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Boats and paddleboards are inspected upon entry and exit Thursday at the Boulder Reservoir. Eurasian watermilfoil, Zebra mussels, and Quagga mussels are among the monitored invasive species. (Olivia Sun, The Colorado Sun via Report for America)

A tiny piece of a fast-growing plant floating in the Boulder Reservoir has heightened concerns among wildlife officials about the invasive species that’s known to disrupt aquatic ecosystems and affect drinking water quality. It’s the first new detection since 2020 of the noxious aquatic weed that the state has been working to control for more than 15 years. 

If Eurasian watermilfoil starts to grow roots in the reservoir, the plant’s green, feather-like strands can grow up to 2 inches a day, reaching upward of 21 feet. As it creeps toward the surface of water, it can form dense canopy-like structures casting deep consequences for the aquatic ecosystem, boaters, anglers and swimmers.

The fragment was found in Boulder Reservoir during Colorado Parks and Wildlife’s semi-annual monitoring visit late last month. A team will conduct a survey this fall, when water levels are lower, to determine if the plant has rooted, said Kate Dunlap, Boulder’s water quality project manager. 

A Colorado Parks and Wildlife sign warns residents of Eurasian watermilfoil Thursday at the Boulder Reservoir. A fragment of the invasive plant was found in August. (Olivia Sun, The Colorado Sun via Report for America)

Though the plant isn’t new to Colorado — it was first identified in 2005 and has since been established in more than 50 reservoirs, ponds and lakes — the recent detection raises the stakes for anyone on the water who can transport the plant unknowingly from one lake to another. A small fragment of the plant, carried by the wind, dropped from a bird’s beak, or clinging to a kayak’s hull, could put an entire reservoir at risk. 

And once the invasive aquatic weed is introduced, it’s nearly impossible to get rid of.

“We’re hoping that if it is established, we have caught this early so that we can manage it at a small scale,” Dunlap said. Next month, she and Colorado Parks and Wildlife will throw a hook over a boat to capture pieces of plants rooted in the reservoir and then analyze them. If they find Eurasian watermilfoil and it is very sparse, they can pull out the roots. 

“But it really does depend on where it is and to what extent,” she said.

Eurasian watermilfoil is one of eight nuisance species identified by Colorado that pose a significant threat to the state’s aquatic resources. It’s illegal to release, plant or transport any of the species, including invasive mussels, clams and snails.

The state has prioritized controlling Eurasian watermilfoil largely because of its rapid growth, Dunlap said. The plant can quickly become tangled, forming dense mats that block the sun from reaching native plants below it. That threatens the habitat for fish, insects and other aquatic life, she said. 

Boat inspector Sequoia Mitra places a wire boat tag onto Arthur ‘Art’ Vandelay’s MasterCraft ski boat upon inspection at the Boulder Reservoir. (Olivia Sun, The Colorado Sun via Report for America)
Arthur ‘Art’ Vandelay shows a receipt following an inspection at Boulder Reservoir. Eurasian watermilfoil, Zebra mussels, and Quagga mussels are among the monitored invasive species.

It can also limit recreational opportunities on the reservoir. The plant can become tangled in boat propellers and engines, and dense plant growth could limit swimming opportunities. 


The plant can also clog drinking water intakes, leading to more maintenance. As the plant starts to decompose, it can affect the taste and smell of drinking water, she said. 

The Boulder Reservoir is an important part of the regional water supply, underscoring the importance of protecting the water of nonnative, invasive species, Dunlap said. The city of Boulder primarily treats water from Carter Lake for the city’s water supply, but Boulder Reservoir can be used as a backup during any maintenance on the Carter Lake pipeline.

The more contact people have with the plant boosts the chances it can spread from one body of water to another. Eurasian watermilfoil has already taken root in several areas that are popular for paddleboarding and boating, including Saint Vrain State Park ponds, Chatfield Reservoir and Eleven Mile State Park along the Front Range, according to data from Colorado Parks and Wildlife.

In this 2019 photo, Eurasian watermilfoil is seen in the Pueblo Reservoir. (Photo provided by Colorado Parks and Wildlife)

“Public education really is a big component of our program because as people understand the ‘why’ they’re more likely to follow the rules in terms of only bringing watercraft through the southern entrance (to Boulder Reservoir) so that it can be properly inspected,” Dunlap said. 

The last time Eurasian watermilfoil was detected by CPW was nearly two years ago at Beckwith Reservoir in Colorado City, said Robert Walters, the invasive species program manager for the agency. While gas-operated motors are prohibited, the reservoir is a popular place for those on kayaks, canoes and pedal boats. 

While its spread isn’t aggressive, the fragment found in Boulder highlights the need for proper decontamination and inspection of watercraft after leaving a body of water, Walters said. 

For 13 years, Boulder’s parks and recreation staff has been inspecting every watercraft that leaves Boulder Reservoir to help prevent the spread of nonnative aquatic species. Boaters are limited to where they enter the water — the north shore is off limits — and violators can be issued a summons and fined. In the summer, staff line the north shore to educate visitors about aquatic nuisance species and wildlife in the area.

Paddleboarders and kayakers enjoy the Boulder Reservoir on Thursday. (Olivia Sun, The Colorado Sun via Report for America)

Colorado Parks and Wildlife has agents inspecting and decontaminating boats at 73 locations across the state before they enter and exit Colorado’s waterways. Since the program started in 2008, almost 500,000 boats have been inspected annually. 

The department encourages all visitors to clean all parts of their water equipment and gear, drain any item that could hold water and allow time for drying before going into another body of water.

“The more waters that become positive in Colorado for the species, the more opportunity there is for people that are coming in contact with the water to interact with the species,” Walters said. “Just due to that increased level of contact, there is an increased level of risk of it spreading into new locations.”

Olivia Prentzel is a general assignment writer for The Colorado Sun. Email: