Randolph County Lake Quality
Illinois, 3 lakes, average grade D (Poor)
Randolph County has 3 graded lakes — a thin enough sample that one or two stressed lakes can move the county average noticeably. Use the per-lake pages below for what actually matters. The county-wide average grade is D — water quality issues are persistent across most of the monitored basins. 1 lakes fall into the F bracket outright.
The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Randolph Lake (D); the most-stressed is Coulterville Lake (F).
Quick Answers for Randolph County
Planning a trip? Check special fishing regulations for Randolph Lake — or check the official Illinois fishing regulations.
All Lakes in Randolph County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Randolph Lake | D | 1.7 ft | 49 µg/L |
| 2 | Sparta Northwest Lake | D | 2.6 ft | 34 µg/L |
| 3 | Coulterville Lake | F | 2.7 ft | 426 µg/L |
Other Counties in Illinois
See how Randolph 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 Randolph County
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Source: EPA National Aquatic Resource Surveys, 2026.