Peterson Lake
Flathead County, MontanaView mapDirections
Not Rated: one monitoring station are on record, but they measured water clarity only. LakeQuality’s A–F method grades total phosphorus, total nitrogen, chlorophyll-a, and turbidity; clarity is shown, not scored.
- Limited data
Peterson Lake in Flathead County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 14.9 ft visibility.
- Clarity
- 14.9 ft
- Trend
- ↓ Declining
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Stays near Peterson Lake
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Watch Peterson Lake’s letter
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Why Peterson Lake has no grade
Peterson Lake has monitoring on record from one station, but only water clarity was measured. LakeQuality’s A–F letter uses total phosphorus, total nitrogen, chlorophyll-a, and turbidity against EPA National Lakes Assessment condition classes. With none of those grading measurements on record, Peterson Lake is Not Rated — not an F. Its 14.9-foot median from 3 readings collected 2020–2023 is still shown as clarity, but clarity alone cannot earn a letter.
Not Rated is EPA Integrated Reporting Category 3 — insufficient information. 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.
Peterson 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 2023-07-17. Grade methodology: LakeQuality methodology.
What Peterson Lake is good for
From access and monitoring records, not editorial picks.
- Clear water14.9 ft of visibility
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Peterson Lake
These readings exist but are not enough for an A–F grade — see why.
Crystal clear, you can see 14.9 ft down. TSI 38.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 14.9 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 38 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Fishing Peterson Lake
Water Quality Trend: ↓ Declining
Based on 12 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.212 m/yr | 3 |
| Phosphorus | ↓ Declining | +0.44 µg/L/yr | 11 |
| Chlorophyll-a | → Stable | -0.03 µg/L/yr | 12 |
Graded lakes near Peterson Lake
Peterson Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Flathead County
Frequently asked questions about Peterson Lake
Where is Peterson Lake?
- Peterson Lake is a lake in Flathead County, Montana. LakeQuality has not rated this lake (UNRATED) — the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity.
What is the water quality grade for Peterson Lake?
- Peterson Lake in Flathead County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 14.9 ft visibility.
Is Peterson Lake safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
Explore Peterson Lake across LakeQuality
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Where this data comes from
3 measurements from 1 monitoring station, 2020–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS3 samples · 2020–2023
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.