Threats facing Minnesota lakes

Many lakes in Minnesota have elevated levels of phosphorus caused by contaminated runoff, erosion, and sediment. Some lakes are also polluted with chloride, which comes from road salt and water softeners.

Contaminated runoff, erosion, and sediment

Aerial view of brown sediment flowing into lake near shore

Runoff from agricultural land and lakeshore development raises the amount of phosphorus in Minnesota lakes, which in turn causes algae to grow. Algae-covered lakes are less attractive for fishing and swimming — highly valued pastimes in Minnesota and uses that are protected under the federal Clean Water Act. In addition, phosphorus can fuel toxic blue-green algal blooms, which are harmful to people and pets.

A green river runs under a white bridge and creates a green waterfall

In 2013, intense algal blooms on southern Minnesota’s Lake Crystal resulted in Minneopa Creek’s waterfall at Minneopa State Park flowing green.

Too much salt

MnDOT truck spreading road saltThe salt applied to roads, parking lots, and sidewalks during our icy winters contains chloride, a water pollutant. When snow and ice melts, the salt goes with it, washing into our lakes. It takes only one teaspoon of road salt to permanently pollute five gallons of water. Lakes that receive municipal wastewater can also have elevated chloride levels from water softener salt. Once in the water, there is no way to remove the chloride, and at high concentrations, chloride can harm fish and plant life. The issue is particularly acute in the metro areas, where there is more pavement and therefore more winter salting.

Invasive species

Invasive species, such as zebra mussels, Eurasian watermilfoil, and Asian carp, are not native to Minnesota and can cause economic or environmental damage or harm human health. Invasive species have taken hold in many Minnesota lakes. The Minnesota Department of Natural Resources is working to curb the spread of these plants and animals. Learn more on the DNR website.