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

In mid-November 2020, a highly observant second grader and her parent reported seeing Egeria densa in Duvall’s Lake Rasmussen. Egeria, also called Brazilian elodea, is a densely growing underwater plant native to Brazil and Argentina that was accidentally introduced to lakes and rivers in Washington. Because of its abundant growth and lack of natural controls, it can overwhelm beneficial native plants, fill in open water areas, and impede fish passage.

Egeria densa in water
Egeria densa is an underwater plant introduced by accident into lakes and rivers in Washington. Photo by Thurston County Noxious Weed Control Board

In 1998, Egeria was found on the Chehalis River. From 1999 to 2014 over 300,000 pounds of Egeria biomass was removed from 35 acres of the Chehalis River (Thurston County Noxious Weed Program).

man holding Egeria plants standing in a river choked with Egeria
Egeria in the Chehalis River. Photo from Thurston County Noxious Weed Control Program.
buckets of Egeria plants on a boat
Many boatloads of Egeria were removed from the Chehalis River over the past 15 years. Photo from Thurston County Noxious Weed Control Program.

In King County, all three large lakes Washington, Sammamish and Union, and the Sammamish River are already heavily infested with Egeria, along with two small lakes in south King County, Dolloff and Fenwick. However, this is the first time Egeria has been found in the Snoqualmie Watershed.

After mapping the locations of the plant in Lake Rasmussen, King County and Duvall staff began developing a plan to eliminate Egeria from the lake and prevent it from spreading downstream to the Snoqualmie River. The King County Noxious Weed Program and the City of Duvall are consulting with the Snoqualmie Tribe, Tulalip Tribes, WRIA 7, Snoqualmie Valley Watershed Improvement District, Sound Salmon Solutions, WDFW, and other King County agencies to ensure a safe, thorough and effective strategy that will result in the eradication of this very harmful plant.

Lake Rasmussen
Lake Rasmussen is a quiet, shallow lake in Duvall that drains into the Snoqualmie River in the winter by way of Cherry Valley. Photo by Ben Peterson.
aerial photo of Lake Rasmussen with dots showing weed locations
Lake Rasmussen in Duvall showing locations of Egeria (green dots) and native Elodea plants (gray dots) in November 2020

Egeria densa is a Class B Noxious Weed according to Washington State’s Noxious Weed List, required to be controlled by State Law RCW 17.10. This weed spreads very quickly, filling the water column with dense vegetation. Once fully established in a lake or river system, Egeria can have devastating impacts on both the health of the water body and its recreational uses. Fishing is ruined since hooks now get snagged every cast, swimming becomes dangerous, boating becomes difficult whether using oars, paddle, or propeller, and fish and other aquatic animals are impacted by decreased oxygen in the water and higher water temperatures. If left alone, Egeria’s growth accumulates sediment and can completely change the habitat and water level of a water body.

Egeria densa stem being held in a hand
Egeria densa is a water plant with whorls of leaves densely covering the stems. Photo by Zach Chupa.
comparison photo of native Elodea and noxious weed Egeria
The native water plant Elodea canadensis on the left looks very similar to the noxious weed Egeria densa on the right. Elodea has smaller leaves, usually with 3 in whorl, and Egeria has larger leaves, with 4-8 in a whorl. Both plants are found in Lake Rasmussen. Photo by Ben Peterson.

It isn’t known how Egeria got to Lake Rasmussen. Although the plant can be carried by wildlife and flowing water, usually it takes human help to get to new areas. In the past, Egeria was sold as an aquarium plant and has been introduced to lakes when people unwittingly released their pet fish. The Washington Invasive Species Council’s Don’t Let it Loose campaign has great information about the problem with releasing pets into the wild. More commonly, aquatic plants move by plant fragment attached to boots, equipment, boats, and trailers as they move between waterbodies.

Egeria stems on an outboard motor
Egeria can spread to new waterways on boats and other equipment; even small fragments can make new plants. Photo by Katie Messick.

To keep Egeria from spreading to new lakes and waterways, visitors to Lake Rasmussen and other infested lakes and rivers should clean off their boots, boats, and dogs before leaving. It just takes a small plant fragment to spread the problem.
Visit the King County Noxious Weeds website for more information on Egeria and other noxious weeds. For more information on aquatic weeds and the Lake Rasmussen project, contact Ben Peterson, ben.peterson@kingcounty.gov.

More information on Egeria densa, also called Brazilian elodea:

Brazilian Elodea – August 2018 Weed of the Month