Mahaska County Lake Quality
Iowa, 3 lakes, average grade D (Nutrient-rich)
Mahaska County has 3 monitored lakes — a thin enough sample that one or two stressed lakes can move the county average noticeably. Use the per-lake pages below for what actually matters. 2 of those lakes carry an A–F grade. The county-level summary describes that subset, not every water body listed. The county-wide average grade is D — water quality issues are persistent across most of the monitored basins. 1 lakes fall into the F bracket outright.
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 Hawthorn Lake (D); the most-stressed is White Oak Lake (F).
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Quick Answers for Mahaska County
Planning a trip? Check special fishing regulations for Hawthorn Lake — or check the official Iowa fishing regulations.
All Lakes in Mahaska County
3 lakes · 2 graded, 1 not rated2 graded lakes · 1 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | DHawthorn LakeMahaska | D | 2.4 ft | 60 µg/L | 27.5 µg/L | No recent samples | ✓ Public |
| 2 | FWhite Oak LakeMahaska | F | 1.4 ft | 125.5 µg/L | 68.7 µg/L | No recent samples | ✓ Public |
| NRLake Keomah BeachMahaska | NR | No recent samples | No recent samples | No recent samples | No recent samples | ✓ Public |
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 Iowa
See how Mahaska 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.