What is EWM?

Eurasian Watermilfoil (EWM) is an invasive aquatic plant that grows densely, shading out other native plants and reducing habitat and food sources for fish, insects, and other aquatic animals. EWM impacts swimming, boating and other recreation and could impact water quality and water supply.

EWM Management FAQS - updated May 2026

The plant was first detected in Boulder Reservoir in 2022 and was already detected in other Boulder County waterbodies at that time. There is no way to know exactly how EWM arrived. It can reproduce from just one fragment and can be transported by humans, dogs, wind, watercraft or connected waterways.

The City of Boulder and Northern Water are using a combination of hand removal in shallow areas and, starting in 2026, mechanical harvesting and Diver-Assisted Suction Harvesting (DASH) in deeper areas of Boulder Reservoir. Our team also performs ongoing plant biomass surveys and mapping inform management strategies and priority areas.

Mechanical Harvesting and Diver-Assisted Suction Harvesting (DASH) strategies:

  • Mechanical harvesting reduces the volume of plant matter, maintaining recreation access and safety.
  • DASH removes EWM at the roots, reducing the population over time.

Visitors will notice Boulder's new DASH boat, which is a pontoon that is specially equipped with suction tubes for a diver to use under the water. The boat slowly moves around the reservoir as a diver harvests the plant material underwater. We kindly ask to keep a careful eye out for a dive flag this season and give our DASH teams space!

Eco-Cycle supported the city in launching an on-site composting program in 2025. Removed EWM biomass is combined with goose waste to eliminate regrowth potential, reduce landfill disposal and produce nutrient-rich compost for future use on turfgrass at Boulder Reservoir.

To help reduce the spread of invasive species, community members are encouraged to use the Clean, Drain, Dry approach with your watercrafts and gear.

We expect that several years of intensive management will reduce the amount of EWM in the reservoir, meaning fewer recreational impacts and avoided water supply impacts. However, we don’t expect to ever fully remove EWM from Boulder Reservoir.

Colorado Parks and Wildlife and other communities primarily rely on herbicide treatments to control EWM, although other communities employ manual harvesting. City of Boulder and Northern Water evaluated herbicide treatments in 2025 and, for multiple reasons, decided to continue to use non-chemical methods to manage EWM.