Lake Water Quality Guides

Each US lake page shows its nutrient and algae readings, total phosphorus, total nitrogen, chlorophyll-a, and turbidity, beside the EPA National Lakes Assessment 2022 condition class (Good, Fair or Poor) for that lake’s ecoregion. Good water quality, in those terms, is phosphorus and chlorophyll-a in the Good class with deep Secchi clarity. Clarity itself is measured and shown in feet with no class. There is no single national phosphorus or chlorophyll-a number that counts as Good. Lakes without enough recent samples show the readings they have and say so. The Carlson Trophic State Index is reported as a measurement and stated as a class only when the sampling floor on the methodology page is met.

Every measurement on LakeQuality comes from the EPA Water Quality Portal, which aggregates each covered state's monitoring agency data — programs like the Minnesota Pollution Control Agency and the Wisconsin Department of Natural Resources are examples — along with U.S. Geological Survey records and various county and tribal monitoring programs, across all United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs. we cover. Supplemental physical attributes are deepest in Minnesota and Wisconsin: depth comes from the Minnesota and Wisconsin DNR lake surveys, fish-species lists cover Minnesota, Wisconsin, and Michigan, and surface area is partial across all United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs.. The 681 Wisconsin DNR Fisheries Survey reports are integrated directly. Last refreshed August 2026.

Yes. The readings reflect long-term water quality based on summer-season measurements, and no reading is a swimming clearance. Day-of conditions can change quickly — a heavy rain can flush sediment and bacteria into a lake, and warm calm weather can trigger a localized cyanobacteria (blue-green algae) bloom even in a lake with the lowest nutrient readings. Whether a lake is safe to swim in today is a call for the state water agency and the county health department: check current conditions and any posted health advisories from the Minnesota Department of Health, Wisconsin DNR, or your state agency before swimming, especially with young children, pregnant family members, or pets.

Zebra mussels, Eurasian watermilfoil, starry stonewort, and other DNR-listed invasive species change a lake's ecology and recreational value but are not part of the nutrient and algae readings. The Minnesota DNR maintains an infested waters list that LakeQuality shows on each lake page so you can see infestation status alongside water quality. The Wisconsin DNR maintains its own infested waters database with similar coverage.

Not necessarily. Good nutrient and algae readings signal strong water quality, which generally supports stronger long-term property values, but other factors matter for any specific purchase: trend direction (are the readings improving or declining), depth (a 4-foot maximum is a different lake than a 60-foot maximum with the same readings), surface area, public access, fishing opportunity, and local watershed development pressure. The "How to Choose a Lake Property" guide walks through how to weigh each factor.