Lake Koshkonong vs Rock Lake
Water quality, depth, fish species, and recreation comparison.
Rock Lake has a higher water quality grade (B, Good) than Lake Koshkonong (D, Poor). Both are in Jefferson County, Wisconsin.
Both Lake Koshkonong and Rock Lake sit in Wisconsin. A same-state comparison strips out the state-level water-quality regime as a variable: any grade differences here are about the lakes themselves, not the agencies grading them. Rock Lake (B) is materially cleaner than Lake Koshkonong (D). A gap that wide is unlikely to close in a single year of remediation work; it reflects multi-decade differences in the lakes themselves.
For a recreational visitor, the wider grade gap is decisive — Rock Lake is the better water-quality choice. For a researcher, the gap is the interesting part: what is different about the two watersheds, and which of those differences is mutable?
Lake Koshkonong
Very murky, less than 1.7 ft of visibility.
Rock Lake
Good clarity, visible to about 12 ft.
Side-by-Side Metrics
| Metric | Lake Koshkonong | Rock Lake |
|---|---|---|
| Overall Grade | D (Poor) | B (Good) |
| Water Clarity | 1.7 ft | 12 ft |
| Phosphorus | 287 µg/L | 18.1 µg/L |
| Chlorophyll-a (Algae) | No data | No data |
| Maximum Depth | 7 ft | 60 ft |
| Surface Area | 10,595 acres | 1,365 acres |
| Public Access | Yes | Yes |
| Fish Species | 7 | 5 |
| Trophic State | hypereutrophic | mesotrophic |
Bold value = better for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better).
Verdict
Rock Lake wins on overall water quality with a Grade B versus Lake Koshkonong's Grade D. Water clarity: 12 ft vs 1.7 ft. For more fish-species variety, Lake Koshkonong edges ahead with 7 documented species.
Source: EPA National Aquatic Resource Surveys, 2026.