Colorado Parks and Wildlife officials started refilling Highline Lake on Monday after a long battle with a tiny invader — the zebra mussel — forced the agency to drain it completely in November.
Colorado has worked for years to keep these rapidly reproducing mussels from settling into the state. But in 2022, Highline Lake, northwest of Grand Junction, became the first body of water in Colorado to be classified as infested after state staff found adult mussels. Then in 2024, zebra mussel larvae were found along the Colorado River in Colorado.
Colorado Parks and Wildlife staff said people need to follow inspection rules to prevent further spread, especially as this summer’s boating season begins to ramp up.
“Eradicating an invasive species once it is established can be difficult, which is why we focus so heavily on preventing their introduction,” Maddie Baker, CPW invasive species specialist, said in a news release Tuesday.
A single female zebra mussel can produce 1 million larvae in a year. If left unchecked, adult zebra mussels will lock onto every inch of docks, boat motors, dam intake pipes, buoys and shoreline rocks — potentially causing millions of dollars in damage and clogging up vital water delivery systems for farmers, ranchers and other water users.
Finding them in Highline Lake was “sickening,” state park staff said in 2022.
Colorado Parks and Wildlife fully drained the lake in November 2024 as part of a larger response to the mussel infestation. The lake, part of Highline Lake State Park near Loma, is a popular destination for boating, sport fishing, swimming and camping.
Water managers are refilling Highline Lake using spillover from the Government Highline Canal. Officials are also monitoring the canal for signs of zebra mussels.
The Grand Valley Water Users Association diverts water from the Colorado River into the 55-mile canal to provide irrigation water to more than 23,000 acres of land. Whatever doesn’t get used by irrigators ends up spilling into the lake.
Adult zebra mussels lining a plastic tube. (Colorado Parks and Wildlife, Contributed)The association did not respond to requests for comment Thursday.
The process to refill the lake is expected to take 35 days or more, depending on irrigation use and water supply, Rachael Gonzales, CPW regional spokeswoman, said in an interview Thursday. The lake’s water level was at 0 feet Thursday. At maximum, its elevation is 65 feet.
In August, Colorado invasive species experts confirmed the Colorado River near Grand Junction was positive for zebra mussels after finding larvae on three different dates. They plan to renew testing along the river in April, CPW staff said.
Boating will open up again this summer, but visitors to the lake should be prepared for wait times: Watercraft inspections are required before they enter the water and before they leave the reservoir.
“We are grateful for the support and understanding of our boating and angling community,” Alan Martinez, Highline Lake State Park Manager, said in the Tuesday news release. “It’s great to see water flowing into Highline Lake again, and we look forward to welcoming our water recreationists back for the 2025 boating season.”
The fight against voracious mussels
Zebra mussels are one of the top targets of Colorado’s invasive species program.
The mussels, which have distinctive black-and-white zigzag stripes, strip plankton from the water en masse, crippling the food supply for native species and sport fisheries. They can spread from lake to lake on boat trailers, boat bottoms, angling equipment, rafts, paddle boards — pretty much any hard, underwater surface.
There are already well-established populations in eastern waterways like the Mississippi River and the Lower Arkansas in Kansas and Oklahoma. In 2008, Colorado passed the toughest “mussel free” legislation in the country to try to keep them out of the state.
Colorado Parks and Wildlife manages 72 boat inspection sites. The effort has stopped 281 watercraft with adult zebra or quagga mussels from entering Colorado waters, the agency’s website said.
But the voracious zebra mussels have still slipped the net.
CPW staff found the first adult zebra mussel at Highline Lake on Sept. 14, 2022. In response, state staff lowered the lake’s water level to expose shorelines and treated the lake with EarthTechQZ, an EPA-registered mollusk-killer, three times in March 2023.
It didn’t work. The zebra mussels were still there in October 2023. They closed all motorized boating for the 2024 season for the first time since the 1960s. Staff applied more EarthTechQZ. They realized they had to drain the lake, and by Nov. 18, it was done.
Highline Lake near Loma, Colorado, sits empty in late 2024 for the first time in over 60 years because of a zebra mussel infestation. (Colorado Parks and Wildlife, Contributed)The process cost about $300,000 as of 2023, not counting funding from federal agencies or the Grand Valley Water Users Association, Gonzales said.
CPW will continue to sample Highline Lake weekly for zebra mussels. Even if no evidence of the invader is found, Highline Lake will be classified as “infested” until it has a clean bill of health for five consecutive years.
“We remain hopeful that draining the lake has eradicated zebra mussels,” Robert Walters, Invasive Species Program manager, said in the news release Tuesday.
Recreation is back on this summer
Once the water level returns to normal, CPW will begin restocking Highline Lake. Initially, anglers will be able to catch trout.
Anglers should also keep their gear free of mud and plants between each use, CPW said.
Colorado Parks and Wildlife had not announced the opening date for boating as of Thursday.
Boaters should expect some delays once the boating season starts. Inspections will be required for all motorized watercraft and trailers before they enter the water and before they leave the reservoir.
People with other watercraft, like kayaks and paddleboards, can do their own inspections by feeling the surface for sandpaper-like bumps that are hard to remove, or they can ask for their watercraft to be inspected.
All boats must be clean, drained and dry before launching at Highline Lake, or Highline Lake State Park staff will have to decontaminate them — basically spraying outside surfaces or flushing internal engine areas with 140-degree water that kills the mussels. No soaps or chemicals are used, CPW staff Baker said. All boats must be decontaminated after they leave the lake.
This process can take anywhere from 10 minutes to over an hour depending on the type of boat. Boaters will be issued a green seal and a blue receipt indicating the boat was last used on a body of water with a known aquatic nuisance species.
“By taking the extra time to complete a few simple steps, you can help prevent zebra mussels, or any other invasive species, from spreading to other bodies of water throughout Colorado,” Baker said.
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