Monona County Lake Quality
Iowa, 2 lakes, average grade D (Poor)
Monona County has 2 graded 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. On average, Monona County lakes grade D, with phosphorus loading and reduced clarity showing up across most of the dataset.
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 Oldham Lake (C); the most-stressed is Blue Lake (F).
Quick Answers for Monona County
Planning a trip? Check special fishing regulations for Oldham Lake — or check the official Iowa fishing regulations.
All Lakes in Monona County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Oldham Lake | C | 5.1 ft | - |
| 2 | Blue Lake | F | 0.9 ft | - |
Other Counties in Iowa
See how Monona County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 15 states.
Where to stay near Monona County
Top-rated places to stay near Monona County
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Source: EPA National Aquatic Resource Surveys, 2026.