Birch Lake vs Thunder Lake
Water quality, depth, fish species, and recreation comparison.
Birch Lake and Thunder Lake both received the same overall water quality grade of A (Excellent). Both are in Cass County, Minnesota.
Both Birch Lake and Thunder Lake sit in Minnesota. 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. The grades are close: Birch Lake (A) and Thunder Lake (A) are within one letter of each other on the LakeGrade rubric. The per-parameter sub-grades below will show where the small differences actually live.
With grades this close, the choice between the two lakes turns on non-water-quality factors: depth, fish species, public access, distance from home. The per-lake pages below cover all of those.
Birch Lake
Good clarity, visible to about 14 ft.
Thunder Lake
Crystal clear, you can see 16.5 ft down.
Side-by-Side Metrics
| Metric | Birch Lake | Thunder Lake |
|---|---|---|
| Overall Grade | A (Excellent) | A (Excellent) |
| Water Clarity | 14 ft | 16.5 ft |
| Phosphorus | 12 µg/L | 10 µg/L |
| Chlorophyll-a (Algae) | 4 µg/L | 2.4 µg/L |
| Maximum Depth | 45 ft | 95 ft |
| Surface Area | 1,267 acres | 1,347 acres |
| Public Access | Yes | Yes |
| Fish Species | 14 | 17 |
| Trophic State | mesotrophic | oligotrophic |
Bold value = better for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better).
Verdict
Both lakes earn the same Grade A. The tiebreakers come down to clarity (Birch Lake: 14 ft, Thunder Lake: 16.5 ft) and what you want from the lake. Birch Lake has fewer fish species than Thunder Lake.
Source: EPA National Aquatic Resource Surveys, 2026.