Union County Lake Quality
Iowa, 6 lakes, average grade F (Algae-prone)
Union County holds 6 monitored lakes. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. Every one of those lakes carries an A–F grade, so the county-level summary covers the full set rather than a monitored subset. 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 Three Mile Lake (D); the most-stressed is Twelve Mile Lake (F).
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Quick Answers for Union County
Planning a trip? Check special fishing regulations for Three Mile Lake — or check the official Iowa fishing regulations.
All Lakes in Union County
6 lakes6 graded lakes
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | DThree Mile LakeUnion | D | 2.7 ft | 43 µg/L | 28.4 µg/L | No recent samples | ✓ Public |
| 2 | FTwelve Mile LakeUnion | F | 2.4 ft | 79 µg/L | 45 µg/L | No recent samples | ✓ Public |
| 3 | FGreen Valley LakeUnion | F | 1.5 ft | 103 µg/L | 61.7 µg/L | No recent samples | ✓ Public |
| 4 | FSummit LakeUnion | F | 1.4 ft | 121 µg/L | 45.7 µg/L | No recent samples | ✓ Public |
| 5 | FMckinley LakeUnion | F | 1 ft | 150 µg/L | 69.3 µg/L | No recent samples | Not recorded |
| 6 | FThayer LakeUnion | F | 3.5 ft | 68 µg/L | 34.3 µg/L | No recent samples | Not recorded |
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Other Counties in Iowa
See how Union 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.