Updated Jul 2026
Illinois Lake Water Quality
Water quality grades for 144 monitored lakes across 45 counties.
Across Illinois, 144 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 3% of the dataset. Most lakes here sit in C or D — productive, often shallow, frequently algal in late summer.
Eutrophic conditions dominate — nutrient-loaded, productive, with algal pressure that shows up most years in late summer. These lakes tend to be shallow, agricultural-watershed, and over-warmed by the time July ends. Winnebago ranks highest on average county grade (A). Sangamon 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
How Illinois’s 144 graded lakes break down from A (excellent) to F.
Top-Graded Illinois Lakes
The highest water-quality grades in Illinois — clarity, depth, and A-F grade at a glance.
Diversey Harbor Lake
Cook County, Illinois
See grade for Diversey Harbor Lake →Loch Lomond Lake
Lake County, Illinois
See grade for Loch Lomond Lake →Axehead Lake
Cook County, Illinois
See grade for Axehead Lake →Powderhorn Lake
Cook County, Illinois
See grade for Powderhorn Lake →Baumann Park Lake
Winnebago County, Illinois
See grade for Baumann Park Lake →Silver Lake
McHenry County, Illinois
See grade for Silver Lake →Silver Lake
McHenry County, Illinois
See grade for Silver Lake →Timber Lake
Lake County, Illinois
See grade for Timber Lake →Boulder South Lake
Perry County, Illinois
See grade for Boulder South Lake →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 71 reservoirs →71 of 144 Illinois lakes (49%) are man-made reservoirs in the USACE National Inventory of Dams. 55 carry a High or Significant hazard rating. The oldest matched dam dates to 1885.
EPA-Impaired Lakes in Illinois
View all 112 impaired lakes →112 of 144 Illinois lakes (78%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 52 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
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Diversey Harbor Lake | Cook | A | 25.6 ft |
| 2 | Loch Lomond Lake | Lake | A | 16.5 ft |
| 3 | Axehead Lake | Cook | A | 11.9 ft |
| 4 | Powderhorn Lake | Cook | A | 10.1 ft |
| 5 | Baumann Park Lake | Winnebago | A | 10.2 ft |
| 6 | Silver Lake | McHenry | B | 7 ft |
| 7 | Silver Lake | McHenry | B | 10 ft |
| 8 | Timber Lake | Lake | B | 10.6 ft |
| 9 | Boulder South Lake | Perry | B | 8.8 ft |
| 10 | Wampum Lake | Cook | B | 6.9 ft |
| 11 | Timber Lake | Carroll | B | 11.5 ft |
| 12 | Round Lake | Lake | B | 7.4 ft |
| 13 | Big Rock Lake | Kane | B | 8.1 ft |
| 14 | Kinkaid Lake | Jackson | B | - |
| 15 | Lake Calumet | Cook | B | - |
| 16 | Flatfoot Lake | Cook | B | 6.3 ft |
| 17 | Opeka Lake | Cook | B | 3.9 ft |
| 18 | One Horse Gap Lake | Pope | B | 4 ft |
| 19 | Glendale Lake | Pope | B | 4.8 ft |
| 20 | Tecumseh Lake | Hardin | B | 3.6 ft |
| 21 | Grays Lake | Lake | B | 3.5 ft |
| 22 | Calumet Lake | Cook | B | 4.3 ft |
| 23 | Lily Lake | McHenry | B | 3.3 ft |
| 24 | Catherine Lake | Lake | B | 7.4 ft |
| 25 | Icebox Quarry Lake | Cook | C | 9 ft |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Boujan Swale Lake | Cass | F | - |
| 2 | Quiver Lake | Mason | F | - |
| 3 | Mauvaise Terre Lake | Morgan | F | - |
| 4 | Vermont City Lake | McDonough | F | - |
| 5 | Paradise Lake | Coles | F | - |
| 6 | Paradise Lake | Pike | F | - |
| 7 | Coulterville Lake | Randolph | F | 2.7 ft |
| 8 | Kinmundy Old Lake | Marion | F | 1.5 ft |
| 9 | Borah Lake | Richland | F | 2.2 ft |
| 10 | Highland Silver Lake | Madison | F | 1.3 ft |
| 11 | Nashville City Lake | Washington | F | 1.3 ft |
| 12 | Governor Bond Lake | Bond | F | 1.2 ft |
| 13 | Decatur Lake | Macon | F | 1.3 ft |
| 14 | Lou Yaeger Lake | Montgomery | F | 1.8 ft |
| 15 | Mattoon Lake | Cumberland | F | 1.3 ft |
| 16 | Glenn Shoals Lake | Montgomery | F | 1 ft |
| 17 | Fairfield Lake | Wayne | F | 1.3 ft |
| 18 | Slm Side Channel Lake | St. Clair | F | 0.8 ft |
| 19 | Carlyle Lake | Clinton | F | 1.8 ft |
| 20 | Raccoon Lake | Marion | F | 1.2 ft |
| 21 | Sam Dale Lake | Wayne | F | 1.2 ft |
| 22 | Skokie Lagoons Lake | Cook | F | 1.6 ft |
| 23 | Holiday Shores Lake | Madison | F | 1.7 ft |
| 24 | Vandalia Lake | Fayette | F | 1.3 ft |
| 25 | Saganashkee Slough Lake | Cook | F | 0.8 ft |
Where to stay near Illinois
Top-rated places to stay near Illinois
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Source: EPA National Aquatic Resource Surveys, 2026.