The prize for the lake in the metro area with poorest water quality goes to Meuwissen Lake, west of Chaska.
The prizes for the cleanest lakes are shared in a 10-way tie, most of them in Washington County.
The Metropolitan Council recently released the top 10 best and worst lakes, according to the monitoring done in 2017. The rankings do not include all lakes — just the 180 lakes that the Met Council tracks.
Brian Johnson, environmental scientist for the Met Council, said the ratings are based on years of monitoring by volunteers in the council’s Citizen-Assisted Monitoring Program.
A lake’s water quality, he said, is like a report card for everything happening around it.
If a lake is clean, it means that nothing washing into the lake is polluting it. The cleanest lakes, said Johnson, are surrounded by development where runoff is controlled — or no development at all.
If a lake has poor water quality, it’s a sign of environmentally harmful activity in the watershed. Runoff from the watershed area carries whatever is left on the ground — fertilizer from lawns, oil from parking lots, chemicals from factories.
Some lakes, however, have built-in advantages.
Lakes with small watersheds tend to collect less runoff pollution. Deep lakes are healthier than shallow ones, because certain fish favor colder water and the dark lake-bottoms don’t produce excessive plant life.
The Met Council has given each lake a letter grade, A through F. To determine the grades, it tests for:
- Phosphorous, an element found in fertilizers. When phosphorous washes into lakes, algae overpopulates. It then dies off and rots, which harms fish and plant life.
- Nitrogen, another polluting element that stimulates plant growth. Nitrogen in lakes often comes from stormwater runoff and agricultural sources such as animal manure.
- Clarity. Clear water is a sign of a healthy lake, without excessive algae growth.
- Chlorophyll, the green substance that plants use to turn light into energy. Chlorophyll is an indicator of algae growth.
- Other signs of unhealthy lakes, including high temperatures, high acidity and low levels of oxygen.
To keep a lake clean, homeowners and businesses in the watershed area can help by limiting runoff pollution. Mary Connor, information officer with the Minnesota Pollution Control Agency, gives these suggestions:
- Homeowners should keep streets and gutters clear of leaves and grass clippings.
- Fertilizers should be applied only on lawns or gardens, not onto paved surfaces.
- Buffer zones — a natural area of aquatic plants — surrounding lakes can stop the flow of polluting runoff.
- Runoff from yards can be slowed by the use of rain gardens and rain barrels.
- Paved surfaces should be kept to a minimum. Any permeable surface, allowing water to seep into the ground, is preferable to asphalt or concrete.
- The use of salt on icy sidewalks and driveways should be kept to a minimum. Salt is a pollutant in lakes.
The top 10 cleanest lakes, out of the 180 lakes monitored by the Metropolitan Council, are listed in no particular order:
- Little Carnelian Lake, north of Stillwater
- Square Lake, north of Stillwater
- Big Carnelian, north of Stillwater
- Big Marine Lake, Scandia
- Keewahtin Lake, Forest Lake
- LakeJane, in Lake Elmo
- Courthouse Lake, Chaska
- Lake Lac Lavon, Apple Valley
- Lake Waconia, Waconia
- Twin Lake, south basin, Arcola
The 10 lakes in the metro area with the poorest water quality are listed in order, starting with the worst:
- Meuwissen Lake, west of Chaska
- Chub Lake, south of Lakeville
- Lake Maria in Wright County
- Benton Lake, west of Chaska
- Big Woods Lake, Chaska
- Goose Lake, north basin, Lake Elmo
- Nielson Lake, Scandia
- Heifort’s Pond, Stillwater
- Lake Augusta, Mendota Heights
- Hazeltine Lake, Chaska
Check your lake
How clean is your lake? For the 2017 Metropolitan Council’s lake-by-lake assessment of water quality, visit metrocouncil.org/Wastewater-Water/Services/Water-Quality-Management/Lake-Monitoring-Analysis.aspx. Then click on “2017 Study of the Water Quality of Metropolitan Area Lakes.”