Destructive Zebra mussels keep turning up in the Colorado River and Western Slope lakes

The spread of invasive zebra mussels has continued on Colorado’s Western Slope this summer, with additional discoveries made in recent weeks in the Colorado River, Highline Lake, Mack Mesa Lake, and a private body of water in Western Eagle County. 

In May, Colorado Parks and Wildlife ramped up testing for the aquatic nuisance species — known for its rapid reproduction rate and highly destructive impact on water ecosystems and infrastructure — in northwest Colorado’s waterways following the discovery of the species’ free-floating larvae or veligers in the Colorado River last summer. 

Despite increased testing last summer and fall, the agency and its partners were unable to detect any additional zebra mussel adults or veligers in 2024. 

However, this year’s sampling efforts have turned up several zebra mussel veligers around Glenwood Springs, officially marking the river — from its confluence with the Roaring Fork River to the Utah border — as “positive” for the invasive species. 

The positive samples came from stretches of the Colorado River between Glenwood Springs and Silt, as well as in the Grand River Park near New Castle in June. To date, none of its sampling in the river has turned up adult zebra mussels, only veligers, puzzling the agency as to the source of the species. 

While Parks and Wildlife reports that it has not had any positive samples in the Eagle and Roaring Fork rivers, it did have a significant discovery in a private body of water in Eagle County. 

On July 3, the agency found a large number of adult zebra mussels in a private lake, as the agency works with Western Slope businesses and individuals to conduct sampling on private properties.   

Parks and Wildlife declined to comment on whether the body of water connects to the Eagle River or the Colorado River system. In a news release, Jeff Davis, the agency’s director, called the discovery “a significant step toward identifying a potential source of zebra mussels and advancing our efforts for eradication.”

Parks and Wildlife will be working on a plan for eradicating the zebra mussels from the private property. 

However, the species is notorious for being nearly impossible to get rid of once discovered, as evidenced by the mussel’s continued presence in Highline Lake following years of attempts to wipe it out. 

Colorado’s first identification of zebra mussels took place in the Grand Junction area lake in 2022. The lake is fed by the Government Highline Canal and connected to the Colorado River. 

In the years since, Parks and Wildlife has attempted to rid Highline Lake of zebra mussels, including applying molluscicide to the lake, water sampling, cleanup efforts and, ultimately, draining the lake entirely this past fall. 

The lake was refilled this spring, but a June 10 sample from the lake tested positive for zebra mussels. 

“While we had maintained hope that our eradication efforts at Highline Lake would be successful, we have always known this was a lofty goal,” said Robert Walters, Parks and Wildlife’s invasive species program manager, in a news release.

With the latest discovery — and the lake’s status as an infested body of water — the agency will continue watercraft inspection and decontamination of every watercraft entering and exiting the lake. 

The mussels were also discovered at another lake in the Highline Lake State Park, which is also fed by the Government Highline Canal. Parks and Wildlife found several zebra mussel veligers at Mack Mesa Lake in June and July, marking the body of water as “positive” as well. 

As the positive samples stack up, Walters said a critical step to stopping the species’ spread is “understanding the current extent of zebra mussels in western Colorado.”

Water users and boaters also have a role to play in preventing further contamination and spread of the invasive species to more waterways. Parks and Wildlife reminds users to clear, drain and dry all water vessels and gear — including fishing gear, motorized and hand-launched vessels like paddleboards and kayaks — after each use.