Marshall County Lake Quality
Iowa, 3 lakes, average grade B (Good)
Marshall 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 lands at B — solid water quality with most lakes carrying clear summer transparency and low-to-moderate phosphorus.
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 Sand Lake (A); the most-stressed is Green Castle Lake (C).
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Quick Answers for Marshall County
Planning a trip? Check special fishing regulations for Sand Lake — or check the official Iowa fishing regulations.
All Lakes in Marshall County
3 lakes · 2 graded, 1 not rated2 graded lakes · 1 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | ASand LakeMarshall | A | 6.4 ft | 25 µg/L | 3.6 µg/L | No recent samples | Not recorded |
| 2 | CGreen Castle LakeMarshall | C | 4.6 ft | 59 µg/L | 6.9 µg/L | No recent samples | Not recorded |
| NRCrab Tree LakeMarshall | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded |
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 Marshall 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.