For the first time, the City of Boulder plans to use herbicide at Boulder Reservoir to control Eurasian watermilfoil, an invasive species. This plant displaces native aquatic plants and poses risks to swimmers and boat motors through entanglement. It can also clog waterways, affecting water flow and the odor and taste of drinking water. The growth of Eurasian watermilfoil has been rampant in parts of the country for decades.
Eurasian watermilfoil was first detected in the reservoir in the summer of 2022 and has since taken over the western coves. The city and Northern Water, which share management of Boulder Reservoir, are now looking to hire a company to apply herbicide before Eurasian watermilfoil threatens drinking water infrastructure, they said.
“Our first priority is ensuring that the milfoil does not impact our ability to deliver drinking water downstream,” reservoir manager Stacy Cole told Boulder Reporting Lab.
Boulder Reservoir is vital to the region’s water management system. Unincorporated Boulder County and Weld County get drinking and irrigation water from the reservoir through the Boulder Creek supply canal. Jeff Stahla, Northern Water’s public information officer, told Boulder Reporting Lab that the spread of Eurasian watermilfoil could reduce the canal’s capacity and require intensive labor to remove.
On top of that, the reservoir houses the 63rd Street water treatment plant on-site. The plant treats water delivered directly from Carter Lake through a dedicated pipeline, which then supplies some customers in the City of Boulder. Water is generally not drawn from Boulder Reservoir into the treatment plant, except when other city water sources are unavailable, typically during maintenance. But it remains a vital backup water resource that must be accessible.
“If left unchecked, Eurasian watermilfoil could … adversely impact water quality and ecosystem health and threaten regional water supply,” said Michael Lawlor, the urban water quality program coordinator for the City of Boulder, during a water advisory board meeting.
Eurasian watermilfoil, like other invasive species, spreads when people or animals inadvertently transfer plant fragments from infested water bodies to clean ones. These fragments take root, grow and disperse seeds into the underwater soil.
According to Cole, the reservoir manager, Eurasian watermilfoil has rapidly spread at Boulder Reservoir over the past year and a half. She watched the plant transform from sporadic patches into dense mats covering most of the western side near 51st Street. The growth is also spreading to the northern areas of the reservoir.
“This plant is around 50% of my job,” Cole said.
Parts of the western reservoir have been closed to motor watercraft to prevent disturbing the milfoil and spreading it. The city also conducts manual removal once every one to two weeks to maintain swim areas. In Colorado, the plant is a List B noxious weed species, requiring a management plan to control its spread.
After consulting with an environmental expert, the city and Northern Water decided to use the herbicide triclopyr, which is recommended by the Colorado Department of Agriculture for controlling Eurasian watermilfoil. Stahla told Boulder Reporting Lab that triclopyr breaks down in sunlight and is certified for use in water sources. It is used across the country to control Eurasian watermilfoil and has been shown to be effective at small doses.
Northern Water and the city have submitted a request for proposal to hire a triclopyr applicator. “We don’t have a date, we don’t have a contractor selected,” Stahla said. “This is the announcement that the process has started.”
Once an applicator is selected, they will propose a management plan detailing the concentration, timing, frequency and location of herbicide application.
Cole acknowledges the use of herbicide is worrisome for many people and mentioned that there have been extensive discussions among reservoir managers about its use. “The herbicide we’ve chosen to look at for application is, in our opinion, the least impactful while still providing the results we’re looking for,” she said.
According to the National Pesticide Information Center, a collaboration between scientists at Oregon State University and the U.S. Environmental Protection Agency, triclopyr is “practically non-toxic” to birds, fish and bees. In humans, triclopyr has low toxicity when inhaled, ingested or absorbed through the skin, although its salt form can cause permanent eye damage. The EPA classifies triclopyr as a Group D carcinogen, indicating insufficient evidence to determine whether it causes cancer in humans.
Cole noted that while herbicide is already used to combat noxious weeds in the reservoir’s feeder canal, this will be the first time any herbicide is applied directly to the reservoir.
“In this case, the risks of letting Eurasian watermilfoil spread and further impact reservoir use outweigh the risk of targeted herbicide on an as-needed basis,” Cole added.
Even with the herbicide, eradication of Eurasian watermilfoil from Boulder Reservoir is “basically impossible,” she said. “So the outcome is not to eradicate but to get rid of as much of it as we can to create a better situation for us to manage moving forward.”
Manual removal of Eurasian watermilfoil is labor-intensive. Cole described the process: raking the shallows to disturb the milfoil, collecting the loose plants and those washed ashore and loading them in barrels for disposal.
Other proposed management methods include lowering the reservoir’s water level to expose the milfoil in shallow water and installing permanent bottom barriers. But the feasibility of these approaches needs to be evaluated.
As part of Boulder Reservoir’s aquatic nuisance species prevention program, all watercraft — from motorboats to paddleboards — must have a permit and be inspected before entering the reservoir. The program, in place since 2009 to prevent quagga and zebra mussels, now also includes inspections of all outgoing craft to prevent the spread of Eurasian watermilfoil. Visitors are urged to clean shoes, equipment, watercraft and pets to prevent Eurasian watermilfoil from spreading to other bodies of water.
Elsewhere in Colorado, the highly invasive and destructive zebra mussel has been identified in the Colorado River near Grand Junction. Zebra mussels harm ecosystems by killing native plankton and can impact water infrastructure when their clusters clog canals. Colorado Parks and Wildlife are asking Colorado River recreationists to clean, dry and drain all vessels and equipment to prevent zebra mussels from entering other bodies of water. The species was first identified in Colorado in 2022.