Mercer County Lake Quality
Missouri, 4 lakes, average grade A (Excellent)
Mercer County holds 4 monitored lakes. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. 1 of those lake carry an A–F grade. The county-level summary describes that subset, not every water body listed. On average, Mercer County lakes grade an A. That puts it near the top of the Missouri county ranking.
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 Lake Marie (A); the most-stressed is Lake Marie (A).
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Quick Answers for Mercer County
Planning a trip? Check special fishing regulations for Lake Marie — or check the official Missouri fishing regulations.
All Lakes in Mercer County
4 lakes · 1 graded, 3 not rated1 graded lake · 3 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | ALake MarieMercer | A | 7.8 ft | 13 µg/L | 2.7 µg/L | No recent samples | Not recorded |
| NRLake PahoMercer | NR | 2.2 ft | 37 µg/L | 48.2 µg/L | No recent samples | ✓ Public | |
| NRTwin LakeMercer | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRBallast Pit LakeMercer | NR | No recent samples | No recent samples | No recent samples | 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 Mercer 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.