For four years, Idaho has taken an aggressive approach to rid the Snake River of harmful invasive mussels.
State and federal crews have monitored constantly. Applied large amounts of molluscicides, a specialized pesticide that kills mollusks. Targeted and adapted their rapid response.
Now, they’ve found 77% fewer larval quagga mussels, known as veligers, compared to last year.
“This level of containment is unprecedented among known quagga mussel infestations. No other state has a proven playbook for how to accomplish this work. Idaho is charting new territory with our eradication effort,” said Chanel Tewalt, director of the Idaho State Department of Agriculture.
Make no mistake, there’s still a long way to go, said Nic Zurfluh, the department’s bureau chief overseeing the Invasive Species, Noxious Weeds, & Range programs.
“ We're in this for the long haul,” Zurfluh said, after almost a month spent treating the infested area of the Snake. “This species is extremely challenging, and other states have had challenges with it before. I believe eradication is possible, and we're moving towards that.”
The quagga quagmire
Once detected, quagga mussels have never been eradicated before. These fingernail-sized mussels reproduce rapidly and spread just as fast. In rivers, they’ll float downstream and eventually settle at the river bottom.
They can cause millions of dollars of damage each year, clogging irrigation pipes and filters. They can also change ecosystems. In the Northwest, they could significantly harm hydropower systems.
Idaho has spent millions so far in an effort to eradicate quagga mussels in the Snake River, but leaders say it could cost much more if the mussels blossomed out of control. In 2023, the state allocated over $2.3 million towards mussel eradication. In 2026, Idaho’s budget for treatment was over $3.5 million.
The concern is: the farther these mollusks make it downstream, the more damage they’ll do. They could eventually end up in the Columbia River if steps aren’t taken to slow their spread. According to the USGS Western Fisheries Research Center, the Northwest is “the last untouched frontier within the continental United States.”
So how has Idaho been so successful at pushing back the quagga mussel spread?
It starts with monitoring, Zurfluh said.
From tows to dives
As planktonic creatures, veligers are invisible to the naked eye. To get an estimated count, they use a plankton net, specific for invasive mussels. The holes in the net are 64 microns in diameter, or about two-thirds the width of a strand of hair.
“ Anything that's smaller than that falls through the net. Anything that's larger, including all plankton, will be captured … in the bottom of the net,” Zurfluh said.
Crews monitor statewide, with an intensive emphasis on the mid-Snake, Zurfluh said. They’ll collect 3,500 plankton tow samples in Idaho, with 500 or more coming from the quagga treatment zone, he said.
They’ll start upstream of the infested area to make sure no new quagga veligers are coming from somewhere else. They also sample for quagga environmental-DNA above the treatment area. So far, upstream sampling has remained clean, Zurfluh said.
Then, crews make their way downstream, collecting three tows per sample across the river’s current. When they find a plume of veligers, that indicates an adult colony is close by, he said.
At that point, scuba divers search for any adult quagga mussels. As of Aug. 14, dive crews found and removed seven adult quagga mussels.
“What really changed this year is we're getting to know this section of river really well,” Zurfluh said. “We've performed several dives this summer, before any veligers were detected.”
Because they found the adults early, they were able to start treatment even sooner this year, he said, before the mussels could spawn.
Next, a microscopy lab tests the samples for any quagga veligers.
“Under a cross-polarized light microscope filter, (quagga veligers) will show up with a distinct ‘x’ pattern,” Zurfluh said.
The lab further identifies the quagga veligers based on shell structure and size, he said.
For backup, another scientist will verify all initial detections, he said. They also have genetics labs test samples.
Treating the infestation
To treat the areas, crews use multiple types of copper treatments, or molluscicides, that work in different ways. Chelated copper, which binds copper to keep it stable in water, hangs down in the water column and gets to the bottom of holes, Zurfluh said. Copper sulfate moves around in the water column differently.
They put each product in for 96 hours at a time. Then, they move downstream and do it all again.
Crews are aiming for 1 part-per-million for copper floating in the water column, which is safe for people.
They also use a granulated copper formula placed inside mesh bags. Divers bring the bags to underwater caverns that are challenging to treat.
“We're getting more specific with treatment areas and a smaller footprint with our treatment areas. Overall, utilizing less copper, less product, within the treatment area, and we're just being a lot more strategic with the placement and the timing,” Zurfluh said.
Treatment challenges
The part of the Snake River Canyon they’re treating is complicated, he said. The water depth can change. There are holes, trenches and rock ledges.
Deep holes and trenches don’t mix well with the flowing river water. So, crews mount underwater boom sprayers onto rock mantles and use a pressurized pump to inject the molluscicide into the depths, Zurfluh said.
In shallow bays, the copper treatments can bind with heavy sediments, he said, which means it's no longer able to kill the mussels.
In addition, freshwater entering the treatment area can dilute the treatments, he said. Typically, that comes from irrigation returns or spring inflows.
So, he said, the team is adapting its treatment strategy each year.
“It’s real-time adaptive management and really pivoting off of what is the data telling us about where the quagga mussels are, about where and how long we're getting effective copper levels,” Zurfluh said.
However, the fewer mussels there are, the harder they will be to detect, he said. Zurfluh compared it to pulling weeds in a garden.
“ The first year you buy a new property, you set up a garden, and you've got 100 weeds, and you pull weeds, and 20 come back,” he said. “Well, you can see those 20, and you spend a lot of time and resources. And three come back. You stay on it. Well, one comes back. It gets harder to find the last one.”
Zurfluh said the biggest thing they’ve learned is that this level of work takes a team. For this rapid response, federal and state resources have had to aggressively treat and monitor the mid-Snake River. They have to treat and monitor the area consistently.
“It takes an army,” he said.
Full eradication would require at least five years of no detections. But Zurfluh said he thinks Idaho can get there one day.
