Foys Lake
Flathead County, MontanaView mapDirections
Nutrients and algae run above the regional average.
↓ down from A at the last regrade
- Mesotrophic
- #38 of 44 in the county
Foys Lake in Flathead County, Montana has a water quality grade of D (Nutrient-rich). Phosphorus: 15.8 µg/L. Chlorophyll-a (algae): 4.1 µg/L.
- Type
- Mesotrophic
- Phosphorus
- 15.8 µg/L
- Chlorophyll-a
- 4.1 µg/L
- Trend
- ↓ Declining
Stays via Stay22
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 grade measures nutrients and algae, not lodging — cabins and rentals around Foys Lake are listed below.
Top-rated places to stay near Foys Lake
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Foys Lake is a mesotrophic lake in Flathead County, Montana. Water quality is graded D based on 3 sampling years through 2023. 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 Safety
Grade onlyElevated nutrients
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 D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
No clarity data. Phosphorus 15.8 µg/L. Chlorophyll-a 4.1 µg/L. TSI 44. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| 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 |
County Ranking
Ranked #38 of 44 lakes in Flathead County
Nearby Lakes in Flathead County
Frequently asked questions about Foys Lake
Where is Foys Lake?
- Foys Lake is a lake in Flathead County, Montana. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Foys Lake?
- Foys Lake in Flathead County, Montana has a water quality grade of D (Nutrient-rich). Phosphorus: 15.8 µg/L. Chlorophyll-a (algae): 4.1 µg/L.
Is Foys Lake safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Explore Foys Lake across LakeQuality
More Montana Lake Rankings
Other Grade D 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)
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.