In early June, Colorado Parks and Wildlife (CPW) found a single zebra mussel veliger while sampling the Colorado River near Grand River Park in New Castle.
Zebra mussels are invasive, bivalve filter feeders that multiply en masse. They originated in Eastern Europe and came to the U.S. in shipping vessels traveling the St. Lawrence Seaway. From there, they made their way to the Great Lakes region, and have caused billions of dollars in damage by multiplying on important infrastructure like irrigation and drinking water systems.
Robert Walters is CPW's Invasive Species Program Manager.
"They will grow on any sort of solid surface that can have significant impacts, not only on our recreational opportunity but also on our use of water, including uses such as hydroelectric production as well as agriculture," he said.
He said they also pose a threat to native species in the Colorado River, including endangered fish.
"From a more ecological perspective, they are also a filter feeder," he said. "A single muscle can filter up to one liter of water per day. When they are filtering the water, they are filtering out the phytoplankton and potentially the zooplankton, which are the basis of our aquatic food web."
The zebra mussel veliger found near New Castle is the furthest upstream this species has been detected on the Colorado River.
CPW found an adult zebra mussel in Highline Lake near Grand Junction in 2022, and last summer, the agency detected veligers (the microscopic larval stage of bivalves) in the Colorado River and canals near the city.
Walters said that the main question CPW is trying to answer is, 'where are the zebra mussels and their veligers coming from?' The sample near New Castle, while disappointing, does provide more evidence for researchers.
"That essentially reduces the probability of the origin of these things in a 40-mile stretch of the river," he said. "So that allows us to really focus in a little more tightly on the areas upstream of New Castle as a potential source for the veligers."
Walters said CPW plans to take samples even further upstream this summer on the Colorado River, from Glenwood Canyon up to Granby Dam in Grand County. He said because there's no effective way to eradicate mussels once they're present, CPW is focusing on early detection to prevent their spread.
"Colorado has one of the largest watercraft inspection and decontamination programs in the nation," he said. "We run over 70 inspection programs around the state, where we have mandatory inspection requirements for trailered and motorized boats. We inspect around 500,000 of them each and every year and decontaminate somewhere around 30,000."
Walters said in addition to increased water sampling and these inspection and decontamination stations, CPW is focusing heavily on education to prevent humans from spreading zebra mussels.
The agency has been targeting vessels used for recreation, encouraging people to clean, drain, and dry their equipment after using, including waders, paddleboards, and boats. However, veligers can be transported through any kind of human equipment or infrastructure.
"Anything that has been in the water somewhere else has the potential to be moving around aquatic nuisance species," Walters said. "Things like boat docks, boat lifts, pumps, barges, heavy equipment — anything that's coming in contact with the water has the potential to move these things around. So even if somebody's out there doing a water-based construction project or some sort of work in or near one of our waterways, we really encourage people to make sure all of that equipment is clean and dry."
Other invasive species, like quagga mussels, have been found further downstream in the Colorado River Basin, particularly in Lake Powell and Lake Mead.
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