Union County Lake Quality
Ohio, 1 lakes, average grade A (Excellent)
Only 1 lakes in Union 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. On average, Union County lakes grade an A. That puts it near the top of the Ohio county ranking.
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. Marysville Upground Reservoir (A) is the cleanest in the county.
Quick Answers for Union County
Planning a trip? Check special fishing regulations for Marysville Upground Reservoir — or check the official Ohio fishing regulations.
All Lakes in Union County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Marysville Upground Reservoir | A | - | 39.7 µg/L |
Other Counties in Ohio
See how Union County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 36 states and 2 Canadian provinces.
Where to stay near Union County
Top-rated places to stay near Union County
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Source: EPA National Aquatic Resource Surveys, 2026.