Water managers and state wildlife officials last year hoped the discovery of a microscopic zebra mussel larva in the Colorado River was a one-time event, not a sign of a larger problem lurking beneath the surface.
It was the first time larvae from the destructive invasive species had been found in the river in Colorado. For nearly a year, despite increased sampling, state wildlife officials didn’t see any more evidence of the mussels.
But their hopes were dashed earlier this month when Colorado Parks and Wildlife detected three more tiny larvae in the stretch of the Colorado River between Glenwood Springs and Silt. The mussels — known to devastate ecosystems and clog critical infrastructure — had once again found their way to the river that is the backbone of Colorado and the Southwest’s water supply.
“We were all hoping against hope that it was an isolated incident,” said Tina Bergonzini, the general manager of the Grand Valley Water Users Association, based in Grand Junction, which manages a Mesa County irrigation system that relies on the Colorado River. “It is scary, from a water management standpoint, when you have something that could affect delivery and have ramifications for our entire community. It’s a scary thought.”
With the discovery of additional larvae this summer, the Colorado River from Glenwood Springs to the Utah border is now considered positive for zebra mussels. The river can shed that designation only once routine testing confirms a lack of zebra mussel larvae for five continuous years. CPW has beefed up its sampling and lab staff to catch any additional larvae — called veligers — quickly.
The invasive species destroys aquatic ecosystems, causes millions of dollars in damage to infrastructure like dams and irrigation pipes, and reproduces at an incredible rate.
Once established, experts said, zebra mussels are nearly impossible to eradicate.
David Strayer, a freshwater ecologist at the Cary Institute of Ecosystems Studies who has studied mussels for decades, said he didn’t know of an example where zebra mussels were eradicated from a river system once adult populations had established themselves.
“They have the potential to radically change the ecosystem,” he said.
The spread of mussels
The threat of zebra mussels has always lurked over Colorado’s borders.
The mussels — about the size of a fingernail once mature — are native to Eastern Europe and first appeared in the United States in the Great Lakes in the 1980s. The species has since established itself in all of the Great Lakes, in all large eastern river systems and in 33 states. Just 150 miles east of the Colorado state line, Kansas’ Cedar Bluff Reservoir has hosted a zebra mussel infestation since 2016.
Quagga mussels — an equally destructive relative of the zebra mussel — have established populations downstream on the Colorado River in the system’s two major reservoirs: Lakes Powell and Mead.
Mussels and their larvae spread in two ways: By floating downstream or when they are transported by people from an infected body of water on boats, boots and bouys.
Veligers are microscopic and a single quart of water can contain hundreds, Strayer said. Each year, a mature female mussel can release up to one million eggs.
Federal and state agencies for decades have fought to keep the mussels from the West’s waterways, but the species has been detected in California, Utah and Colorado. The species failed to establish itself in Utah but survived in California.
In Colorado, CPW has detected veligers in Grand Lake and in Pueblo Reservoir, but the species did not establish sustained populations.
The state’s first adult mussel was found in 2022 in Highline Lake, northwest of Grand Junction. In 2023, CPW treated the lake with a pesticide, but mussels were found again a few months later. In 2024, the agency drained the lake completely to kill off the mussels.
But just weeks after the lake was refilled this spring and despite strict decontamination protocols for visitors, samplers found more mussels — and, for the first time, they also found some in neighboring Mack Mesa Lake.
CPW officials have not yet decided what the next steps are for the two lakes, said Robert Walters, CPW’s invasive species program manager.
The discovery of additional veligers in the Colorado River has prompted CPW to bulk up its sampling and testing staff. The agency dedicated a team of three technicians based in Grand Junction to sample the river and doubled the size of its Aquatic Nuisance Species laboratory so that samples could be processed more quickly. It also dedicated staff members from its Denver office to sample the river all the way from the Granby Dam to the mouth of Glenwood Canyon.
The river is now being tested weekly, as are two of its tributaries, the Eagle and Roaring Fork rivers.
At any given time, CPW could dedicate up to 12 staff members to zebra mussel detection, Walters said. In 2024, CPW collected 275 samples from the river for testing. Since mid-April this year, CPW has already collected 279 samples.
The U.S. Bureau of Reclamation, too, is on high alert.
The federal agency owns irrigation canals in Mesa County and has increased testing in those systems, said Ethan Scott, the lands and recreation division manager from Reclamation’s Western Colorado Area Office.
“There’s definitely a concern that if they’re getting in our river, it won’t be hard for them to move to lakes and reservoirs from there,” he said.
Federal and state officials, as well as water managers and ecologists, are urging everyone who recreates or works in rivers and lakes to take steps to kill any mussel larvae that may be stuck on them or their equipment. They should drain, wash and dry all equipment and keep an eye out for adult mussels, which often have black and white stripes.
“If everyone is doing this, we have a pretty good chance of stopping this from spreading farther than it has,” Walters said.
‘Almost everything transformed’
Once established, zebra mussels filter huge quantities of plankton and other organic matter from the water — eliminating food sources for other species.
In New York’s Hudson River system, which Strayer studied, the invasive mussels filtered the river’s entire water supply every day, halved the amount of fish food available, shrank fish populations, reduced oxygen levels in the water, changed the river’s chemistry and decimated the native mussel population.
“Almost everything we measured about the river changed,” he said. “Almost everything transformed.”
Outside of mass ecological change, the mussels can wreak havoc on the valves, pumps and pipes that make up irrigation systems and dams. Adult mussels attach themselves to hard surfaces in incredible densities — up to 1,000 per square foot. They can constrict water flow in pipes and jam moving parts.
As general manager of the Grand Valley Water Users Association, Bergonzini is tasked with running an irrigation system that delivers water to 23,000 acres of land. That includes the Government Highline Canal, where CPW detected veligers last year.
Adult mussels could quickly and easily clog the irrigation system’s 150 miles of pipes as well as the smaller tubes farmers use to drip water directly on crops, like the region’s famed Palisade peaches. The pipes and tubes are meant to conserve water by replacing open ditches and reducing evaporation.
But they are an Achilles’ heel in a mussels infestation, Bergonzini said.
Adult mussels could also clog the association’s fish screen, which keeps fish — including endangered species — from getting trapped in the system’s canals, instead returning them to the river.
The association paid $80,000 to treat the entire system with an ionized copper solution at the end of the last irrigation season and will likely do so again, Bergonzini said
“It’s something that we’re going to have to work with our water users to raise the money for,” she said. “And that’s just for the prevention — it’ll be even more if we end up having adult populations and have to mitigate throughout the year.”
Similar treatment is not possible in the Colorado River itself. There’s just too much water, said Strayer, the ecologist.
“You would need a line of rail cars to dump the substance in the river,” he said.
Bergonzini urged Coloradans and visitors to be vigilant when they work or play in the state’s waters.
“There’s a mindset that they’re already here, but that’s incredibly short-sighted,” she said. “We all need to look at the communities and recreation we have — and realize that all of that could be affected by people’s unwillingness to help stop the spread of this invasive species.”
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