The agency and partners have taken weekly water samples and worked to educate river users

Despite taking around 350 additional samples along the Colorado River, Colorado Parks and Wildlife officials have yet to find any further discoveries of zebra mussels in the state.

The wildlife agency discovered evidence of several zebra mussel veliger, the species' free-floating larvae, in early July in the Colorado River near Grand Junction and the Government Highline Canal. No adult zebra mussels were found.

"As of this time, we haven't found any additional adults or zebra mussel veligers since July, which is really good news, but it also sort of confounds us a bit in terms of discovering the cause and source of the original discovery," said Rob Harris, the water resources section manager for Parks and Wildlife, at a Colorado Water Conservation Board meeting on Wednesday, Sept. 18.

The invasive species, which is known for its rapid reproduction rate, pose "extreme risk of ecological impacts to Colorado," according to Parks and Wildlife.

Their presence in the Colorado River and other waterways can have negative impacts on fish populations since they can kill off "essential prey items" including plankton. Once adults age, zebra mussels can also clog water infrastructure.

The July discovery prompted Parks and Wildlife to deploy an Invasive Species Rapid Response Plan.

Working with the Bureau of Reclamation, U.S. Fish and Wildlife Service, United States Geological Survey and local water users, Harris said the wildlife agency has collected around 350 samples from Glenwood to the Utah border. This included weekly sampling between De Beque and Grand Junction and two multi-day rafting trips between Glenwood and Grand Junction.

As these samples found no additional zebra mussel veligers or adults, Parks and Wildlife is "not that much closer to discovering the causes here, but we'll keep looking and provide any updates as we have them," he added.

Paula Stepp, executive director of Middle Colorado Watershed Council, told the Garfield County Board of County Commissioners on Monday that based on information from Parks and Wildlife, eventually, the region "will probably see upstream movement of those zebra mussels."

"The concern is what that means to our public water intakes and our agricultural community, because once those are established, then we're looking at a long-term detrimental impact," Stepp said.

In addition to testing, Parks and Wildlife and its partners have spent time and resources on education and outreach. This included 3,800 face-to-face contacts with recreators for voluntary inspections and education of Colorado River boat launches in the Grand Junction area, a Bureau of Land Management advertising campaign, posting informational signs at river access areas and surveying boat users.

The goal of surveying is "to better understand what kind of water craft are going into the river -- where they're coming from, where they're going and to better understand what kind of risk this presents," Harris said.

Part of Parks and Wildlife's outreach effort includes asking individuals boating, floating, paddling or fishing in the Colorado River to clean, drain and dry their vessels and equipment after exiting the river. This includes motorized boats, rafts, paddle boards, kayaks and fishing gear.

Zebra mussels were first found in Colorado in 2022 at the Highline Lake near Grand Junction. This lake is fed by the Government Highline Canal. Parks and Wildlife has been working to stop the spread of the species by treating the water and slowly draining the lake.