How A–F Lake Water Quality Grades Are Calculated

Of the 37,546 lakes we track, 5,737 carry an A–F letter and 31,809 are Not Rated. The letter is a repeatable classification of nutrients and algae, not a subjective judgment and not a swim clearance. This guide is the short version; the cutpoint table, the sampling a grade requires, and the ATTAINS cross-check are all on the methodology page. Figures here match the published freeze (37,546 tracked, 5,737 graded, 31,809 Not Rated, 1,178 A's). Not a new grade run.

Step 1 — Pull the measurements

The raw data comes from the EPA Water Quality Portal, which aggregates state, federal, and tribal monitoring records into one standardized feed, plus licensed international chemistry catalogs. For the letter we use total phosphorus, total nitrogen, chlorophyll-a, and turbidity. Secchi disk depth is measured and shown on every lake page; it is not scored. For each parameter we take the median of recent surface samples so a single anomalous reading cannot swing the grade.

Step 2 — Classify against EPA ecoregion cutpoints

Each median is classified Good / Fair / Poor against the EPA National Lakes Assessment 2022 condition classes for that lake's ecoregion. Reference phosphorus varies roughly 30× across U.S. ecoregions, so a concentration that is Good in the Coastal Plains can be Poor in the Western Mountains. There is no single national Secchi, phosphorus, or chlorophyll-a number that equals an A. The transcribed cutpoint table is on our methodology page.

Florida is the deliberate profile exception: subtropical lakes use Florida LAKEWATCH breakpoints so naturally tannic, productive waters are not flunked by a northern key.

Step 3 — Composite the letter — or refuse to invent one

Gradable nutrient and algae classes become the letter. Clarity never enters that composite. In a sediment-driven lake, phosphorus is reported but not graded, because clay-bound phosphorus is not a trustworthy algae signal. A lake that falls short of the minimum — at least three samples within ten years on a gradable parameter, and an EPA ecoregion so the cutpoints apply — is UNRATED. That is EPA Integrated Reporting Category 3, not an F. Another 7,677 rated lakes carry a limited-data confidence flag, which is not the same as unrated.

Step 4 — Classify trophic state separately

Independently of the letter, the same chemistry can feed the Carlson Trophic State Index, a logarithmic scale that places a lake on a productivity spectrum: oligotrophic (TSI under 40, clear and nutrient-poor), mesotrophic (40–50), eutrophic (50–70, nutrient-rich and algae-prone), or hypereutrophic (over 70). TSI is reported as a measurement. The trophic class is stated as fact only when the lake is rated. TSI never alters the A-through-F result.

How the grades come out across the dataset

Applied to 5,737 US lakes with a letter. Ontario and Québec have 0 letters. The method produces the distribution below. The Not Rated segment is part of the claim — a five-letter bar renormalised to 100% would silently treat unmeasured lakes as if they had been graded:

Letters are comparable only inside the same ecoregion key (or the Florida profile). A phosphorus reading that is Fair in Minnesota's Upper Midwest key is not automatically Fair in Ohio's Temperate Plains key.

Frequently Asked Questions

Sources: EPA Water Quality Portal (measurements); EPA National Lakes Assessment 2022 Technical Support Document (ecoregion condition classes); R.E. Carlson, “A trophic state index for lakes,” Limnology & Oceanography (1977); Florida LAKEWATCH (subtropical profile). Grades are a classification of measured data, not a safety certification.