Lake Water Quality Guides

Across the U.S., lake water quality is judged primarily on three indicators: Secchi disk depth (water clarity, in feet), total phosphorus (the nutrient that drives algae growth, in micrograms per liter), and chlorophyll-a (a direct measure of algae concentration, also in µg/L). LakeQuality follows Metropolitan Council grading thresholds — an A lake has clarity over 14.8 feet, phosphorus under 20 µg/L, and chlorophyll-a under 5 µg/L. The Carlson Trophic State Index combines these into oligotrophic (clear, low-nutrient), mesotrophic (moderate), eutrophic (high-nutrient), or hypereutrophic (very high) categories.

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 16 states 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 16 states. The 681 Wisconsin DNR Fisheries Survey reports are integrated directly. Last refreshed July 2026.

Yes. The grade reflects long-term water quality based on summer-season measurements. 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 an A-graded lake. Always 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 do not directly drive the LakeQuality water quality grade. The Minnesota DNR maintains an infested waters list that LakeQuality joins to 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. An A grade signals strong water quality, which generally supports stronger long-term property values, but other factors matter for any specific purchase: trend direction (is the grade improving or declining), depth (a 4-foot maximum is a different lake than a 60-foot maximum at the same grade), 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.