Eurasian watermilfoil (EWM) is an invasive aquatic plant that creates canopy-like structures as it grows towards the surface of the water, shading out other native plants and reducing habitat and food sources for fish, insects and other aquatic animals.
EWM looks very similar to a native species (Northern Watermilfoil) that is beneficial for the environment and a part of the Boulder Reservoir ecosystem.
There is no way to know how the plant got into the Boulder Reservoir. EWM can reproduce and spread from just one fragment that has been introduced into a body of water, and can be easily transported by humans, dogs, wind, watercrafts or connected waterways. The plant is already in other waterbodies within Boulder County. It was first detected in Boulder Reservoir in 2022, and the city has continued to monitor and manage its presence.
Boulder Reservoir is an important water supply and recreational resource. The plant has spread significantly in the western coves and swim beach, impacting recreational uses such as swimming and boating. EWM forms thick underwater mats, which can entangle propellers, oars and paddles making it difficult for watercraft to navigate. It can also be dangerous to swimmers as the dense vegetation poses a risk of entanglement.
Without management, EWM could lead to water quality and water supply impacts. Potential impacts include taste and odor in drinking water, clogging of the 63rd Street water treatment plant intake or Northern Water’s supply canal for downstream water users and/or reduced water storage capacity due to plant biomass.
Impacts to aquatic life could include alteration to fish habitats, species composition changes or die-offs. EWM can alter nutrient cycling in aquatic ecosystems. Its decay can release nutrients back into the water column, potentially causing algal blooms.
Northern Water and the city are working to build on existing hand removal in designated swim areas, by using broad-scale harvesting EWM removal across the reservoir. The partners evaluated the use of chemical treatment (herbicides) but have paused pursuing that approach due to implementation challenges associated with the reservoir’s primary function as a regional water supply. Large-scale harvesting will likely be conducted primarily through Diver Assisted Suction Harvesting (DASH) to reduce EWM biomass, limit it’s spread, and maintain safe recreational access in the reservoir.
The city will take to continue to manage the established EWM by:
Continuing the existing watercraft inspection and decontamination program, which includes inspecting large watercraft, and small watercraft as allowed, entering and exiting the Reservoir.
Encouraging all visitors to follow a “Clean, Drain, and Dry (PDF)” practice to confirm that they are not transporting EWM to or from the Reservoir.
Educating visitors about EWM, including those recreating on the North Shore.
Maintaining the ban on possessing or launching watercraft from all areas other than the designated South Shore recreation area. Anyone with watercraft or a flotation device in these areas including around Coot Lake will be issued a summons with a potential fine.
Maintaining the closure on the west side of Boulder Reservoir
Conducting routine vegetation surveys and partnering with CPW to assess any changes in EWM.
Colorado Parks and Wildlife and other communities primarily rely on herbicide treatments to control EWM in recreational areas across the state. Some communities also use EWM-eating weevils, and some employ manual harvesting.
Northern Water and city staff evaluated a suite of management options and determined that in the near-term, herbicide will be the most effective at reducing EWM and limiting further growth. Weevils are not currently available, and manual harvesting is not a sustainable approach for the density and extent of EWM in the Reservoir. Other approaches, such as Reservoir drawdown and bottom barriers, would require longer term planning and may prove infeasible do to expense or other constraints.
There are no other documented aquatic nuisance or invasive plant species in the Reservoir.
EWM is established in waterbodies throughout Colorado and in Boulder County. EWM has been detected in Boulder Creek, downstream from Boulder Reservoir, ponds on the University of Colorado Boulder campus, the Panama Reservoir, Sawhill Ponds, Legget Ditch, Whiterock Ditch and in Lower Boulder Ditch.
We encourage all visitors to the Reservoir to follow a “Clean, Drain, and Dry (PDF)” practice to confirm they are not transporting EWM to or from the Reservoir.
These practices should be used by visitors to any waterbody to avoid potentially transporting and spreading any type of aquatic nuisance species.
Preventing introduction is critical – that’s why the city has such a robust watercraft inspection program at Boulder Reservoir. ALL Watercraft are only allowed entry through the southern main entrance. All small and large watercraft are inspected for aquatic nuisance species and decontaminated or quarantined if needed. Staff also educate visitors about the Reservoir and aquatic nuisance species to prevent introduction.
Reminder: 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.
