Highline Lake was drained because of invasive mussels. Water — and vigilance — returns this summer.
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LOMA, Colo. — Right now, water is flowing into Highline Lake for the first time in four months. Colorado Parks and Wildlife drained the lake, a sprawling centerpiece in the state park of the same name in western Colorado, last fall as part of the plan to eradicate an invasive mussel.
Zebra mussels, the invasive species, were found in the lake during routine sampling in 2022, threatening to contaminate the entire state.
That mollusk is an aquatic nuisance species. Colorado is surrounded by states dealing with infestations of zebra and quagga mussels. The animals are filter feeders, and effectively remove plankton from the food chain, disrupting lake ecosystems from the bottom up.
Here’s what happened.
A bad day
On September 14, 2022, Highline Lake State Park manager Alan Martinez received a phone call he had been working to avoid for more than a decade. “I still remember that phone call I got from Robert Walters, who's the head of our [invasive species program,]” said Martinez, who has worked for Colorado Parks and Wildlife for 36 years, starting his career at Highline and eventually coming back to manage it.
Walters told Martinez that sampling crews had found a zebra mussel on the bottom of the lake. Thanks to legislation from 2008, park officials have been able to inspect boats coming into Colorado for aquatic nuisance species, largely keeping the state’s lakes and rivers free of contamination.
The reservoir closes to boating every year at the end of September. In 2022, as staff removed docks and other equipment for their seasonal closure, they found more of the zebra mussels — signs of a full infestation.
“That was the first adult mussel ever found in Colorado,” said Madeline Baker, an invasive species specialist who started working for CPW six days before crews discovered the mussels at Highline.
CPW doesn’t know where the zebra mussel came from.
What are zebra mussels anyway?
The small mollusks are native to freshwater in Europe and Asia, and likely arrived in the U.S. in the 1980s, setting up shop in the Great Lakes after being discharged in ballast water from large ships that traveled across the Atlantic.
Zebra mussels, and their cousin, the quagga mussel, attach to hard surfaces such as boats, underwater pipes, or buoys. They can clog water intakes at municipal and agricultural infrastructure, and are expensive to remove.
Invasive mussels have been found in Lake Powell, and farther west in the Snake River in Idaho. Microscopic zebra mussel larvae, known as veligers, were found in the Colorado River last year. CPW is looking for the source of those juvenile mussels.
Unprecedented problem, experimental solution
Baker worked on the plan to deal with the mussels. The creatures colonize the top 25 vertical feet of a body of water. That fact, and an effort to maintain the remaining fish population at the lake, lead to the first attempt at killing the zebra mussels.
“We drained about 25, 30 vertical feet, in the hopes of exposing as many mussels as possible to dry conditions, freezing conditions, outside of the water,” said Baker.
That was in March of 2023. The lake remained open to boating with containment protocols in place. Every boat entering and exiting the lake received a wash with hot water.
“That hot water is enough to kill these invasive species. So, every single boat was decontaminated on its way out [to the water],” said Baker.
Another part of the plan: thousands of gallons of molluscicide, a chemical that targets the zebra mussels.
“Our first approach with the lighter handed treatment of four parts per million, we're really trying to find that middle ground where we had a chance at eradicating the mussels but still allowing the fishery to persist,” said Ben Felt, a senior aquatic biologist with CPW, of the largemouth bass, bluegill, and crappie in the lake.
The mussels stuck around after the first treatment of the chemical.
“We changed gears a little bit and then went to this two pronged approach of doubling up on the concentration for the fall treatment and then ultimately draining the lake to give ourselves the best chance at removing the mussels,” said Felt.
The lake was empty by the end of 2024. Biologists, technicians, Colorado Mesa University students and others scoured the shoreline, docks, and breakwaters for the mussels. They can’t survive long outside of the water, but Baker still scans pieces of concrete as she walks along the empty lake, hypervigilant for the invasive species.
Highline summer
Water will hit the silty, sandy lakebed the first week of April. When full, Highline Lake holds roughly 3,400 acre feet of water. An acre-foot is equivalent to 326,000 gallons.
It will take one to two months to refill, after that, visitors will hear the refrain “clean, drain, dry,” as CPW tries to keep invasive species out of all of Colorado’s water.
Baker says all water users, from anglers to paddle boarders, to people with ski boats, can help keep out invasive species by cleaning equipment, draining it and letting it dry, before traveling to another body of water.
Parks and wildlife will increase sampling at the lake, upstream, and downstream to keep watch for the microscopic, juvenile life stage of the mussel, called veligers.
“We also perform shoreline surveys where we check infrastructure, shore rocks, hard surfaces, anywhere we believe that adult mussels can be attaching,” she said.
Previously, sampling crews traveled from Denver to conduct tests on the Western Slope, but because of the infestation at Highline, and the importance of stopping it from happening again, a dedicated crew will be based in the region.
“Last summer was unusual, not to have full parking areas […] we’re ready to have people back and enjoying the park for what it’s here for,” said Martinez.
He says about 80% of visitors to Highline are from Colorado, and half of those are locals in Mesa County. Over 2,000 boats launch on the lake every year. According to the CPW website, the agency has intercepted 281 contaminated boats throughout the state, likely preventing other mussel infestations.
Highline will be listed as an infested body of water until it records five consecutive mussel-free years.
Type of story: Explainer
Provides context or background, definition and detail on a specific topic.
Provides context or background, definition and detail on a specific topic.
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