This summer, Colorado Parks and Wildlife (CPW) has ramped up testing and treatment of Colorado waters to prevent the further spread of zebra mussels in rivers and lakes.
Between April and mid-August, the agency collected some 318 water samples from different spots along the Colorado River, plus 58 samples from the Eagle River and 26 samples from the Roaring Fork River. Four of the Colorado River samples contained zebra mussel veligers, which are free-swimming larvae and the last juvenile stage of the mollusks. No adult zebra mussels were detected in any of the water samples.
Zebra mussels are an aggressive aquatic nuisance species that is invasive to North America. The species has been disrupting U.S. ecosystems since being discovered in the Midwest at Lake St. Clair in 1988, spreading to 33 states in the intervening decades. Zebra mussels were first noticed in the western states in Lake Mead in 2007. Colorado waters remained uncontaminated by the species until 2022, when zebra mussels were identified in Highline Lake near the Utah border.
Over the past three years, CPW officials have been monitoring Western Slope waters closely in efforts to halt the spread of the invasive species. Adult zebra mussels attach themselves in dense groups to water storage, treatment and distribution infrastructure. This type of spread can clog and destroy water systems, wreaking havoc on agricultural, municipal and industrial resources. Though the Colorado Front Range faces issues with other invasive aquatic species not present in Western Slope watersheds, CPW has named mussels as the most serious threat to Colorado waters.
Zebra mussels are resilient and have spread primarily by attaching to trailered watercraft that wasn't sufficiently cleaned after use in contaminated bodies of water. The species can also attach to other hard surfaces exposed to water, including fishing waders, bait buckets, live wells and other transportable equipment. They can survive for up to 30 days without water. The larvae are microscopic and cannot be seen by the naked eye.
The contaminated Highline Lake, near Loma, Colorado, was treated with a copper-based molluscicide in 2022 and then drained in 2024 to combat the infestation. Upon refilling the lake, the water once again tested positive for zebra mussels, and the Grand Valley Water Users Association stated in July that they are looking into ways to repeat the copper treatment. Though expensive, it is a preferable option to removing and replacing infrastructure that would be destroyed if the mussels are not eradicated.
Also in July, CPW announced that through the extensive water testing, its teams identified the probable source of zebra mussel contamination: a privately-owned lake in western Eagle County. The single lake is being viewed as the source which has contaminated not only Highline Lake, but the Government Highline Canal, Mack Mesa Lake and the western stretches of the Colorado River.
In a mid-August press release, CPW invasive species program manager Robert Walters shared updates about the situation and a treatment plan for the private lake, the name of which has not been released to the public. "We are grateful for the continued collaboration with the private landowner," Walters said. "They understand the dangers zebra mussels pose to our environment and water infrastructure."
The lake has been treated with EarthTec QZ, the EPA-registered, copper-based molluscicide used on Highline Lake. Following this initial treatment, CPW staff will routinely monitor the water to evaluate its effectiveness. "Eradicating invasive mussels is extremely difficult," explained Walters, "which is why we focus so heavily on preventing their introduction." CPW plans to continue extensive testing of adjacent waters with the goal of catching and preventing further infestation.
CPW is working with local partners around the region to provide resources to stop the spread. Two Rivers Park in Glenwood Springs has a new watercraft cleaning station to help combat the invasive species. The agency also continues to improve and expand information resources. In addition to revamping signs posted near popular water access points, the CPW website is regularly updated with information on mussels and other aquatic nuisance species, as well as locations for cleaning boating and fishing gear.
CPW asks any recreationists who see evidence of zebra mussels or other invasive species to send photos and location details to Invasive.Species@state.co.us for identification.
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