Big Thunder Lake
Big Thunder Lake in Beltrami County, Minnesota has a water quality grade of A (Excellent). Water clarity: 11.5 ft visibility. Phosphorus: 12 µg/L. Chlorophyll-a (algae): 3.7 µg/L.
- Clarity
- 11.5 ft
- Typical in August
- 72°F
- Mean depth
- ~13 ft est.
- Type
- Mesotrophic
- Trend
- → Stable
Next lake in Beltrami County
Big Thunder Lake is a mesotrophic lake in Beltrami County, Minnesota with an estimated average depth of about 13 feet. Water quality is graded A based on 2 sampling years through 2025. Water clarity is declining over recent sampling years. Compared to the 84 other monitored lakes in Beltrami County, Big Thunder Lake ranks #49 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-10. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLow algae
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A (Excellent)
Good clarity, visible to about 11.5 ft. Phosphorus level: 12 µg/L. Chlorophyll-a: 3.7 µg/L. Trophic State Index: 42.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 11.5 ft | Not graded |
| Phosphorus | 12 µg/L | Good |
| Chlorophyll-a (Algae) | 3.7 µg/L | Good |
| Trophic State Index (TSI) | 42 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Big Thunder Lake
Compare lakeside cabins, hotels, and vacation rentals near Big Thunder Lake in Beltrami County, Minnesota — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near Big Thunder Lake
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Water Temperature
Big Thunder Lake typically sits around 72°F at the surface in August. Summer average is about 73°F. Usually warmest in Jul (about 76°F) and coldest in Feb (about 34°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 58 field readings (2010–2025) via the EPA Water Quality Portal. See the full Big Thunder Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~13 ft est. |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Minnesota has not published bathymetry for Big Thunder Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing Big Thunder Lake
Water Quality Trend: → Stable
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.586 m/yr | 6 |
| Phosphorus | ↑ Improving | -0.33 µg/L/yr | 6 |
| Chlorophyll-a | → Stable | +0.03 µg/L/yr | 6 |
County Ranking
Ranked #49 of 85 lakes in Beltrami County
Nearby Lakes in Beltrami County
Frequently asked questions about Big Thunder Lake
What is the water quality grade for Big Thunder Lake?
- Big Thunder Lake in Beltrami County, Minnesota has a water quality grade of A (Excellent). Water clarity: 11.5 ft visibility. Phosphorus: 12 µg/L. Chlorophyll-a (algae): 3.7 µg/L.
Is Big Thunder Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Big Thunder Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 3.7 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Big Thunder Lake?
- Big Thunder Lake typically sits around 72°F at the surface in August, from 58 EPA Water Quality Portal field readings (2010–2025). That is a typical value for the month, not a live reading.
How deep is Big Thunder Lake?
- No published bathymetric survey covers Big Thunder Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 13 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
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Where this data comes from
87 measurements from 1 monitoring station, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Red Lake DNR
REDLAKE_WQX87 samples · 2020–2025
Analytical methods
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- Turbidity by Nephelometry
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder