O'Brien County Lake Quality
Iowa, 2 lakes, average grade D (Nutrient-rich)
O'Brien County has 2 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. Every one of those lakes carries an A–F grade, so the county-level summary covers the full set rather than a monitored subset. On average, O'Brien County lakes grade D, with phosphorus loading and reduced clarity showing up across most of the dataset.
Most of the county's monitored lakes are shallow — under 50 feet — which means wind keeps the water column mixed all summer and sediment-bound phosphorus releases back into the surface layer whenever the bottom is disturbed. Dog Creek Lake (D) is the cleanest in the county.
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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
2 lakes2 graded lakes
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | DDog Creek LakeO'Brien | D | 3.4 ft | 58 µg/L | 25 µg/L | No recent samples | Not recorded |
| 2 | DMill Creek LakeO'Brien | D | 3 ft | 62 µg/L | 28.4 µg/L | No recent samples | Not recorded |
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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 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.