The State of America's Lakes 2026

The 2026 grade distribution

Lake water quality in the United States is not evenly distributed. We grade every lake A through F using three measurements that public agencies collect: Secchi depth (how far you can see into the water), total phosphorus (the nutrient that feeds algae), and chlorophyll-a (a direct measure of algae concentration). Here is how the 5,469 monitored lakes break down.

The middle of the distribution tells the real story. A large block of lakes grade C, meaning fair water quality with moderate algae during warm months, water that is generally safe but visibly greener than a pristine northern lake. The tails matter too: A-graded lakes are clear enough to see the bottom in many areas, while F-graded lakes can have visibility under three feet and recurring blooms.

Water quality state by state

The cleanest lakes cluster where the geology and land use favor them. Deep, cold, forested watersheds in the northern states hold clarity; shallow lakes ringed by row-crop agriculture carry the phosphorus loads that turn water green. The table below ranks the states we cover by their average lake grade.

On this measure, Michigan posts the highest average grade (B), and Iowa the lowest (D). Read state averages as a reflection of the underlying landscape, not a report card on any one agency — a state of shallow prairie lakes will always average greener than a state of deep glacial ones, no matter how well either is managed.

Averages also hide range. Every state on the list has clear A-graded lakes and impaired F-graded ones, sometimes within a few miles of each other. To see where a specific lake falls, open its state page or the portfolio-wide cleanest-lakes ranking.

What is improving, and what is slipping

Water quality changes slowly. We flag a trend only where a lake has several years of repeated sampling, then look at the direction of its clarity, phosphorus, and chlorophyll-a over time. Of the 4,123 lakes with enough history to call, 1,768 are improving and 1,349 are declining. The rest are holding steady.

Improvements usually trace back to fewer nutrients reaching the water: upgraded wastewater treatment upstream, shoreline buffers that catch runoff, and agricultural practices that keep phosphorus on the field. Declines run the other way — new development, intensifying farm runoff, invasive species, and warmer summers that lengthen the algae season. The two stories are covered in depth in the comeback lakes and the lakes where quality is slipping.

How these grades are built

Every number here is computed from public monitoring data, not estimates. Grades follow long-established Metropolitan Council thresholds: an A requires phosphorus under 20 µg/L and chlorophyll-a under 5 µg/L, while an F reflects heavy nutrient loads and dense algae. A lake's overall grade blends its clarity, phosphorus, and algae scores. Full detail is on the methodology page.

Two caveats apply to any national read. First, data density varies by state, so a state average built on a few hundred lakes carries more uncertainty than one built on thousands. Second, a grade is a long-term water-quality signal, not a day-of forecast — a clean lake can still have a temporary bloom after a warm, calm spell. For same-day risk, always check your state's beach and algae advisories before you swim.