Foys Lake
Flathead County, MontanaView mapDirections
Montana DEQ publishes the water-quality measurements for Foys Lake. Here is what has been measured.
- Mesotrophic
Ask about Foys Lake
Answers come straight from the public measurements and survey records on this page.
Foys Lake in Flathead County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 15.8 µg/L. Chlorophyll-a (algae): 4.1 µg/L.
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Stays near Foys Lake
Compare lakeside cabins, hotels, and vacation rentals near Foys Lake in Flathead County, Montana — 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. The water-quality readings cover nutrients and algae, not lodging. Cabins and rentals around Foys Lake are listed below.
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Foys Lake is a mesotrophic lake in Flathead County, Montana. Overall water quality is declining over recent sampling years. Compared to the 43 other monitored lakes in Flathead County, Foys Lake ranks #38 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2023-08-09. Grade methodology: LakeQuality methodology.
Swimming conditions
Readings onlyElevated nutrients
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 Foys 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 Foys Lake
These are the measurements Montana DEQ publishes for Foys 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 15.8 µg/L. Chlorophyll-a 4.1 µg/L. TSI 44.
| Metric | Value | EPA condition class |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 15.8 µg/L | Good |
| Chlorophyll-a (Algae) | 4.1 µg/L | Poor |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Fishing Foys Lake
Water Quality Trend: ↓ Declining
Based on 11 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.123 m/yr | 5 |
| Phosphorus | ↓ Declining | +1.75 µg/L/yr | 10 |
| Chlorophyll-a | ↑ Improving | -0.48 µg/L/yr | 11 |
Nearby Lakes in Flathead County
Frequently asked questions about Foys Lake
Where is Foys Lake?
- Foys Lake is a lake in Flathead County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Foys Lake?
- Foys Lake in Flathead County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 15.8 µg/L. Chlorophyll-a (algae): 4.1 µg/L.
Is Foys Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Explore Foys Lake across LakeQuality
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Other Lakes in Montana
Mesotrophic Lakes in Montana
Where this data comes from
24 measurements from 1 monitoring station, 2012–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana Watershed
MTWTRSHD_WQX24 samples · 2012–2023
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.