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Eradication of Milfoil from Walsh Lake in Seattle’s Cedar River Municipal Watershed

Josh Wozniak and Rob Zisette, Herrera Environmental Consultants, 2200 Sixth Avenue, Suite 1100, Seattle, Washington 98121; phone: 206-441-9080, email: jwozniak@herrerainc.com

In the late summer of 2005, Herrera Environmental Consultants (Herrera) was tasked by the City of Seattle to follow up on reports of a Eurasian watermilfoil (milfoil) (Myriophyllum spicatum) population that had become established in Walsh Lake, in King County, Washington.  Walsh Lake is located within Seattle’s Cedar River municipal watershed, which has been closed to public access for several decades.  It is a low elevation lake (700 feet) that supports a robust native plant population, including a large floating bog mat.  The discovery of milfoil in this lake was of particular concern because Seattle would like to re-establish the original lake outlet that discharges to the Cedar River at a location upstream of intake to Lake Youngs (where milfoil was eradicated by Herrera in 1996).  Herrera immediately mobilized to conduct a thorough aquatic plant survey of the lake, map the extent of the milfoil population, and develop an eradication plan that was compatible with the municipal watershed restrictions (i.e. no herbicides).  A total of 104 native plant species were identified, including 26 native floating and submerged plant species.  Boat and diver surveys found one established patch of robust milfoil plants (several years old) covering approximately 200 square feet, and several dozen small milfoil plants in outlying areas.  The established milfoil patch was isolated with a turbidity curtain to prevent further spread of milfoil by the removal operation.  Divers pulled 580 pounds of milfoil from the containment and outlying areas.  Herrera will continue survey and removal activities in the summer of 2006 to achieve complete eradication of milfoil from Walsh Lake.