PlainLakes

EPA ATTAINS + USGS NWIS · extract August 17, 2026

US lake assessment statistics

Key findings compiled live from EPA ATTAINS assessment decisions and linked USGS gauge records. Every figure below is computed from the PlainLakes database at request time. These are filed assessment counts and gauge statistics, not recreation safety advice or real-time water quality.

60,590
Assessment units
24,429
Definitive verdicts
50
States + DC

Key findings

Standalone, citable statements from the linked ATTAINS extract. Free to reuse with attribution to PlainLakes (CC BY 4.0; cite the page).

  • 60,590 lake-type assessment units are filed to EPA ATTAINS across 50 states by state agencies and tribal nations. The counting unit is an assessment unit on the Integrated Report, not a physical lake; large lakes split across states appear once per state unit.
  • 52.5% of the 24,429 units carrying a definitive overall verdict are not supporting at least one designated use (12,831 impaired vs 11,598 fully supporting with reported-use backing). Comparative shares exclude verdicts resting only on unreported use groups.
  • 15,611 waterbodies meet PlainLakes' tier-1 index predicate (42,622 named units in the register). Tier-1 is an enrichment and index-surface contract, not an EPA field.
  • 2,961 "Mercury in Fish Tissue" leads the filed-cause label ranking on 24 states. This ranks the strings states file, not distinct pollutants; mercury appears under 7 labels and reaches 6,851 waterbodies when counted as one family.
  • 78% of 8,914 units with a fish-consumption use verdict are not supporting that use (6,956 impaired). Fish consumption is one designated-use group among several this portal reports.
  • 0.3% of named waterbodies carry a linked USGS daily level record (121 of 42,622; 106 tier-1 indexed). Level readings describe gauge statistics, not safety or water quality.
  • 2,783 New Hampshire reports the most lake-type units assessed as not supporting at least one use (2,786 units in its register). High counts can reflect broad monitoring as much as water quality.
  • 19,871 Wisconsin files the largest register (19,871 lake-type units; 1,219 tier-1 indexed). Register size and impaired count measure different things.
  • 2,233 New Hampshire leads tier-1 indexed coverage (2,233 of 2,786 filed units).

States with the most impaired lake-type units

Raw counts of units assessed as not supporting at least one designated use. Not a per-capita or water-quality rank.

1. New Hampshire2,7832. Minnesota1,7983. Michigan8074. Massachusetts6785. Florida6366. New Jersey5837. New York5638. Wisconsin493
Top state impaired-unit counts (EPA ATTAINS)

Source: EPA ATTAINS Integrated Report assessment decisions State rollup of lake-type assessment units · 2026

Full ranking: states by impaired share · research analysis.

How to read these numbers

ATTAINS records assessment units on each state's Integrated Report, including tribal nations filing under Clean Water Act section 518 authority. Impaired means not supporting at least one designated use; fully supporting counts exclude verdicts resting only on use groups this portal does not report. Cause labels are filed vocabulary and can split one pollutant family across several strings. USGS level records describe daily gauge statistics for linked sites; they are not a safety verdict. Methods: methodology.

Download the state-level snapshot, or see the full dataset page for CSV/JSON downloads, provenance, and a stable citation.

This portal is rendered directly from its public source data, no number is typed in by an editor. National impaired share uses 24,429 definitive verdicts; tier-1 and gauge linkage use PlainLakes index predicates documented on methodology. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of ATTAINS extract August 17, 2026.