Some of these decisions are not final. 124 of these 783 records carry a verdict Illinois decided in its 2026 Integrated Report, which is open for public comment, so they can change before the cycle is submitted to EPA. They are counted in every figure on this page.
Some of these decisions are EPA's. 1 of these 783 records carries a Clean Water Act cleanup plan EPA established rather than Illinois. They are counted in every figure on this page, and each record page names which.
State water-quality picture · #11 by reported lake-type waterbodies
Illinois lakes and reservoirs
Every figure on this page is computed from the assessment decisions Illinois reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 1988–2026), plus daily water-level observations from the U.S. Geological Survey.
- 783
- Reported waterbodies
- 434
- Not supporting a use
- 215
- Fish use not supporting
- 0
- Gauged lakes
Illinois in one line
783 lake-type waterbodies in Illinois are reported to EPA ATTAINS across reporting cycles 1988–2026: 434 assessed as not supporting at least one designated use, 63 fully supporting, and 286 without a definitive overall verdict. The most-listed impairment cause is Phosphorus, Total, on 303 waterbodies.
- 783
- reported waterbodies
- 434
- not supporting a use
- 63
- fully supporting
- 87%
- of definitive verdicts impaired
An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.
Illinois's large register still carries a heavy not-supporting share
Among the largest third of state lake-type registers (783 units), Illinois also sits in the upper third for ruled not-supporting share (87%). The size and the load move together here, not every large inventory does.
Data updated 2026-08-17
According to the U.S. Environmental Protection Agency's ATTAINS program, Illinois reported 783 lake-type waterbodies in Illinois in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.
How Illinois's large inventory splits across overall status
Each of the 783 lake-type units reported by Illinois 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 Illinois
Waterbodies carrying each listed cause, most first
- Phosphorus, Total
Phosphorus, Total
303 waterbodies
- Mercury
Mercury
206 waterbodies
- Total Suspended Solids (TSS)
Total Suspended Solids (TSS)
171 waterbodies
- Polychlorinated Biphenyls (PCBs) 89
Polychlorinated Biphenyls (PCBs)
89 waterbodies
- Aquatic Plants (Macrophytes) 64
Aquatic Plants (Macrophytes)
64 waterbodies
What this shows Phosphorus, Total is the most-listed impairment cause in Illinois, on 303 of the lake-type waterbodies reported there.
Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.
Illinois: 87% of ruled verdicts are not-supporting, against a national share of 53%, 1.6× the national rate.
Reporting practice
How much of the Illinois inventory carries 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.
Illinois holds the 11th-largest reported lake-type inventory in the country, 783 assessment units, carrying a definitive verdict on 63.5% of it. The counts above describe the 497 units carrying a verdict, not all 783 reported. Illinois has also retired 5 lake-type assessment units, superseded filings that are kept on the site but not counted here.
The states reporting at the nearest rates are Nebraska (63.5%) and Tennessee (65%).
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.
Illinois against every comparable state
Share of reported lake-type waterbodies carrying a definitive verdict
63.5% 26th of 47 higher than 20 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 Illinois
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 Michigan Nearshore Not Supporting | 125,440 |
| 2 | Carlyle Not Supporting | 24,580 |
| 3 | Rend Not Supporting | 18,900 |
| 4 | Shelbyville Not Supporting | 11,000 |
| 5 | Crab Orchard Not Supporting | 6,965 |
| 6 | Clinton Not Supporting | 4,895 |
| 7 | Springfield Not Supporting | 4,000 |
| 8 | Thompson Not Supporting | 3,783 |
| 9 | Chautauqua Not Assessed | 3,562 |
| 10 | Spring (Carroll) Not Assessed | 3,550 |
| 11 | Kinkaid Not Supporting | 3,475 |
| 12 | Senachwine Not Supporting | 3,324 |
By cause count
Most-listed waterbodies in Illinois
Waterbodies carrying the most listed impairment causes among the lake-type units reported by Illinois.
| # | Waterbody | Listed causes |
|---|---|---|
| 1 | Depue Not Supporting | 17 |
| 2 | Campus Not Supporting | 10 |
| 2 | Fox Not Supporting | 10 |
| 2 | Frank Holten 1 Not Supporting | 10 |
| 2 | Grass Not Supporting | 10 |
| 6 | Evergreen Not Supporting | 9 |
| 6 | Horseshoe (Madison) Not Supporting | 9 |
| 6 | Lake Michigan Nearshore Not Supporting | 9 |
| 6 | Marion Not Supporting | 9 |
| 6 | Sam Parr Not Supporting | 9 |
9 of the 10 rows in this table tie on the ranked value: 4 at 10 (rank 2), 5 at 9 (rank 6). 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.
States near Illinois's heavier not-supporting load
Two EPA ATTAINS peer sets outside Illinois. The reporting-practice chart above stays on definitive-verdict share; the neighborhoods below emphasize cross-state nearest neighbors when the not-supporting count is in the upper third.
States with a similar reported inventory
Nearest states by lake-type units on the register (783 here).
- Washington · 832 units · 49 off
- Vermont · 712 units · 71 off
- Idaho · 630 units · 153 off
- Missouri · 623 units · 160 off
Similar not-supporting load
Nearest states by units assessed not supporting a use (434 here). Inventory peers above are omitted.
- Wisconsin · 493 not supporting · 59 off
- New York · 563 not supporting · 129 off
- New Jersey · 583 not supporting · 149 off
- Kansas · 241 not supporting · 193 off
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: 63.5% of the inventory reported from Illinois carries a definitive verdict, 26th 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 Illinois 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 Illinois 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 Illinois, 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.