Updated Aug 2026 · 37,603 lakes
Find Your Next Great Lake Adventure
Stay near A-grade lakes
Cabins and rentals near lakes graded A for nutrients and algae. Pick a town or lake, or search as-is.
The letter
What A through NR means
The letter is nutrients and algae — phosphorus, nitrogen, chlorophyll-a, and turbidity — against EPA condition classes for that lake's ecoregion. Clarity is shown in feet; it is not the letter.
- AExcellent1,178 lakes · 3.1%
EPA Good class for that ecoregion.
- BGood1,754 lakes · 4.7%
Favourable class — not the cleanest bracket, not stressed.
- CFair1,014 lakes · 2.7%
Middle of the EPA scale. Algae pressure is common in late summer.
- DNutrient-rich1,150 lakes · 3.1%
Poorer nutrient class. Often a productive warm-water fishery.
- FAlgae-prone641 lakes · 1.7%
Poorest nutrient class. Check advisories; skip the water if you see scum.
- NRNot rated31,866 lakes · 84.7%
Not enough recent summer samples for a letter. Never an F.
The letter covers nutrients and algae only. It is blind to mercury, PFAS and bacteria, so it is not a swimming clearance and not a fish-consumption clearance.
Read the methodologyNew · Annual data report
The State of America’s Lakes 2026
5,737 graded lakes ranked — the cleanest, the worst algae hotspots, and the biggest improvers among the 5,737 lakes with a letter.
Cleanest lake
Sunset Lake
Rutland County, VT
Worst algae hotspot
Marais des Cygnes Wildlife Area Lake
Linn County, KS
Of the lakes we can grade
51.1%
grade A or B. Share of the 5,737 graded lakes, not of all 37,603.
Late season
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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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Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
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Canada
Alberta, Saskatchewan, Manitoba, New Brunswick, Prince Edward Island, Yukon, Northwest Territories and Nunavut are not listed. No licensed lake-chemistry record for those places is published here.
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Frequently asked questions
How are lake grades calculated?
Lakes are graded A to F on nutrients and algae — total phosphorus, total nitrogen, chlorophyll-a, and turbidity — measured against the EPA National Lakes Assessment 2022 condition classes for that lake's own ecoregion, so a lake is judged against the lakes it would naturally resemble. Water clarity (Secchi depth) is measured and shown in feet on every lake page but is not scored: EPA assigns no condition class to Secchi depth, and dark water can come from algae, from natural dissolved colour, or from suspended clay. A lake without enough recent summer sampling is published as Not Rated rather than given a letter the data cannot support. Not Rated is never an F. The full cutpoint table, the composite rule and the sampling a grade requires are on our methodology page.
Why are some lakes not rated?
A lake is Not Rated when it has no usable nutrient or algae measurements — usually because the only reading available is water clarity, or because there is no recent sample at all. 31,866 of 37,603 lakes are Not Rated. Of those, 1,524 have never been sampled but do carry a water-clarity rating measured from satellite imagery, which shows how far you can see into the water without standing in for a grade. Every Not Rated lake still shows whatever measurements exist, rather than being given a letter the data cannot support.
Where does the data come from?
Water quality data for US lakes comes from the EPA Water Quality Portal, which aggregates monitoring results from state environmental agencies across all 49 covered states (including the Minnesota Pollution Control Agency and Wisconsin DNR), the USGS, and other federal programs. Canadian lakes come from the Ontario Ministry of the Environment, Conservation and Parks and Québec's RSVL programme instead. 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 measurement mean?
Total phosphorus and total nitrogen are the nutrients that feed algae growth. Chlorophyll-a measures the algae itself, so it is the response the grade leans on hardest. Turbidity measures how much suspended material is in the water. Secchi depth measures water clarity in feet — how far down you can see — and is reported alongside the cause limiting it, rather than scored.
Is it safe to swim in a lake graded C or below?
Not on the letter alone. The grade classifies nutrients and algae — it is not a swimming clearance, and it does not measure E. coli, cyanotoxins, or a posted beach closure. A C means moderate nutrient and algae levels; D and F mean higher ones. Check current local advisories before you get in, especially in late summer when blue-green algae blooms peak, and skip the water if you see scum. The grade also measures nothing about whether the lake is worth a trip; cabins and rentals around these lakes are listed on each lake page.
What is trophic state?
Trophic state describes how productive a lake is. The published class comes from median chlorophyll-a against EPA's national bands: oligotrophic at or below 2 µg/L (very low productivity, clear water), mesotrophic above 2 and up to 7, eutrophic above 7 and up to 30 (high productivity, frequent algae), and hypereutrophic above 30 (dense algae). The classic Carlson Trophic State Index is still calculated and shown for continuity, but neither the class nor the index feeds the letter grade.
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