Big Carnelian Lake vs St. Croix Lake
Water quality, depth, fish species, and recreation comparison.
Big Carnelian Lake has a higher water quality grade (A, Excellent) than St. Croix Lake (C, Fair). Both are in Washington County, Minnesota.
Big Carnelian Lake and St. Croix Lake are both in Minnesota — a same-state head-to-head where the comparison comes down to lake-specific differences in depth, watershed, and monitoring history rather than the broader state-level water-quality regime. Big Carnelian Lake (A) is materially cleaner than St. Croix Lake (C). 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 — Big Carnelian 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?
Big Carnelian Lake
Crystal clear, you can see 15 ft down.
St. Croix Lake
Moderate clarity, visible to about 8.7 ft.
Side-by-Side Metrics
| Metric | Big Carnelian Lake | St. Croix Lake |
|---|---|---|
| Overall Grade | A (Excellent) | C (Fair) |
| Water Clarity | 15 ft | 8.7 ft |
| Phosphorus | No data | No data |
| Chlorophyll-a (Algae) | No data | No data |
| Maximum Depth | 66 ft | 78 ft |
| Surface Area | 457.03 acres | 8.4K acres |
| Public Access | Yes | No |
| Fish Species | 12 | 36 |
| Trophic State | oligotrophic | mesotrophic |
Bold value = better for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better).
Verdict
Big Carnelian Lake wins on overall water quality with a Grade A versus St. Croix Lake's Grade C. Water clarity: 15 ft vs 8.7 ft. For more fish-species variety, St. Croix Lake edges ahead with 36 documented species.