PlainLakes

State water-quality picture · #1 by reported lake-type waterbodies

Wisconsin lakes and reservoirs

Every figure on this page is computed from the assessment decisions Wisconsin reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 2022–2026), plus daily water-level observations from the U.S. Geological Survey.

19,871
Reported waterbodies
493
Not supporting a use
148
Fish use not supporting
9
Gauged lakes

Wisconsin in one line

19,871 lake-type waterbodies in Wisconsin are reported to EPA ATTAINS across reporting cycles 2022–2026: 493 assessed as not supporting at least one designated use, 4,225 fully supporting, and 15,153 without a definitive overall verdict. The most-listed impairment cause is Phosphorus, Total, on 256 waterbodies.

19,871
reported waterbodies
493
not supporting a use
4,225
fully supporting
10%
of definitive verdicts impaired

An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.

A large Wisconsin inventory with a lighter ruled-impairment share

19,871 reported lake-type units put Wisconsin in the largest third of state registers, while its ruled not-supporting share (10%) sits in the lowest third. Scale and load pull apart on this page.

Data updated 2026-08-17

According to the U.S. Environmental Protection Agency's ATTAINS program, Wisconsin reported 19,871 lake-type waterbodies in Wisconsin in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.

How Wisconsin's large inventory splits across overall status

Each of the 19,871 lake-type units reported by Wisconsin lands in exactly one overall-status bucket. On a register this large, the bar is an inventory partition, the cause chart and national gauge below answer different questions.

Most-listed impairment causes in Wisconsin

Waterbodies carrying each listed cause, most first

waterbodies

What this shows Phosphorus, Total is the most-listed impairment cause in Wisconsin, on 256 of the lake-type waterbodies reported there.

Source EPA ATTAINS As of Reporting cycles 1987–2026
10 N

Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.

Share of Wisconsin waterbodies with a verdict on fish, recreation, aquatic life or drinking water assessed as not supporting a use

Wisconsin: 10% of ruled verdicts are not-supporting, against a national share of 53%, 5.3× below the national rate.

Reporting practice

Wisconsin's inventory: still mostly without a definitive verdict

Each reporting authority decides how much of the water it files to EPA it actually assesses in a cycle. That decision sets the denominator behind every count on this page.

Wisconsin holds the largest reported lake-type inventory in the country, 19,871 assessment units, carrying a definitive verdict on 23.7% of it. The counts above describe the 4,718 units carrying a verdict, not all 19,871 reported. Wisconsin has also retired 818 lake-type assessment units, superseded filings that are kept on the site but not counted here.

The states reporting at the nearest rates are Minnesota (25.5%) and Alaska (20.8%).

Assessing more of an inventory is a reporting choice, set by cycle length, monitoring budget and how finely a state splits its waters into units. It is not a measure of water quality, and it is why a raw count from one state does not sit beside a raw count from another. How assessment coverage varies.

Wisconsin against every comparable state

Share of reported lake-type waterbodies carrying a definitive verdict

23.7% 43rd of 47 higher than 4 of 47 states

the 47 comparable states, by definitive-verdict share

Each bar is a band; taller bars hold more states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count and share, and where it sits relative to this entry.

Source EPA ATTAINS · Reporting cycles 1987–2026

Each bar counts states whose definitive-verdict share falls in that 10-point band. The 47 states shown are those reporting at least 25 lake-type units; a share off a smaller inventory is precision the data does not have.

By size

Largest assessed lakes in Wisconsin

By assessed acreage as reported in ATTAINS. Acreage is the assessed unit's size, which can differ from a lake's full surface area.

# Waterbody Assessed acres
1 Lake Winnebago Not Supporting 131,942
2 Lake Pepin Not Supporting 25,503
3 Petenwell Lake Not Supporting 23,001
4 Lake Poygan Not Supporting 14,024
5 Lake Chippewa Fully Supporting 13,616
6 Turtle Flambeau Flowage Not Supporting 12,943
7 Castle Rock Lake Not Supporting 12,386
8 Lake Koshkonong Not Supporting 10,596
9 Lake Mendota Not Supporting 9,781
10 Lake Butte des Morts Not Supporting 8,569
11 Lake Onalaska Fully Supporting 8,391
12 Lake Saint Croix Not Supporting 7,696

By cause count

Most-listed waterbodies in Wisconsin

Waterbodies carrying the most listed impairment causes among the lake-type units reported by Wisconsin.

# Waterbody Listed causes
1 Petenwell Lake Not Supporting 5
2 Saratoga Lake Not Supporting 4
3 Lake Butte des Morts Not Supporting 3
3 Lake Monona Not Supporting 3
3 Lake Poygan Not Supporting 3
3 Lake Superior Not Supporting 3
3 Lake Wingra Not Supporting 3
3 Lake Winneconne Not Supporting 3
3 Lake Wisconsin Not Supporting 3
3 Neshonoc Lake Not Supporting 3

8 of the 10 rows in this table tie on the ranked value: 8 at 3 (rank 3). Tied rows share a rank and are listed in no meaningful order; where a tie reaches the top the medal is shared, because nothing separates those rows.

Gauge network

Lakes with a daily level record in Wisconsin

Lakes with a linked USGS gauge and a daily water-level record. Readings are daily-batch snapshots, not real-time.

Wisconsin on the national scale-vs-impaired-share map

Assessed-corpus size does not predict impaired share: a state with a large register can still post a low not-supporting rate, and a small register can post a high one.

Source: EPA ATTAINS, states above the sample floor 2×2 strategic matrix plotting 36 entities by Definitive verdicts (assessed corpus) (X) and Impaired share (%) (Y), with a crosshair dividing the plot into four quadrants. Large corpus, high shareSmall corpus, high shareLarge corpus, low shareSmall corpus, low share 01,0002,0003,0004,0005,000 0%20%40%60%80%100%120% Definitive verdicts (assessed corpus) Impaired share (%) New Hampshire · 2,783, 100%Washington · 393, 100%New Jersey · 604, 96.5%Massachusetts · 718, 94.4%Oregon · 178, 92.7%California · 224, 90.2%New York · 642, 87.7%Illinois · 497, 87.3%Rhode Island · 140, 85%Oklahoma · 195, 78.5%Virginia · 231, 78.4%South Dakota · 186, 78%Kansas · 316, 76.3%Iowa · 203, 67.5%Florida · 1,003, 63.4%Nebraska · 352, 62.2%Indiana · 241, 60.2%North Carolina · 152, 59.2%Utah · 134, 57.5%Missouri · 267, 56.6%Kentucky · 129, 55.8%Colorado · 163, 54.6%Minnesota · 3,364, 53.4%Arkansas · 117, 53%Alabama · 273, 52.4%Georgia · 119, 39.5%South Carolina · 540, 39.3%Michigan · 2,339, 34.5%Connecticut · 139, 31.7%North Dakota · 176, 29.5%Texas · 433, 28.9%Pennsylvania · 516, 26.2%Vermont · 712, 21.6%Idaho · 237, 21.5%Wisconsin · 4,718, 10.4%WisconsinMaine · 303, 10.2%
Source: EPA ATTAINS, states above the sample floor
NameDefinitive verdicts (assessed corpus)Impaired share (%)
New Hampshire2,783100%
Washington393100%
New Jersey60496.5%
Massachusetts71894.4%
Oregon17892.7%
California22490.2%
New York64287.7%
Illinois49787.3%
Rhode Island14085%
Oklahoma19578.5%
Virginia23178.4%
South Dakota18678%
Kansas31676.3%
Iowa20367.5%
Florida1,00363.4%
Nebraska35262.2%
Indiana24160.2%
North Carolina15259.2%
Utah13457.5%
Missouri26756.6%
Kentucky12955.8%
Colorado16354.6%
Minnesota3,36453.4%
Arkansas11753%
Alabama27352.4%
Georgia11939.5%
South Carolina54039.3%
Michigan2,33934.5%
Connecticut13931.7%
North Dakota17629.5%
Texas43328.9%
Pennsylvania51626.2%
Vermont71221.6%
Idaho23721.5%
Wisconsin (this record)4,71810.4%
Maine30310.2%
Source: EPA ATTAINS, states above the sample floor

The crosshair sits at the median across the 36 qualifying states on each axis, not the range midpoint, so it splits the group evenly. Wisconsin is highlighted.

States near Wisconsin's lighter not-supporting load

Two EPA ATTAINS peer sets outside Wisconsin. The reporting-practice chart above stays on definitive-verdict share; these neighborhoods highlight register-size and not-supporting peers while Wisconsin's ruled impairment share sits in the lowest third.

States with a similar reported inventory

Nearest states by lake-type units on the register (19,871 here).

States with a similar not-supporting load

Nearest states by units assessed not supporting a use (493 here). Inventory peers above are omitted.

Peers are nearest-neighbor matches on EPA ATTAINS register size and not-supporting counts among states with at least 25 reported units. Impairment peers skip states already listed in the inventory column so the two link sets stay distinct.

Using this state picture

  • Comparisons across states hold on shares, not counts: 23.7% of the inventory reported from Wisconsin carries a definitive verdict, 43rd of 47 comparable states. National rankings
  • Fish-consumption verdicts here are assessment decisions. Species-level meal advice comes from the state fish-advisory program. Consumption guide
  • Browse every named Wisconsin waterbody with a populated record. Browse waterbodies

PlainLakes renders state assessment decisions and federal gauge observations without modification.

Figures on this page are computed from the assessment decisions Wisconsin reported to EPA's ATTAINS system and from USGS National Water Information System daily values, no number is typed in by an editor. State counts aggregate every lake-type assessment unit reported by Wisconsin, including unnamed units that never receive their own page. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of 2026-08-17. Primary sources: EPA ATTAINS and USGS National Water Information System.