Mcgilvray Lake
Flathead County, MontanaView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from A at the last regrade
- Oligotrophic
- #25 of 44 in the county
Mcgilvray Lake in Flathead County, Montana has a water quality grade of C (Fair). Phosphorus: 5.9 µg/L. Chlorophyll-a (algae): 1.7 µg/L.
- Type
- Oligotrophic
- Phosphorus
- 5.9 µg/L
- Chlorophyll-a
- 1.7 µg/L
- Trend
- ↓ Declining
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Watch Mcgilvray Lake’s letter
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Mcgilvray Lake is a cold, clear oligotrophic lake in Flathead County, Montana. Water quality is graded C 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, Mcgilvray Lake ranks #25 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2023-08-03. 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 5.9 µg/L. Chlorophyll-a 1.7 µg/L. TSI 33. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 5.9 µg/L | Good |
| Chlorophyll-a (Algae) | 1.7 µg/L | Good |
| Trophic State Index (TSI) | 33 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Fishing Mcgilvray Lake
Water Quality Trend: ↓ Declining
Based on 11 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | +0.011 m/yr | 3 |
| Phosphorus | ↓ Declining | +0.81 µg/L/yr | 10 |
| Chlorophyll-a | ↓ Declining | +0.19 µg/L/yr | 11 |
County Ranking
Ranked #25 of 44 lakes in Flathead County
Nearby Lakes in Flathead County
Frequently asked questions about Mcgilvray Lake
Where is Mcgilvray Lake?
- Mcgilvray Lake is a lake in Flathead County, Montana. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Mcgilvray Lake?
- Mcgilvray Lake in Flathead County, Montana has a water quality grade of C (Fair). Phosphorus: 5.9 µg/L. Chlorophyll-a (algae): 1.7 µg/L.
Is Mcgilvray Lake safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Explore Mcgilvray Lake across LakeQuality
Species activity outlook by water temperature and season
Where to stay at Montana's top lakes, each with its grade
Low nutrients, clear water. The letter is not a swimming clearance.
Montana's cleanest and most-polluted lakes, ranked
More Montana Lake Rankings
Other Grade C Lakes in Montana
Oligotrophic Lakes in Montana
Where this data comes from
25 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_WQX25 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.