Boulder to deploy divers to remove invasive Eurasian watermilfoil at reservoir after herbicide plan stalls

The City of Boulder is escalating efforts to contain an invasive aquatic plant at Boulder Reservoir, pivoting to a labor-intensive removal strategy after a planned herbicide treatment fell through. Officials warn the plant’s spread could threaten recreation and drinking water infrastructure. 

Eurasian watermilfoil was first detected in the reservoir in 2022. Since then, it has taken over western coves, extending 10 to 20 feet from the shoreline, according to reservoir manager Stacy Cole. The plant displaces native species, can tangle swimmers and boat motors, and can clog water infrastructure. It may also affect the odor and taste of drinking water. 

While Boulder does not currently draw drinking water directly from the reservoir, it remains a backup supply that must be maintained. The reservoir also provides irrigation and drinking water for parts of unincorporated Boulder County.  

The city and Northern Water, which share management of the reservoir, had previously planned to apply herbicide to control the milfoil but ultimately abandoned the effort.

“We created a plan and after a lot of conversation of trying to figure out how to make it work, we came to the conclusion that it was just a lot,” Cole told Boulder Reporting Lab. A central concern was Northern Water’s commitment to guarantee that no chemicals enter downstream drinking water supplies. 

In the absence of herbicide, seasonal staff have relied on manual removal along the shoreline, supplemented with occasional dredging. About 577 55-gallon barrels of milfoil were removed in 2025, roughly equivalent to three tanker trucks.

The plant’s spread appears to have slowed since the summer of 2024. “It’s already spread to most of the places that it feels comfortable,” Cole said. Still, officials warn the plant could expand further if left unmanaged and could see explosive growth. 

Milfoil also poses a growing risk to recreation. “Eurasian milfoil is another added pressure,” said Parks and Recreation Director Ali Rhodes, referring to the strain on one of Boulder’s only places for water recreation. “Nowhere else in our community can you go out with a stand-up paddle. Nowhere else can you sit on the beach,” Rhodes said. She noted that the reservoir is the city’s only public body of water. 

If Eurasian watermilfoil begins to drive visitors away, it could reduce revenue at the reservoir, adding strain to a Parks and Recreation budget already facing potential service cuts in 2027. 

City officials now believe manual removal, once considered too costly, is a more feasible option. The new approach centers on a DASH program, short for diver-assisted suction harvesting. Boulder has contracted with a Wisconsin-based company, Aquatic Plant Management, to develop the plan. 

Under the program, teams of divers will remove milfoil directly from the reservoir floor. The material will be suctioned onto a boat, where workers will separate out debris before processing the plant. Removed milfoil will be bagged and combined with goose waste to create compost, which will be reused as fertilizer on reservoir grounds. 

“The goal is to hit hard for 2026 and really try to see significant management of the milfoil to lessen our needs moving forward,” Cole said. 

The DASH program will target popular recreation areas, including the swim beach, to limit impacts on swimmers and paddleboarders. Divers will also remove milfoil near the inlet to the treatment plant, Meghan Wilson, water quality senior manager, told the Water Resources Advisory Board in March. While DASH has been used in other parts of the country, its use at Boulder Reservoir would be the first in Colorado, according to city staff. 

Funding for the effort will come from multiple sources, including a $300,000 grant to the Colorado Youth Corps Association to help transport plant material. A $14,000 composting contract with Eco-Cycle is funded by the city’s water utilities, while Boulder Parks and Recreation will contribute $50,000. Northern Water is expected to cover a larger share of the remaining costs. 

The reservoir will continue its aquatic nuisance program, requiring inspection of all boats entering and leaving to prevent further spread of milfoil. 

Cole said the work is not expected to disrupt recreation, though visitors will likely see crews operating on the water as the project begins this spring. 

“We will be notifying the public when these operations will be happening,” she said. The city also plans to recruit certified divers in the coming months to support the effort.