Union County Lake Quality
Wisconsin, 6 lakes, average grade F (Very Poor)
Only 6 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. 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. Three Mile Lake (D) is the cleanest in the county.
Quick Answers for Union County
Planning a trip? Check special fishing regulations for Three Mile Lake, whether the fish are safe to eat, and the best times to fish — or browse the full Wisconsin regulations index.
All Lakes in Union County
| Rank | Lake | Grade | Clarity | Max Depth | Phosphorus |
|---|---|---|---|---|---|
| 1 | Three Mile Lake | D | 2.7 ft | - | - |
| 2 | Thayer Lake | D | 3.5 ft | - | - |
| 3 | Twelve Mile Lake | F | 2.4 ft | - | - |
| 4 | Green Valley Lake | F | 1.5 ft | - | - |
| 5 | Summit Lake | F | 1.4 ft | - | - |
| 6 | Mckinley Lake | F | 1 ft | - | - |
Source: EPA National Aquatic Resource Surveys, 2026.