Lake Water Quality Grades Explained: What A

Lake Water Quality Grades Explained: What A–F Means

LakeQuality grades a lake A through F on nutrients and algae, using the condition classes EPA publishes for least-disturbed reference lakes in the same ecoregion. Of 13,297 monitored lakes across the 49 states and 2 Canadian provinces, 9,479 carry a letter and 3,818 are Not Rated. An A is not a swimming clearance.

What the letter actually measures

The grade is built from four measured quantities: total phosphorus, total nitrogen, chlorophyll-a, and turbidity. Those describe how much nutrient a lake carries and how much algae it grows. That is the entire scope. The cutpoints come from Table 6-3 of the EPA National Lakes Assessment 2022 Technical Support Document (EPA 841-R-24-006). Every sample is from the public EPA Water Quality Portal.

A phosphorus number that would be alarming in a Rocky Mountain lake is unremarkable in a Nebraska Sand Hills lake, which was hypereutrophic before anyone farmed near it. EPA solves that by setting Good / Fair / Poor separately in each ecoregion, from the 75th and 95th percentiles of its own least-disturbed reference lakes. The Good/Fair phosphorus cutpoint runs from 16 µg/L in the Northern Appalachians to 486 µg/L in naturally hypereutrophic Southern Plains lakes. The full table, and how a lake is assigned its ecoregion, lives on the methodology page.

Right now the highest-graded lake in the dataset is Sunset Lake (VT), with chlorophyll-a around 1.3 µg/L and phosphorus around 9 µg/L. The live ranking is on the cleanest-lakes list.

What the letter does not measure

The grade does not measure mercury, PFAS, or bacteria. It is not a swimming clearance and it is not a fish-consumption clearance. Cross-checked against EPA ATTAINS, 53.0% of the lakes we grade A are on a state's Clean Water Act 303(d) impaired list. There is no contradiction in that: a lake can have textbook nutrient chemistry and still be listed for mercury in fish tissue, for PFAS, or for E. coli. Pair the letter with the impairment panel on the lake page, the swim-advisory explainer, and the fish-consumption guide.

Why water clarity is shown, not scored

This is the largest change from the old methodology, and it is deliberate. EPA assigns condition classes to phosphorus, nitrogen, chlorophyll-a, and turbidity — and none to Secchi depth. Dark water has three quite different causes, and only one of them is a nutrient problem:

Algae. The water is green because it is full of phytoplankton. Chlorophyll-a already measures that. Scoring clarity too would count the same fact twice.
Natural dissolved colour. Tannins from peat and conifer forest stain bog lakes brown. A Boundary Waters lake can be tea-coloured, nutrient-poor, and pristine.
Suspended sediment. Wind-resuspended clay or a silty inflow scatters light. A reservoir on clay soils can be permanently turbid with almost no algae in it.

Under the previous system those were indistinguishable, and hundreds of lakes failed a letter on a clarity reading alone. Clarity is still measured in feet on every lake page — see the Secchi depth guide and Water Clarity Explained — with a driver named (algae, stain, sediment) instead of a fake penalty.

How the letter is assembled

The grade is not an average of parameter grades. Averaging ordinal letters is not a valid operation, and the old implementation of it let one Secchi reading publish a letter. The engine uses the cause-and-response logic state agencies use for eutrophication listing: chlorophyll-a is the response (the algae the lake is actually growing); phosphorus and nitrogen are the cause. Cause and response have to agree before a lake is called degraded. When a lake is classified sediment-driven, phosphorus is reported but not graded — clay carries phosphorus that algae cannot use — and chlorophyll-a becomes the sole graded indicator.

A parameter needs at least three summer samples with the most recent inside a 10-year window before it can enter the grade, and at least one nutrient or chlorophyll-a measurement has to meet that minimum. A lake with only clarity readings, only a single sample, or only decades-old data is published as Not Rated. An F is a finding. Not Rated is the absence of one.

Grade distribution in the current dataset

Letters are criterion-referenced, not graded on a curve. A year in which every lake improves is a year in which every letter can improve. The table below is the rated set only — 3,818 additional lakes are Not Rated and are not folded into these percentages.

Grade	Meaning	Rated lakes	Share of rated
A	Excellent — Good nutrients and algae, low chlorophyll-a	3,314	35.0%
B	Good — nutrients and algae in Good class, chlorophyll-a above the mesotrophic ceiling	3,063	32.3%
C	Fair — mixed cause and response, not Good, not Poor	1,220	12.9%
D	Nutrient-rich — higher nutrients and/or algae in its ecoregion. Cabins and rentals around these lakes are still listed.	1,231	13.0%
F	Algae-prone — the worst combined nutrient–algae scores. The letter is not a stay-or-skip rating.	651	6.9%
Rated total		9,479	100%

For the live ends of that distribution, see Cleanest and Dirtiest Lakes in America 2026 and the worst-grade ranking. Trophic class — oligotrophic through hypereutrophic — is a separate productivity scale; it is only stated as fact on rated lakes. That distinction is unpacked in Trophic State Explained.

Frequently Asked Questions

A C means the lake is not in EPA’s Good condition class for nutrients and algae in its ecoregion, but it is not in Poor either. Expect moderate algae in a typical summer, not a guarantee of a bloom and not a day-of swim call. Read the lake page alongside current beach and algae advisories.

No. The letter is a nutrient-and-algae statement. It is blind to mercury, PFAS, and bacteria. Cross-checked against EPA ATTAINS, 53.0% of the lakes we grade A still carry a Clean Water Act 303(d) impairment listing — usually for a toxic or a pathogen the grade never looks at.

Because EPA’s National Lakes Assessment does not grade Secchi depth. Dark water can mean algae (already captured by chlorophyll-a), natural tannin stain, or suspended clay. Scoring clarity as if it were pollution failed bog lakes and clay reservoirs. Clarity is still measured and shown on every lake page; it is explained, not scored.

A letter needs enough summer samples, recent enough, of a nutrient or chlorophyll-a. 3,818 lakes in this dataset are published as Not Rated rather than guessed. That is EPA Integrated Reporting Category 3 — insufficient information — never an F.

No. D is Nutrient-rich and F is Algae-prone: higher nutrients and algae in that ecoregion. Check advisories before swimming, and skip the water if you see scum. The letter is not a stay-or-skip rating — cabins and rentals around those lakes are still listed on each lake page.

Every measurement comes from the EPA Water Quality Portal, which aggregates records submitted by state agencies, USGS, and county and tribal programs across the 49 states and 2 Canadian provinces. Each lake page names the organizations that submitted its samples. The letter itself is judged against EPA National Lakes Assessment 2022 ecoregion condition classes, not a homemade scale.