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May 1, 2020

Herbicide treatment planned for Lake Riley, Mitchell Lake and Red Rock Lake

Treatment will occur on Monday, May 11th.

For the past several years, the District has been monitoring an aquatic invasive species known as curly-leaf pondweed.  This plant originally from Eurasia and Australia, and was most likely introduced at the same time as common carp in the late 1800s.  Curly-leaf is a plant that outcompetes many of our native aquatic plants by growing through the winter months and in essence takes over the space before our native plants can grow.  One method to control curley-leaf pondweed is through early season herbicide treatment.  This treatment is done early attacking the already growing plant and before our native plants start to grow.  Curly-leaf pondweed is also detrimental for water quality in that it starts dying in August which releases phosphorus back into the water column.  And too much phosphorus is unhealthy for our lakes.

The District will be treating 3 lakes this Spring as part of an ecological restoration and water quality improvement program.  Mitchell Lake, Red Rock Lake and Lake Riley will be treated in early May when weather conditions are optimal (60F- low wind conditions).  This will maximize the efficiency of the treatment.  We will be using a herbicide known as diquat.  “There are no restrictions on swimming or eating fish from water bodies treated with diquat. Treated water should not be used for drinking water for one to three days, depending on the concentration used in the treatment. Do not use treated water for pet or livestock drinking water for one day following treatment. The irrigation restriction for food crops is five days, and for ornamental plants or lawn/turf, it varies from one to three days depending on the concentration used.” (WI DNR) 

Following are maps of the three treatment areas.  If you have any questions on this project, please feel free to contact Claire Bleser, 952-607-6512


Treatment Area Maps

Green dots represent locations sampled for curlyleaf pondweed, yellow lines represent treatment areas.

Lake Riley


Mitchell Lake


Red Rock Lake