Johnson County Lake Quality
Illinois, 2 lakes, average grade C (Fair)
Only 2 lakes in Johnson County carry an active grade. The county-level summary is therefore representative of those specific lakes, not necessarily of every water body in the county. The county average is C — the middle of the rubric, with one or two lakes pulling the average up or down depending on year.
Most of the county's monitored lakes are shallow — under 50 feet — which means wind keeps the water column mixed all summer and sediment-bound phosphorus releases back into the surface layer whenever the bottom is disturbed. Vienna Correctional Center Lake (C) is the cleanest in the county.
Quick Answers for Johnson County
Planning a trip? Check special fishing regulations for Vienna Correctional Center Lake — or check the official Illinois fishing regulations.
All Lakes in Johnson County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Vienna Correctional Center Lake | C | 4.5 ft | 20 µg/L |
| 2 | Dutchman Lake | D | 2.1 ft | 37.5 µg/L |
Other Counties in Illinois
See how Johnson County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 15 states.
Where to stay near Johnson County
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Source: EPA National Aquatic Resource Surveys, 2026.