Lake Water Quality Explained: What the Readings Mean

LakeQuality reads a lake's nutrient and algae measurements against the condition classes EPA publishes for least-disturbed reference lakes in the same ecoregion. Of 38,731 monitored lakes across the United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs., 5,873 carry nutrient and algae readings and 32,858 do not. No reading is a swimming clearance.

What the readings actually measure

Four quantities are measured on a lake: 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-measured 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 readings do not measure

These readings do 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, 51.5% of the lakes with the lowest nutrient readings 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 readings 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 were classified 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 readings are classified

Each parameter is read against the EPA National Lakes Assessment condition class for that lake's ecoregion, so a reading is compared with the lakes it would naturally resemble rather than with a national average. Clarity is shown in feet beside them.

A parameter needs at least three summer samples, the most recent inside a 10-year window, before it is published, and at least one nutrient or chlorophyll-a measurement has to meet that minimum. A lake with only a clarity reading shows that reading alone.

For the cleanest end of the range, see the cleanest lakes 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

It means the lake is not in EPA’s Good condition class for nutrients and algae in its ecoregion, and 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. These are nutrient and algae measurements. They are blind to mercury, PFAS, and bacteria. Cross-checked against EPA ATTAINS, 51.5% of the lakes with the lowest nutrient readings still carry a Clean Water Act 303(d) impairment listing — usually for a toxic or a pathogen these readings never look at.

Because EPA’s National Lakes Assessment assigns no condition class to 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.

Publishing a nutrient or algae figure needs enough summer samples, recent enough. 32,858 lakes in this dataset are published with no nutrient reading rather than a guessed one. That is EPA Integrated Reporting Category 3 — insufficient information — never an F.

No. Higher phosphorus and chlorophyll-a readings mean a more productive lake for its ecoregion. Check advisories before swimming, and skip the water if you see scum. The readings are 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 United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs.. Each lake page names the organizations that submitted its samples. The readings are judged against EPA National Lakes Assessment 2022 ecoregion condition classes, not a homemade scale.