St. Clair County Lake Quality
Illinois, 1 lakes, average grade F (Very Poor)
Only 1 lakes in St. Clair 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 F — the most-stressed bracket in our rubric. Most monitored lakes here show persistent nutrient loading and algal pressure.
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. Slm Side Channel Lake (F) is the cleanest in the county.
Quick Answers for St. Clair County
Planning a trip? Check special fishing regulations for Slm Side Channel Lake — or check the official Illinois fishing regulations.
All Lakes in St. Clair County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Slm Side Channel Lake | F | 0.8 ft | 275.5 µg/L |
Other Counties in Illinois
See how St. Clair County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 15 states.
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Source: EPA National Aquatic Resource Surveys, 2026.