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.
Once introduced, the plant generally cannot be eradicated. We are implementing numerous management actions, including watercraft exit inspections, posting signs, closing coves to boating, vegetation surveys and hand-pulling in the swim beach area.
Northern Water and the city hired a subject-matter expert in aquatic nuisance species to recommend management options and develop an adaptive management plan, which includes using a targeted herbicide within the next year to manage EWM’s spread and minimize its impacts. In the coming months, Northern Water and the city will work with a licensed herbicide contractor to create a targeted herbicide application plan to inform how to move forward with treatments on an as-needed basis.
The selected herbicide will be triclopyr, because it is effective and has the fewest impacts to water quality. It is already used by Northern Water in areas adjoining the Boulder Feeder Canal upstream of the Reservoir.
Once Northern Water and the city have a final application plan, they will evaluate and track triclopyr’s effectiveness on EWM to determine future, targeted applications, with the goal of reducing its use over time. The city will continue to pull EWM by hand near the swim area and near water intakes and outlets to protect public safety, water operations, and recreation use. The city and Northern Water will continue to evaluate long-term strategies that limit the use of herbicide.
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.
Given the amount of EWM growth in the Reservoir, our consultants recommend herbicide as the most effective method to get the plant to a level that will allow the use of other non-herbicide methods to assist with the ongoing management.
The city aims to avoid herbicide use and scrutinizes any proposed herbicide use carefully according to our Integrated Pest Management (IPM) plan. However, sometimes it’s the best choice to mitigate impacts like the recreation and water supply impacts at Boulder Reservoir. In this case, the risks of letting EWM spread and further impact reservoir use outweigh the risk of targeted herbicide application on an as-needed basis.
We hope to use herbicides to manage the plant in the short term, and then over time increasingly use other non-chemical strategies to minimize impacts from this plant.
Triclopyr herbicide is extremely effective at controlling EWM and typically provides multiple years of control. Triclopyr as an active ingredient is already on the city’s approved pesticide list for land applications and was approved through the city's IPM program for aquatic use to control EWM specifically in Boulder Reservoir.
Pesticides on the city's approved list have been assessed through a review of regulatory data, as well as peer-reviewed literature to determine safety and effectiveness.
Triclopyr is a systemic herbicide that will kill the plant roots and provide improved long-term control. Many of the other herbicides on the city’s approved list are contact herbicides that would only kill the EWM shoot without killing the roots. After EWM is killed by the herbicide, it sinks to the bottom and decays. Triclopyr photodegrades relatively quickly in water, typically breaking down within a few days of application.
Staff evaluated other herbicides that are effective at controlling EWM, such as fluridone or procellaCOR. However, these herbicides contain per- and polyfluoroalkyl substances (PFAS), chemicals that are known to have human health impacts and that the city will not allow to be intentionally introduced in Boulder Reservoir.
One to three years of large-scale treatment with triclopyr should reduce the need for ongoing treatment. It will reduce fragments and provide longer control compared to other approaches. Northern Water and the city will evaluate its effectiveness and continue to evaluate other long-term strategies as part of the adaptive management plan.
Northern Water and the city will work together to hire a licensed applicator who will determine exact dosage and timing based on our guidelines and surveys of the plant. The first treatment is planned for spring 2025, though this may be re-evaluated pending the conditions of EWM in the Reservoir. Northern Water, the city, and the licensed applicator will work together to address any potential water use restrictions during or after application.
Risks to water quality, recreation and irrigation are minimal and can be managed through operational protocols, such as established concentration thresholds, application areas and timing, to prevent public health concerns. In general, application of triclopyr does not require any recreation restrictions. Drinking water quality can be maintained by following application protocols. As the application plan is developed, the city will update community members on any restrictions during and after application.
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.
