Big Sky Lake
Missoula County, MontanaView mapDirections
Montana DEQ publishes the water-quality measurements for Big Sky Lake. Here is what has been measured.
- Mesotrophic
Big Sky Lake in Missoula County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 10 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
Big Sky Lake is a mesotrophic lake in Missoula County, Montana. 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. Methodology: how these readings are compared.
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Swimming conditions
Readings onlyModerate algae
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Big Sky Lake is safe to swim in today is a call for the Montana Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Big Sky Lake
These are the measurements Montana DEQ publishes for Big Sky Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
No clarity data. Phosphorus 10 µg/L. Chlorophyll-a 2.1 µg/L. TSI 38.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | No reading How deep a black-and-white disk stays visible from the surface. Higher is clearer. | |
| Total phosphorus | 10µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 2.1µg/L | Fair GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 38 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
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Fishing Big Sky Lake
Water Quality Trend: Stable
Based on 12 years of monitoring (2012–2025).
Water clarity
Stable8 years
Phosphorus
Worsening+24.59 µg/L/yr4 years
Chlorophyll-a
Improving-1 µg/L/yr2 years
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. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Big Sky Lake?
- Big Sky Lake in Missoula County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 10 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
Is Big Sky Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Explore Big Sky Lake across LakeQuality
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.