Washington County Lake Quality
Illinois, 1 lakes, average grade F (Very Poor)
Washington County has 1 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 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. Nashville City Lake (F) is the cleanest in the county.
Quick Answers for Washington County
Planning a trip? Check special fishing regulations for Nashville City Lake — or check the official Illinois fishing regulations.
All Lakes in Washington County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Nashville City Lake | F | 1.3 ft | 517 µg/L |
Other Counties in Illinois
See how Washington 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 Washington County
Top-rated places to stay near Washington County
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Source: EPA National Aquatic Resource Surveys, 2026.