Impaired Lakes Explained: What an EPA Listing Means

How many lakes are listed

Of 9,514 lakes matched against the EPA's ATTAINS assessment database across 36 states, 5,662 carry at least one impairment listing — 60%. Lakes that have not been through that match are excluded from the figure entirely; a lake nobody has assessed is unknown, not clean, and folding it in would push the rate down for no reason but silence.

A listing is not a single label. Listed lakes average 1.6 distinct causes each, and the mix is the part that decides what it means for you.

What lakes are listed for

The top two behave completely differently. Nutrient and algae listings describe the water you would swim in and are the same signals behind the letter grade, which is why they and the grade usually agree. Mercury describes the tissue of a walleye that has spent eight years eating smaller fish, and it has almost nothing to do with how the water looks.

Why an impaired lake can still be clear

1,522 lakes here are listed for mercury and nothing else. Among those with a full grade, the distribution is not what the word “impaired” suggests:

Mercury arrives from the atmosphere, mostly from combustion, and settles across whole regions rather than on individual shorelines. Lake bacteria convert it to methylmercury, which concentrates as it moves up the food chain, so the longest-lived predators carry the most. A remote lake with an undeveloped forested shore is exactly the kind of place that gets a mercury listing and an A grade in the same year. What to do about it is in Mercury, PFAS, and the Fish You Shouldn't Eat.

The clearest A-graded lakes here that carry a mercury-only listing:

Where listings concentrate

Listing rates say as much about how thoroughly a state assesses its water as about the water itself. A state that samples aggressively finds more to list; a state that assesses a fraction of its lakes reports a smaller number and a cleaner-looking one. Read the column of assessed lakes next to the column of listed ones.

What a listing should change about your day

Read the cause, not the word. A nutrient or algae listing is a reason to check the current advisory before swimming and to expect green water in late summer; that mechanism is in Why Lakes Turn Green. A bacteria listing is about contact after rain, covered in Is It Safe to Swim in a Lake? A mercury or PCB listing is a reason to follow the state's fish-consumption guidance and no reason at all to skip the swim.

One more thing worth knowing: 3,032 of these lakes already have a cleanup budget written for them. Listings are the mechanism that starts that work, which is why a lake appearing on the list is a step in the process rather than the end of the story. The lakes where it has demonstrably worked are in Lakes Making a Water-Quality Comeback.