Updated Aug 2026
Illinois Lake Water Quality
Water quality grades for 148 monitored lakes across 45 counties.
Across Illinois, 148 lakes carry an active water-quality grade spread over 45 counties. The underlying measurements come from the Illinois Environmental Protection Agency, alongside federal programs and volunteer monitoring networks. A-grade lakes are scarce in Illinois: about 6% of the dataset. Most lakes here sit in C or D — productive, often shallow, frequently algal in late summer.
A meaningful share of the state's monitored 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. Winnebago ranks highest on average county grade (A). Marion sits lowest. With only 45 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
All 148 Illinois lakes in the index: 96 (64.9%) carry an A–F grade and 52 (35.1%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
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Top-Graded Illinois Lakes
The highest water-quality grades in Illinois — clarity, depth, and A-F grade at a glance.
Lake Map
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 68 reservoirs →68 of 148 Illinois lakes (46%) 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 116 impaired lakes →116 of 148 Illinois lakes (78%) 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 45 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 |
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
The top-graded lake in every state we cover, picked grade-first with water clarity as the tie-breaker — plus the states whose winner is still not clean.
State-by-state Secchi champions: the clearest measured lake in each U.S. state we grade, which is not the same as the cleanest nutrient grade.
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.