Updated Aug 2026 · 14,370 lakes · 39 states and 2 Canadian provinces
Find Your Next Great Lake Adventure
A–F report cards on clarity, algae, and fish — we grade the water, not the view.
LakeQuality grades every monitored lake across 39 states and 2 Canadian provinces — water clarity in feet, swimming safety, and A–F report cards from EPA and state agency data — plus lake depth and fish species where state DNR data is available. Where the measurements do not support a grade, the lake is marked Not Rated rather than given a letter.
See every grade at a glance on the interactive lake map, or go deep on our most detailed region — the Boundary Waters canoe country of northern Minnesota.
New · Annual data report
The State of America’s Lakes 2026
14,370 monitored lakes ranked — the cleanest, the worst algae hotspots, and the biggest improvers across 39 states and 2 Canadian provinces.
Cleanest lake
Rainy Lake
Cass County, MN
Worst algae hotspot
Marais des Cygnes Wildlife Area Lake
Linn County, KS
Of the lakes we can grade
66.2%
grade A or B. Share of the 10,087 graded lakes, not of all 14,370.
Late season
Ice safety
No ice is ever completely safe. Thickness guidelines, what changes them, and how to read a lake before you walk out.
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Lake Match
Which lake suits how you actually use one?
Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
The cleanest lakes right now
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Water quality rankings
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Top walleye lakes by water quality
Largemouth & smallmouth bass
Top muskellunge lakes
Coldwater lake rankings
Latest DNR fisheries reports
Browse by state
2,394 lakes across 77 counties
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Frequently asked questions
How are lake grades calculated?
Grades are based on three water quality metrics: water clarity (Secchi depth), phosphorus levels, and chlorophyll-a concentration. Each metric is graded A-F against EPA National Lakes Assessment condition classes for the lake’s own ecoregion. The overall grade combines the available metrics.
Why are some lakes not rated?
A lake is Not Rated when no nutrient or algae measurement meets the evidence bar — usually because the only reading available is water clarity, or because there is no recent sample at all. 4,283 of 14,370 lakes are Not Rated. We show what data does exist rather than assigning a letter the measurements do not support.
Where does the data come from?
Water quality data comes from the EPA Water Quality Portal, which aggregates monitoring results from state environmental agencies in all 38 covered jurisdictions (including the Minnesota Pollution Control Agency and Wisconsin DNR), the USGS, and other federal programs. Lake depth comes from the Minnesota DNR LakeFinder and Wisconsin DNR; fish species data covers Minnesota, Wisconsin, and Michigan.
How often is the data updated?
Data is sourced from the EPA Water Quality Portal covering the most recent years of monitoring. Lake grades reflect the median summer season (June-September) measurements.
What does each metric mean?
Secchi depth measures water clarity in feet (how deep you can see). Phosphorus fuels algae growth, higher levels mean more algae risk. Chlorophyll-a directly measures algae concentration in the water.
Is it safe to swim in a lake graded C or below?
Lakes graded C have moderate water quality and are generally safe for swimming, though algae may be present. Lakes graded D or F have higher nutrients and algae — check local advisories before swimming, and skip the water if you see scum. The grade measures nutrients and algae, not whether the lake is worth a trip; cabins and rentals around these lakes are listed on each lake page.
What is the Trophic State Index?
The Carlson Trophic State Index (TSI) classifies lakes by nutrient levels: oligotrophic (TSI < 40, clear and clean), mesotrophic (40-50, moderate nutrients), eutrophic (50-70, high nutrients and algae), and hypereutrophic (> 70, excessive algae). It is calculated from Secchi depth, phosphorus, and chlorophyll-a.
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