St. Louis County Lake Quality
Missouri, 9 lakes, average grade F (Very Poor)
Only 9 lakes in St. Louis 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.
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 Simpson Park Lake (D); the most-stressed is Creve Coeur Lake (F).
Quick Answers for St. Louis County
Planning a trip? Check special fishing regulations for Simpson Park Lake — or check the official Missouri fishing regulations.
All Lakes in St. Louis County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Simpson Park Lake | D | 1.8 ft | 73 µg/L |
| 2 | Simpson Park Lake | D | 2.5 ft | 36 µg/L |
| 3 | Mallard Lake | F | 1.9 ft | 84 µg/L |
| 4 | Creve Coeur Lake | F | 1.6 ft | 101 µg/L |
| 5 | Koeneman Park Lake | F | 3.1 ft | 270.8 µg/L |
| 6 | Spanish Lake | F | 1.6 ft | 347.8 µg/L |
| 7 | Creve Coeur Lake Nr. Boat Ramp | F | 1.2 ft | 100 µg/L |
| 8 | Bee Tree Lake | F | 0.6 ft | 263 µg/L |
| 9 | Tilles Park Reservoir | F | - | 434.5 µg/L |
Other Counties in Missouri
See how St. Louis 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.