Linn County Lake Quality
Wisconsin, 4 lakes, average grade C (Fair)
Only 4 lakes in Linn County carry an active grade. The county-level summary is therefore representative of those specific lakes, not necessarily of every water body in the county. Linn County averages to a C. The distribution behind that average matters more than the letter: a few stressed lakes versus many decent ones can produce the same average as the inverse.
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. Brookfield Lake (B) is the cleanest in the county.
Quick Answers for Linn County
Planning a trip? Check special fishing regulations for Brookfield Lake, whether the fish are safe to eat, and the best times to fish — or browse the full Wisconsin regulations index.
All Lakes in Linn County
| Rank | Lake | Grade | Clarity | Max Depth | Phosphorus |
|---|---|---|---|---|---|
| 1 | Brookfield Lake | B | 4.4 ft | - | 19.7 µg/L |
| 2 | Jo Shelby Lake | C | - | - | 48 µg/L |
| 3 | Pershing State Park Lake | D | - | - | 75 µg/L |
| 4 | Marceline Lake | F | 1.9 ft | - | 110.9 µg/L |
Source: EPA National Aquatic Resource Surveys, 2026.