St. Louis County Lake Quality
Missouri, 7 lakes, average grade F (Algae-prone)
St. Louis County holds 7 monitored lakes. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. 5 of those lakes carry an A–F grade. The county-level summary describes that subset, not every water body listed. 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. Simpson Park Lake (D) is the cleanest in the county.
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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
7 lakes · 5 graded, 2 not rated5 graded lakes · 2 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | DSimpson Park LakeSt. Louis | D | 1.9 ft | 72.3 µg/L | 23.8 µg/L | No recent samples | ✓ Public |
| 2 | FMallard LakeSt. Louis | F | 1.9 ft | 84 µg/L | 32.2 µg/L | No recent samples | Not recorded |
| 3 | FCreve Coeur LakeSt. Louis | F | 1.6 ft | 101 µg/L | 35.4 µg/L | No recent samples | Not recorded |
| 4 | FKoeneman Park LakeSt. Louis | F | 3.1 ft | 270.8 µg/L | 35.9 µg/L | No recent samples | Not recorded |
| 5 | FSpanish LakeSt. Louis | F | 1.6 ft | 347.8 µg/L | 62.6 µg/L | No recent samples | ✓ Public |
| NRBee Tree LakeSt. Louis | NR | No recent samples | No recent samples | No recent samples | No recent samples | ✓ Public | |
| NRTilles Park ReservoirSt. Louis | NR | No recent samples | 434.5 µg/L | 66.5 µg/L | No recent samples | Not recorded |
NR = not rated: there is not enough recent sampling to support a letter. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.
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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 United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs..
Source: EPA National Aquatic Resource Surveys, 2026.