Lost Loon Lake
Flathead County, MontanaView mapDirections
Not enough samples to grade — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). No letter yet. Here is what we do have.
- Limited data
Lost Loon Lake in Flathead County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Phosphorus: 10.1 µg/L. Chlorophyll-a (algae): 2.6 µg/L.
- Phosphorus
- 10.1 µg/L
- Chlorophyll-a
- 2.6 µg/L
- Trend
- ↓ Declining
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Stays near Lost Loon Lake
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Top-rated places to stay near Lost Loon Lake
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Watch Lost Loon Lake’s letter
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Why Lost Loon Lake has no grade
We grade a lake only when the monitoring record supports a letter: at least three phosphorus or chlorophyll-a samples in the last ten years, scored against EPA National Lakes Assessment thresholds for this lake’s ecoregion. For Lost Loon Lake, no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). Sampling here has recorded phosphorus and algae (chlorophyll-a), but not water clarity.
Not Rated is not an F. It is EPA Integrated Reporting Category 3 — insufficient information. The water here may be excellent or poor; nobody has measured enough of it to say. Everything below that has been measured is still reported on this page.
Sampling is run by Montana DEQ and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Lost Loon Lake is a lake in Flathead County, Montana. Overall water quality is declining over recent sampling years.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2023-08-07. Grade methodology: LakeQuality methodology.
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Lost Loon Lake
These readings exist but are not enough for an A–F grade — see why.
No clarity data. Phosphorus 10.1 µg/L. Chlorophyll-a 2.6 µg/L. TSI 39.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 10.1 µg/L | Not graded |
| Chlorophyll-a (Algae) | 2.6 µg/L | Not graded |
| Trophic State Index (TSI) | 39 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Fishing Lost Loon Lake
Water Quality Trend: ↓ Declining
Based on 12 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.054 m/yr | 3 |
| Phosphorus | ↓ Declining | +0.32 µg/L/yr | 11 |
| Chlorophyll-a | ↓ Declining | +0.04 µg/L/yr | 12 |
Graded lakes near Lost Loon Lake
Lost Loon Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Flathead County
Frequently asked questions about Lost Loon Lake
Where is Lost Loon Lake?
- Lost Loon Lake is a lake in Flathead County, Montana. LakeQuality has not rated this lake (UNRATED) — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years).
What is the water quality grade for Lost Loon Lake?
- Lost Loon Lake in Flathead County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Phosphorus: 10.1 µg/L. Chlorophyll-a (algae): 2.6 µg/L.
Is Lost Loon Lake safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
Explore Lost Loon Lake across LakeQuality
Planning a trip around Glacier?
Lost Loon Lake is about 32 miles from Glacier National Park. Crowds there swing hard by season, so the month you pick changes both how busy the park is and what lodging costs.
Best time to visit Glacier →More Montana Lake Rankings
Other Lakes in Montana
Oligotrophic Lakes in Montana
Where this data comes from
6 measurements from 1 monitoring station, 2022–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana Watershed
MTWTRSHD_WQX6 samples · 2022–2023
Analytical methods
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 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.