Eurasian Watermilfoil

Eurasian Watermilfoil (EWM) is perhaps the most dangerous unwanted aquatic weed because it is extremely aggressive. Its dense weed beds grow rapidly, choke native plants and spread easily to new areas. In the northwest, EWM currently flourishes in Idaho, Washington, and British Columbia. In Montana EWM is found in Broadwater, Flathead, Gallatin, Jefferson, Lake, Sanders, and Valley counties. On some stretches of the Pend Oreille River in Washington, navigation and recreation have become virtually impossible due to EWM.

Eurasian Watermilfoil. Photo courtesy Kate Wilson


EWM reproduces successfully and very rapidly by a process called “fragmentation.” The plants easily break into small pieces and each piece can form roots. It can easily spread between lakes and rivers by boaters unwittingly carrying plant fragments in their hulls and on their trailers. If left untreated, EWM forms dense mats of vegetation on the surface of the water that can interfere with recreational activities such as fishing, swimming, and boating, and that threatens the health of the water body. The resulting effect can be the loss of recreational use, decline in ecosystem health, and a decrease in lakefront property values.




The Beaver Lake Story
In October 2011, Eurasian Watermilfoil (EWM) was discovered by the Montana Department of Natural Resources and Conservation near the boat ramp on Beaver Lake near Whitefish in Northwestern Montana. Beaver Lake is hydrologically connected to Whitefish Lake and there are numerous methods for the plant to be spread from Beaver Lake to Whitefish Lake. An AIS response team responded to the discovery for further investigation. Bottom barriers were placed over the identified patch and a control/eradication plan was developed by a multiple agency workgroup in which the City of Whitefish and the Whitefish Lake Institute participated.

Whitefish Lake Staff working at Beaver Lake. Photo courtesy Whitefish Lake Institute

Since 2012, WLI and the City of Whitefish have taken the lead in addressing the EWM issue at Beaver Lake. As part of the Whitefish AIS Management Plan, WLI coordinated a suction dredging operation to eradicate plants. In 2012, 23.5 pounds of EWM were removed.

The program has proven successful and in 2016, <0.25 pound of EWM was removed from 5 plants hidden under a submerged tree. This atypical AIS success story is the result of very early detection coupled with rapid and aggressive eradication techniques. Because of the real threat to Whitefish Lake and the watershed, suction dredging will continue indefinitely until there is confidence that the EWM has been eradicated.



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