efforts being implemented by the State are also quite impactful from an environmental standpoint. The copper-based aquatic pesticide used to eradicate the quaggas mussels also kills pretty much every living thing in the affected stretch of river. A U.S. Geological Survey study published this summer showed that the 2023 application of the aquatic pesticide killed most of the invertebrate life that fish rely on for food in that section of the river and substantially altered the base of the food web. The study also concluded that while copper concentrations in the water column dissipated pretty quickly, significant amounts of copper settled in the sediments at the bottom or the river, which could have harmful impacts to aquatic life for years to come.
What we know about the latest detection
The good news is that ISDA’s extensive quagga sampling program (over 450 tests this year in the Mid-Snake River) shows that the impacted area has shrunk to 3.5 river miles, down from 7.2 miles in 2024.That means areas like the popular Centennial Park boat launch in Twin Falls are no longer considered to be actively infested by quaggas. According to ISDA, the success of prior treatment efforts have limited the extent of the infestation to just two isolated adult populations in the reservoirs behind the Twin Falls Dam and the Shoshone Falls Dam. In addition, the veliger count per tow sample (essentially the “density” of veligers in the water) has decreased significantly compared to 2024. Overall, this represents significant progress compared to the extent of the infestation the past two years, but full eradication has not yet been achieved (and is not guaranteed).
What’s next?
ISDA has announced plans for another round of “treatment” later this fall — the third in as many years. Although details aren’t finalized, it will involve more of the Natrix copper pesticide used in the prior two large-scale applications that cost a combined $3.6 million according to prior reporting by the Idaho Capital Sun. In particular, this year’s treatment will target the potential “refugia” areas within the river—e.g. deep underwater pools, areas where springs flow into the river—with special methods to ensure that 100% of the river is brought up to a sufficient copper concentration to effectively kill off any adult mussels that could be living there. ISDA has said that their full treatment plan will be made available to the public soon, and treatment is likely to start in the first week of October.
ICL recognizes the extraordinary work by state agencies to keep quaggas from taking hold in Idaho. At the same time, we must have a broader conversation about the future of these efforts. If quaggas continue to be found year after year in the Snake, are we okay with continuing to poison the river on an annual basis? We also want to emphasize that each occurrence of invasive mussels warrants site-specific consideration. For example, what has been done here on the Mid-Snake would not be appropriate for, say, the Salmon River. ICL will continue to monitor Idaho’s quagga eradication efforts to ensure that their response to quagga mussels is effective without causing undue and avoidable environmental harm in the process.
Prevention is the best defense
Whether or not this year’s treatment succeeds, one thing is clear: prevention is critical. We cannot afford to let quaggas spread to other waterways in our state.That means Idaho needs stronger measures, such as:
- Mandatory boat check and decontamination stations at all popular boat ramps statewide
- Consideration of more extensive temporary boating restrictions in the Mid-Snake
- Regular evaluation of prevention programs by ISDA, the Governor, and the Legislature
As the old saying goes: an ounce of prevention is worth a pound of cure. In the case of quaggas, prevention may be worth millions.For more information and ongoing updates regarding quagga mussels in the Snake River, visit ISDA’s official site at https://invasivespecies.idaho.gov/quagga-mussels.