O'Brien County Lake Quality
Iowa, 2 lakes, average grade D (Poor)
O'Brien 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, O'Brien 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 Dog Creek Lake (D); the most-stressed is Mill Creek Lake (F).
Quick Answers for O'Brien County
Planning a trip? Check special fishing regulations for Dog Creek Lake — or check the official Iowa fishing regulations.
All Lakes in O'Brien County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Dog Creek Lake | D | 3.4 ft | - |
| 2 | Mill Creek Lake | F | 3 ft | - |
Other Counties in Iowa
See how O'Brien 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 O'Brien County
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Source: EPA National Aquatic Resource Surveys, 2026.