Impaired waters in the United States.

It means a state has assessed the water against its own water quality standards and found that at least one designated use — swimming, fishing, drinking water supply, or supporting aquatic life — is not being supported. The judgement is the state's, made under sections 303(d) and 305(b) of the Clean Water Act and reported to EPA through a system called ATTAINS. It is a regulatory determination about specific uses, not a general verdict that the water is dirty: a river can be perfectly safe to boat on and still be listed as impaired for fish consumption because of mercury in the tissue of the fish that live in it.

The 303(d) list is the subset of impaired waters that are still awaiting a cleanup plan — a Total Maximum Daily Load, or TMDL, which sets how much of a pollutant the water can receive and still meet standards. Once that plan is written, the water moves to category 4A and comes OFF the 303(d) list. It is still impaired. So the listed count is the regulatory backlog and the impaired count is the state of the water, and they are not two views of the same number: nationally about a third of impaired waters are no longer on the list. This map reports both and makes you choose which one you are looking at, because picking one for you would mean mislabelling the other.

No, and this is the single most important thing to understand about this dataset. Grey means no water in that subbasin has been assessed at all. Clean Water Act assessment is done by the states, on their own budgets and schedules, and no state assesses everything — nationally, only about 53,000 of the country's 103,000 subwatersheds contain a water anybody has assessed. An unassessed river is not a river known to be safe; it is a river nobody has tested. Every percentage on this site is a share of what HAS been assessed, and the unassessed share is shown next to it for exactly that reason.

From EPA's ATTAINS geospatial service, which publishes the assessment decisions states and tribes report under the Clean Water Act. Driftwise harvests the national inventory into its own store and serves this map from that copy, so EPA receives one harvest per refresh regardless of how many people are looking at the page — the site itself never calls EPA. We re-serve the determinations; we do not make them, recompute them, or reinterpret them. Every named water on the state pages links back to EPA's own waterbody report for it.

Because states are on independent Integrated Report cycles. A state submits its assessment to EPA roughly every two years, EPA then takes action on it, and the geospatial service is refreshed at each of those moments — so a national map genuinely carries 2018, 2022 and 2024 assessments side by side. Each state page names its own state's cycle and how far through EPA approval it got, because a single national "as of" date would be false for most of the country.

EPA groups every cause of impairment into 36 categories — pathogens, mercury, nutrients, sediment, temperature, PFAS, habitat alteration and so on — and this map shows which of those a state reported for each water. Roughly eight per cent of all cause records are some form of "cause unknown": the state has established that a designated use is not supported, often through biological sampling that found the aquatic life community degraded, without identifying what is doing it. Those are kept in the legend rather than folded into an "other" bucket, because "we know this water is broken and not why" is a real and common finding, and hiding it would overstate how well understood the problem is.

Those are EPA category 4C: impaired by something that is not a pollutant at all. A dam that changed the flow regime, a channel that was straightened decades ago, an invasive fish that displaced the native community — each can stop a water supporting its designated uses without any pollutant being involved. Because a TMDL sets a pollutant limit, it cannot fix a 4C water, which is why those never appear on the 303(d) list. This map counts them as impaired, reports them separately, and never calls them polluted.

Two reasons, one practical and one honest. Practically, EPA crosswalks each assessment to the HUC12 subwatersheds its geometry touches, and there are 103,068 of those nationally — far too many to draw as individual shapes at national zoom, so the map draws the coarser HUC8 subbasins and computes every figure from the HUC12s inside them. Honestly, the crosswalk is an "intersects" relation: a listed river running forty miles is indexed to every subwatershed it passes through, so a shaded subbasin means "intersected by a listed water", not "this whole area is polluted". The state pages name the actual waters, which is the level at which the question usually gets asked.

Driftwise checks daily and republishes when EPA's inventory changes, which in practice is a handful of times a year — this is biennial regulatory data, not a sensor feed. That means the map is never more than a day behind EPA, and EPA itself may be a year or more behind conditions on the ground: an assessment reflects sampling done during that reporting cycle, not this week's water.

Not on its own. An assessment describes a multi-year regulatory determination about designated uses, not today's conditions, and an unassessed water tells you nothing either way. Beach closures and swimming advisories come from your state or county health department, drinking water quality comes from your utility's Consumer Confidence Report, and EPA's How's My Waterway carries the authoritative record for any individual water. Use those for anything you intend to act on.

They answer different questions from different agencies. The streamflow map ranks each USGS gauge against its own historical record, so it describes how much water is moving right now. The drought map carries the Drought Monitor's weekly assessment of how dry the land is. This map is about water QUALITY — what is in the water and whether it supports the uses people expect of it — and it moves on a two-year regulatory cycle rather than hourly or weekly. A river can be running at a healthy flow, in a region with no drought, and still be listed for pathogens.

Yes. EPA water quality designations are one of more than a dozen federal inputs Driftwise watches on every monitored river reach, alongside USGS gauge readings, the National Water Model, drought classification, NOAA flood forecasts, wildfire proximity, and air quality. In the product, an impairment is resolved to the specific reaches and access points a crew is responsible for, and it feeds the Environmental Risk engine's severity and confidence components — so a listing becomes a factor in what a steward is told to do this week, rather than a page somebody has to remember to check.