Updated Aug 2026
Illinois Lake Water Quality
Water quality grades for 96 of the 594 lakes we list here, across 96 counties.
Grade Distribution
All 594 Illinois lakes in the index: 96 (16.2%) carry an A–F grade and 498 (83.8%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
Top-Graded Illinois Lakes
The highest water-quality grades in Illinois — clarity, depth, and A-F grade at a glance.
Lake Map
Get Illinois water-quality updates
New grades, clarity readings, and algae advisories for Illinois lakes as samples come in. Unsubscribe anytime.
Where to stay near Illinois's top-graded lakes
Cabins, resorts, and vacation rentals around Lake Opeka and the other top-graded Illinois lakes pinned on the map. Live availability and prices.
Top-rated places to stay near Lake Opeka
Prices for Sep 11–13, 4 adults. Change dates on the map below.
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Explore Illinois Lakes
Illinois reports, fishing & travel
Illinois's cleanest and most-polluted lakes, ranked A–F
Species activity outlook by water temperature and season
Where to stay at Illinois's top lakes, each with its grade
Best-graded lakes within 50 miles of Chicago
Best-graded lakes within 50 miles of Rockford
Reservoirs in Illinois
View all 67 reservoirs →67 of 594 Illinois lakes (11%) are man-made reservoirs in the USACE National Inventory of Dams. 52 carry a High or Significant hazard rating. The oldest matched dam dates to 1885.
EPA-Impaired Lakes in Illinois
View all 114 impaired lakes →114 of 594 Illinois lakes (19%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 56 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Illinois Fishing & Recreation
Licenses, regulations, records, and where to fish across Illinois.
Counties
All 96 Illinois counties, ranked by average lake water quality. Compare them against every county nationwide in the county rankings, browse the full directory of graded lakes, or pick a different state.
Illinois Lake Water Quality Dataset (2026)
Every graded Illinois lake in one spreadsheet — A–F grade, water clarity, phosphorus, chlorophyll-a, trophic state, size, public access, and a link to each lake's report card.
Top 25 Cleanest Lakes
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity | Trophic | Access |
|---|---|---|---|---|---|---|
| 1 | ALake Opeka | Cook | A | 4 ft | Mesotrophic | Not recorded |
| 2 | ALake Calumet | Cook | A | 4.1 ft | Mesotrophic | Not recorded |
| 3 | ASilver Lake | McHenry | A | 7 ft | Mesotrophic | ✓ Public |
| 4 | APowderhorn Lake | Cook | A | 10.1 ft | Mesotrophic | ✓ Public |
| 5 | ABaumann Park Lake | Winnebago | A | 10.2 ft | Mesotrophic | Not recorded |
| 6 | AGrays Lake | Lake | A | 3.5 ft | Mesotrophic | Not recorded |
| 7 | ACatherine Lake | Lake | A | 7.4 ft | Mesotrophic | ✓ Public |
| 8 | ACedar Lake | Lake | A | No recent samples | Mesotrophic | Not recorded |
| 9 | ADeep Lake | Lake | A | No recent samples | Mesotrophic | Not recorded |
| 10 | BAxehead Lake | Cook | B | 11.9 ft | Eutrophic | Not recorded |
| 11 | BBoulder South Lake | Perry | B | 8.8 ft | Eutrophic | ✓ Public |
| 12 | BWampum Lake | Cook | B | 6.9 ft | Eutrophic | Not recorded |
| 13 | BShabbona Lake | DeKalb | B | 4.9 ft | Eutrophic | ✓ Public |
| 14 | BBig Rock Lake | Kane | B | 8.1 ft | Eutrophic | Not recorded |
| 15 | BRound Lake | Lake | B | 7.4 ft | Eutrophic | ✓ Public |
| 16 | BChannel Lake | Lake | B | 8 ft | Eutrophic | ✓ Public |
| 17 | BZurich Lake | Lake | B | No recent samples | Eutrophic | Not recorded |
| 18 | BWest Loon Lake | Lake | B | No recent samples | Eutrophic | Not recorded |
| 19 | CGeorge Lake | Rock Island | C | 4.2 ft | Eutrophic | ✓ Public |
| 20 | CWayne City Lake | Wayne | C | 2.1 ft | Eutrophic | Not recorded |
| 21 | CWoods Creek Lake | McHenry | C | 3.9 ft | Eutrophic | Not recorded |
| 22 | CKinkaid Lake | Jackson | C | No recent samples | Eutrophic | ✓ Public |
| 23 | CTower Lake | Lake | C | No recent samples | Eutrophic | Not recorded |
| 24 | CEast Loon Lake | Lake | C | No recent samples | Eutrophic | Not recorded |
| 25 | DFlatfoot Lake | Cook | D | 6.2 ft | Eutrophic | ✓ Public |
25 Lakes Needing Attention
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | FIsland Lake | McHenry | F | No recent samples |
| 2 | FSlocum Lake | Lake | F | No recent samples |
| 3 | FLouise Lake | Lake | F | No recent samples |
| 4 | FValley Lake | Lake | F | No recent samples |
| 5 | FDiamond Lake | Lake | F | No recent samples |
| 6 | FMauvaise Terre Lake | Morgan | F | No recent samples |
| 7 | FVermont City Lake | McDonough | F | No recent samples |
| 8 | FLoveless Lake | Macoupin | F | No recent samples |
| 9 | FCharleston Side Channel Lake | Coles | F | No recent samples |
| 10 | FParadise Lake | Coles | F | No recent samples |
| 11 | FParadise Lake | Pike | F | No recent samples |
| 12 | FCoulterville Lake | Randolph | F | 2.7 ft |
| 13 | FKinmundy New Lake | Marion | F | 2.9 ft |
| 14 | FNippersink Lake | Lake | F | 1.3 ft |
| 15 | FKinmundy Old Lake | Marion | F | 1.5 ft |
| 16 | FCanton Lake | Fulton | F | 2.5 ft |
| 17 | FPistakee Lake | McHenry | F | 2.7 ft |
| 18 | FHighland Silver Lake | Madison | F | 1.3 ft |
| 19 | FJacksonville Lake | Morgan | F | 1.8 ft |
| 20 | FMarie Lake | Lake | F | 2.7 ft |
| 21 | FNashville City Lake | Washington | F | 1.3 ft |
| 22 | FGovernor Bond Lake | Bond | F | 1.2 ft |
| 23 | FNewton Lake | Jasper | F | 1.7 ft |
| 24 | FGrass Lake | Lake | F | 1.4 ft |
| 25 | FLou Yaeger Lake | Montgomery | F | 1.8 ft |
96 of the 594 Illinois lakes in this index carry an A–F water-quality grade, spread over 96 counties. Those grades rest on measurements from the Illinois Environmental Protection Agency, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, without enough recent nutrient or algae sampling behind them to support a letter. A-grade lakes are scarce among the Illinois lakes that carry a letter: about 9% of them. Most sit in C or D, productive, often shallow, frequently algal in late summer.
By surface area the largest lake here is Carlyle Lake, at about 24,580 acres, and 66 of the lakes in this index cover more than 500 acres. Size is worth knowing before you go: a large deep basin buffers nutrient inputs by sheer volume, while a shallow one warms quickly and can turn over its whole water column in a single summer storm. A meaningful share of the state's graded lakes register as hypereutrophic: heavily nutrient-loaded, frequent blooms, low summer transparency. These are usually shallow, surrounded by row-crop agriculture, with little volume to buffer the inputs. The cleanest county on average is Winnebago (grade A); the most-stressed is Marion. The gap between the top and bottom counties is large enough that a single figure for Illinois lake water quality hides real local variation.
Lake Match
Which lake suits how you actually use one?
Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
Source: EPA National Aquatic Resource Surveys and the EPA Water Quality Portal.
More on lake water quality
A data snapshot of the lakes we grade: how the A–F distribution breaks down, the clearest and the greenest states, and where the water is improving.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.
The cleanest and dirtiest U.S. lakes in the monitoring record, ranked on EPA nutrient and algae condition classes across thousands of graded lakes.
The clearest lakes in America, ranked by measured Secchi depth in feet from EPA monitoring data — and why the famously clear lakes are missing.