Big Lake vs Blackduck Lake
Water quality, depth, fish species, and recreation comparison.
Big Lake has a higher water quality grade (A, Excellent) than Blackduck Lake (D, Poor). Both are in St. Louis County, Minnesota.
Big Lake and Blackduck 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. The grades are meaningfully apart: Big Lake grades a A while Blackduck Lake grades a D. That two-letter spread reflects real underlying differences — different watershed inputs, different depth profiles, or different monitoring rigor.
For a recreational visitor, the wider grade gap is decisive — Big 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 Lake
Moderate clarity, visible to about 8.2 ft.
Blackduck Lake
Moderate clarity, visible to about 6.8 ft.
Side-by-Side Metrics
| Metric | Big Lake | Blackduck Lake |
|---|---|---|
| Overall Grade | A (Excellent) | D (Poor) |
| Water Clarity | 8.2 ft | 6.8 ft |
| Phosphorus | 14 µg/L | 43 µg/L |
| Chlorophyll-a (Algae) | 4.9 µg/L | 23.3 µg/L |
| Maximum Depth | 22 ft | 30 ft |
| Surface Area | 1,904 acres | 1,250 acres |
| Public Access | Yes | Yes |
| Fish Species | 8 | 8 |
| Trophic State | mesotrophic | eutrophic |
Bold value = better for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better).
Verdict
Big Lake wins on overall water quality with a Grade A versus Blackduck Lake's Grade D. Water clarity: 8.2 ft vs 6.8 ft. For fishing diversity, Big Lake also leads with 8 species.
Source: EPA National Aquatic Resource Surveys, 2026.