Big Sky Lake
Missoula County, MontanaView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from A at the last regrade
- Mesotrophic
- #33 of 40 in the county
Big Sky Lake in Missoula County, Montana has a water quality grade of C (Fair). Phosphorus: 10 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
- Type
- Mesotrophic
- Phosphorus
- 10 µg/L
- Chlorophyll-a
- 2.1 µg/L
- Trend
- → Stable
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Watch Big Sky Lake’s letter
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Big Sky Lake is a mesotrophic lake in Missoula County, Montana. Water quality is graded C based on 3 sampling years through 2025. Phosphorus level is declining over recent sampling years. Compared to the 39 other monitored lakes in Missoula County, Big Sky Lake ranks #33 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2025-09-09. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyModerate
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.
The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: C (Fair)
No clarity data. Phosphorus 10 µg/L. Chlorophyll-a 2.1 µg/L. TSI 38. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 10 µg/L | Good |
| Chlorophyll-a (Algae) | 2.1 µg/L | Fair |
| Trophic State Index (TSI) | 38 | Mesotrophic |
Moderate nutrients, good water quality
Fishing Big Sky Lake
Water Quality Trend: → Stable
Based on 12 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.026 m/yr | 8 |
| Phosphorus | ↓ Declining | +24.59 µg/L/yr | 4 |
| Chlorophyll-a | ↑ Improving | -1 µg/L/yr | 2 |
County Ranking
Ranked #33 of 40 lakes in Missoula County
Nearby Lakes in Missoula County
Frequently asked questions about Big Sky Lake
Where is Big Sky Lake?
- Big Sky Lake is a lake in Missoula County, Montana. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Big Sky Lake?
- Big Sky Lake in Missoula County, Montana has a water quality grade of C (Fair). Phosphorus: 10 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
Is Big Sky Lake safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Explore Big Sky Lake across LakeQuality
More Montana Lake Rankings
Other Grade C Lakes in Montana
Mesotrophic Lakes in Montana
Where this data comes from
29 measurements from 1 monitoring station, 2023–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana Volunteer Water Quality Monitoring
MTVOLWQM_WQX29 samples · 2023–2025
Analytical methods
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
- 4500 N C ~ Persufate Method for Total Nitrogen
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.