Last week, Colorado Parks and Wildlife confirmed the presence of Eurasian watermilfoil (EWM), an invasive plant, in the Boulder Reservoir during their semi-annual invasive species monitoring visit.

If it becomes established, EWM grows quickly, forms thick mats and disrupts aquatic ecosystems. The invasive plant is not harmful to humans.

With this discovery, the city will work closely with Colorado Parks and Wildlife this fall to determine whether the plant has been established in the Reservoir.

The city has watercraft inspection and aquatic nuisance species (ANS) programs in place at the Reservoir to protect water quality and recreation. Despite these robust efforts, EWM may have arrived at the Reservoir via wind, fishing, animals, humans or watercraft. This plant can easily be transmitted without the carrier’s knowledge.

“The extent and location of EWM in the Reservoir is not yet known as just a fragment was discovered,” said Parks and Recreation Director Ali Rhodes. “We’ll work with Colorado Parks and Wildlife, who will conduct a more comprehensive aquatic plant survey this fall to determine the extent of the invasive plant’s growth at the Reservoir.”

If EWM establishes and spreads at the Reservoir, the following services could be impacted:

  • Swimming – Dense plant growth could limit swimming opportunities. As of now, the plant has not been observed in the Reservoir’s swim beach area.
  • Large and small watercraft – EWM can get tangled in boat propellers and engines, which can increase its spread. Dense growth can also make paddle craft use difficult.
  • Adjacent bodies of water – EWM may also spread to nearby Coot Lake which would impact fishing. Since EWM is already present in Boulder Creek and several other bodies of water in Boulder County, the impact to these is minimal.
  • Drinking water – The Reservoir is a water supply jointly managed by the city and Northern Water. EWM can cause taste and odor issues in drinking water and clog water intakes and outlets. The Boulder Reservoir Water Treatment Plant primarily treats water directly from Carter Lake through a pipeline. However, the Reservoir serves as a water supply for the region and a backup water supply for the city, so Northern Water and the city are partnering on planning and future management.

At this time, the city and Northern Water are not planning to make any changes to water supply operations or recreation activities at the Reservoir. The city will continue to run its existing watercraft inspection and decontamination program and implement the following additional precautionary measures to minimize EWM’s spread and educate Reservoir visitors:

  • City staff will continue to inspect large watercraft exiting the Reservoir. Boaters will receive a blue paper receipt indicating that their boat was last inspected at an ANS-positive body of water.
  • City staff will inspect small watercraft exiting the Reservoir, as allowed, and educate visitors about EWM.
  • Watercraft are not allowed on the North shore. Anyone with watercraft or a flotation device in this area and around Coot Lake will be issued a summons with a potential fine.
  • Signage about EWM will be posted around the Reservoir.

The city encourages all visitors who enjoy the water to follow a “Clean, Drain, and Dry” practice to confirm that they are not transporting any ANS, such as EWM, to or from the Reservoir.

With more information from Colorado Parks and Wildlife’s follow-up survey, the city will evaluate EWM mitigation strategies or future adjustments to recreation and water utilities operations.

“Although this invasive plant doesn’t affect public health, because it could impact water quality and recreation activities in the future, we will work closely with Colorado Parks and Wildlife to get more information and keep the community informed about any impacts and changes at the Reservoir,” said Rhodes.

More information will be posted at and