US States With the Most Lake Waterbodies Assessed as Impaired
EPA ATTAINS data ranking states by lake-type waterbodies their agencies assessed as not supporting at least one designated use.
Research period:
According to the U.S. Environmental Protection Agency's ATTAINS program, state agencies and tribal nations report 60,590 lake-type waterbodies nationwide in the August 2026 extract, and PlainLakes renders those filed assessment decisions alongside linked U.S. Geological Survey daily gauge records where a conservative name match exists. See our methodology for reporting-unit limits, filer attribution, and refresh cadence.
Research question
Which US states report the most lake waterbodies assessed as not supporting a designated use?
Methodology
The ranking reads the per-state rollup of lake-type assessment units: for each state it carries the number of units assessed as Not Supporting at least one designated use, the total number of units the state reported, and the number assessed as Fully Supporting. States are ordered by the count of Not Supporting units and the ten highest are shown.
This is a count, not a rate, and the difference decides how the page should be read. A state with many assessed lakes can lead a count ranking while sitting mid-table on the share of its own assessments that failed. The reported total and the Fully Supporting count sit beside the impaired count on every row so the denominator is never hidden, and the share-based ranking with a minimum-sample floor lives on its own page for readers who want the rate.
The count reflects each state's assessment programme as much as its water. A state that assesses aggressively, revisits waters often, and divides them into many small assessment units will file more impaired units than a state that assesses a fraction of its waters at a coarser grain. Reading the top of this list as "the states with the worst lake water" is the one conclusion the data cannot support.
One listing practice can dominate a state's entire count. Where a state issues a fish-consumption advisory that applies across nearly all of its lakes, that single decision marks almost every assessed unit as not supporting a designated use, and the state rises to the top of a national count on the strength of one advisory rather than a lake-by-lake finding. The state pages carry the breakdown by use, which is where that pattern becomes visible.
"Not Supporting" is a regulatory verdict, not a description of a lake's condition to a visitor. It means the state found the waterbody does not meet the water-quality standard for at least one of its designated uses, which may be fish consumption, recreation, aquatic life or drinking-water supply, and the failing use matters as much as the fact of failure. A lake that fails a fish-consumption standard because of mercury may be perfectly safe to swim in.
Only units the state assessed can appear. Most lake-type units in the national file carry no assessment at all, and an unassessed lake is absent from both the impaired and the fully-supporting column. A state's reported total is therefore the size of its assessment programme, not a census of its lakes.
Refresh cadence follows the source. States report to EPA on their own cycles, typically every two years, and some file late, so a state's counts can rest on decisions from several years ago. The database behind this page spans reporting cycles 1987 through 2026 and the vintage PlainLakes has loaded is shown on the page; no turnaround between an EPA release and its appearance here is promised.
Cross-state comparison has a hard limit worth naming. Each state writes its own water-quality standards, chooses its own monitoring strategy, and decides how finely to divide its waters into units, so two states can assess the same lake condition and file different verdicts. The ranking is a reliable answer to "which states have filed the most impaired lake units", and it is not a national league table of water quality.
Every figure is traceable. Each state row links to that state's page, which lists the assessed waterbodies behind the count, and the same records are published by EPA through its own ATTAINS interface, so any total here can be checked against the source directly.
PlainLakes is published by an organization, not by a newsroom of named reporters, and no individual byline or human review is claimed for these pages. The ordering follows the documented query above rather than editorial selection. Corrections are welcome through the address in the footer and are reconciled against the upstream record.
See the methodology page for the complete data-update process, source vintage, and field definitions.
US States With the Most Lake Waterbodies Assessed as Impaired
Live data, updated automatically as the source dataset is revised
The ranked top 10
50 states carry at least one lake waterbody assessed as not supporting its designated uses. The 10 with the highest counts are ranked below. Figures update automatically as the source data is refreshed.
| # | State | Assessed not supporting | Reported waterbodies | Fully supporting |
|---|---|---|---|---|
| 1 | New Hampshire | 2,786 | 0 | |
| 2 | Minnesota | 13,176 | 1,566 | |
| 3 | Michigan | 4,307 | 3,457 | |
| 4 | Massachusetts | 1,191 | 40 | |
| 5 | Florida | 1,475 | 367 | |
| 6 | New Jersey | 611 | 21 | |
| 7 | New York | 2,231 | 79 | |
| 8 | Wisconsin | 19,871 | 4,225 | |
| 9 | Illinois | 783 | 63 | |
| 10 | Washington | 832 | 0 |
Source: U.S. Environmental Protection Agency, ATTAINS Integrated Report assessment decisions. Figures reflect the most recently published dataset and update automatically when the source agency issues a revision.
Findings
Top entity in the ranking
The top-ranked record in this dataset is New Hampshire, with a value of 2,783 on the Assessed not supporting column. The full top-10 set is rendered in the table above. Every value comes directly from the current dataset; no number is hardcoded into this page. When U.S. Environmental Protection Agency publishes a revision, the ranking and the prose around it update automatically.
Distribution shape
The gap between the top-ranked record (2,783) and the 10th-ranked record (393) characterizes how concentrated the top of the distribution is. Where the top value is many multiples of the median value of the visible set, the population is highly concentrated, a small number of entities accumulate the bulk of the measured quantity. Where the top and bottom of the visible set are close together, the distribution is relatively flat across the top end. The full distribution beyond this top-10 cut is summarized in the aggregate context section below and explored in the linked entity profiles.
Aggregate context
The 50 qualifying states report 12,831 impaired lake waterbodies between them, out of 60,590 lake waterbodies reported in total, a mean of 257 impaired units per state. The ten states ranked above hold 71.5% of the national impaired count, which says as much about the size of their assessment programs as about their water: a state that assesses more of its inventory finds more, and the per-state share is the comparable measure rather than the count ranked here. These figures come from the same query that produces the ranking, with the ten-row cut removed and nothing else changed.
Source provenance
The records in this ranking originate from U.S. Environmental Protection Agency, specifically the ATTAINS Integrated Report assessment decisions. PlainLakes loads the source vintage the agency has published and reads the page's figures from it, so no static export carries stale numbers; the vintage this page was built from is named in the citation block, and no turnaround between an EPA release and its appearance here is promised. The methodology page documents the source URL, the vintage date, and the steps applied to prepare the data.
Why this ranking matters
Rankings like this one let a reader scan a population quickly and identify outliers, concentrations, and patterns that warrant deeper investigation. The detail pages linked from each entity in the table above give the full per-entity context: time-series history where available, related metrics from adjacent tables, and links onward to the underlying source records. The methodology page explains how an entity earns inclusion in the dataset and how the ranking column is computed at the source.
What this analysis cannot tell us
Counts reflect each state's own assessment program and reporting cycle, so a high count can indicate broad monitoring as much as poor water quality. Waterbodies without a definitive overall verdict are counted as reported but not as impaired. Assessment units, not physical lakes, are the counting unit; large lakes split across states appear once per state unit.
Most-listed lake impairment cause labels nationally
The five most-listed cause labels across state lake assessments nationally. These rank the names states file, not distinct pollutants: Mercury reaches this list under more than one label, so two bars can be one substance.
Sources
- EPA ATTAINS (Assessment, Total Maximum Daily Load Tracking and Implementation System) - https://www.epa.gov/waterdata/attains
PlainLakes is rendered directly from state Clean Water Act assessment decisions in EPA ATTAINS and lake level records from USGS NWIS, no number is typed in by an editor. Every figure on this page is computed at render time from the PlainLakes database; no number is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of 2026-08-14. Primary sources: EPA ATTAINS and USGS National Water Information System.