Lake Rasmussen’s new aquatic noxious weed gets speedy action – July 21 community open house to discuss control plans

When second grader Maggie Carosino noticed a strange plant in in the shallow water of her neighborhood lake last fall, she gave the lake a fighting chance. By notifying state and county officials, Maggie and her mother, Reagan, allowed noxious weed experts and city staff to move quickly, hopefully in time to prevent Egeria from spreading into the Snoqualmie River. As we covered in an earlier post, the population in Duvall’s Lake Rasmussen was the first known occurrence of the Class B Noxious Weed Egeria densa in the Snoqualmie Watershed.

On July 21, King County and Duvall are holding a community open house at Lake Rasmussen Park from 6:30 to 7:30 p.m. Project leads will explain the impacts of Egeria, how people can prevent spreading it further, and the scope of work to eradicate it from the lake. The park access is at the end of 4th Avenue NE and NE Cherry Street in Duvall. For more information, contact Ben Peterson, King County Noxious Weed Control Program, 206-477-4724; or Alana McCoy, City of Duvall, 425-939-8045.

Young person holding a stem of Egeria densa in their hand
Maggie discovered invasive Egeria growing in Lake Rasmussen and alerted Ecology and King County (with a little help from her mom). Photo by Reagan Carosino.
Closeup of young person holding a stem of Egeria densa in their hands
Maggie holding an Egeria stem she found in Lake Rasmussen. Photo by Reagan Carosino.

Egeria, also called Brazilian elodea, becomes so abundant underwater in our climate that it can fill the entire water column. Egeria’s dense growth leaves little room for fish or beneficial vegetation, significantly reducing the fish habitat and recreational value of a lake or river.

To get a sense of how bad it can get, visit the Sammamish River, Lake Union, or Lake Fenwick in King County, and areas of the Chehalis River in Thurston County. Once Egeria gets established, you can’t really get rid of it. Instead, you are stuck with costly ongoing maintenance and damage control.

Because of the potential devastating impact to fish habitat in the Snoqualmie Watershed, Maggie’s discovery set off alarm bells for fish biologists and organizations working hard to preserve and restore fish habitat in Cherry Creek and the Snoqualmie River area.

King County Noxious Weed Control Program and the City of Duvall Public Works Department acted quickly on Maggie’s report with lake and outlet stream surveys in the fall of 2020 and spring of this year. Fortunately, their surveys showed the Egeria in Lake Rasmussen is still relatively sparse. And perhaps even more important, it appears that the plants hadn’t yet spread downstream to Cherry Creek or the Snoqualmie River, both highly important waterways for salmon habitat, tribes, fishing, and recreation.

People at the outlet to Lake Rasmussen
Ben Peterson, King County Aquatic Weed Specialist, checks outlet of Lake Rasmussen with lakeside homeowner Brady Allard. Photo by Sasha Shaw.
Two people searching for Egeria in the outlet stream of Lake Rasmussen
Alana McCoy, City of Duvall Project Manager, and Matt Martin, King County Noxious Weed Specialist, inspect Lake Rasmussen’s outlet stream for Egeria. Photo by Ben Peterson.
Person with a sampling rake inspects outlet of Lake Rasmussen for Egeria
King County Noxious Weed Specialist Matt Martin inspects outlet for Egeria. Photo by Ben Peterson.

It isn’t known how Egeria got to Lake Rasmussen, but this plant is often spread by small fragments carried on boats or from fish aquariums emptied into lakes. To keep Egeria from spreading, visitors to Lake Rasmussen and other infested areas should clean off their boots, boats, and dogs before leaving.

Because the Egeria is still limited in the watershed, aquatic plant experts with King County and the State of Washington believe that there is a small window of opportunity to eradicate it from the lake and prevent it from spreading downstream.

After consulting with tribes, fish biologists, state agencies, conservation organizations, lakeside residents, and local stakeholders, King County and Duvall developed a rapid response plan to treat the Egeria this summer, and if needed, again next year.

Map of Lake Rasmussen showing sample locations with Egeria present
Map of Lake Rasmussen showing sample locations with Egeria present (the 4 red dots) and absent (the green dots).
Map of the creek downstream of Lake Rasmussen showing areas surveyed for Egeria in March and May 2021
Map of the creek downstream of Lake Rasmussen showing areas surveyed for Egeria in March and May 2021.

King County has received approval to control the Egeria, a Class B Noxious Weed in Washington, through Ecology’s Aquatic Plant and Algae Management General Permit. The permit allows a licensed aquatic weed control contractor to treat the lake with a safe herbicide to hopefully eradicate the Egeria plants. The herbicide applicator will treat half the lake at a time, and the treatment dates are scheduled for Aug. 3 and Aug. 18.

Signs will be posted around the lake and adjacent landowners will be notified two days before each treatment. Diquat is a contact herbicide that is highly effective on Egeria but has less impact on native lake vegetation.

Three plants, from left to right: Egeria (Egeria densa), common waterweed (Elodea canadensis), Coontail (Ceratophyllum demersum)
The introduced species Egeria is larger and grows more densely than the similar aquatic plants that are native to Lake Rasmussen. From left to right: the noxious weed Egeria (Egeria densa), the native look-alike common waterweed (Elodea canadensis), and another similar native plant Coontail (Ceratophyllum demersum).

Egeria plants will show effects within a week of treatment. Egeria has shallow roots and is unlikely to grow back following the treatment. The native plants in the lake will be temporarily affected by the treatment but will grow back in following years due to their deeper root systems. There are no fishing or swimming restrictions associated with diquat herbicide, and the chemical does not move into the groundwater. There are restrictions for irrigation and drinking that last up to five days. Although the lake water is not known to be used for these purposes, the lake will be closed with signage posted to educate the public.

The City of Duvall will receive funding from the Cooperative Watershed Management Grant Program to help pay for half of the Egeria treatments. Funding for the other half of the cost will come from the King County Noxious Weed Control Program.

Person fishing on the shore of Lake Rasmussen
Lake Rasmussen is mostly undeveloped with easy access for the public to visit, fish, and generally enjoy the lake’s natural features.

More Information:

City of Duvall Lake Rasmussen Park

Egeria Identification and Control, King County, Washington

Washington State Department of Ecology Permit Information

Duvall and King County plan rapid response to new aquatic weed found upstream of the Snoqualmie River



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