Monona County Lake Quality
Wisconsin, 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, whether the fish are safe to eat, and the best times to fish — or browse the full Wisconsin regulations index.
All Lakes in Monona County
| Rank | Lake | Grade | Clarity | Max Depth | Phosphorus |
|---|---|---|---|---|---|
| 1 | Oldham Lake | C | 5.1 ft | - | - |
| 2 | Blue Lake | F | 0.9 ft | - | - |
Source: EPA National Aquatic Resource Surveys, 2026.