Williamson Reservoir
Chouteau 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
Williamson Reservoir in Chouteau County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Phosphorus: 410 µg/L. Chlorophyll-a (algae): 6.7 µg/L.
- Phosphorus
- 410 µg/L
- Chlorophyll-a
- 6.7 µg/L
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Watch Williamson Reservoir’s letter
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Why Williamson Reservoir 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 Williamson Reservoir, 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.
Williamson Reservoir is a lake in Chouteau County, Montana.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2022-09-20. 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 Williamson Reservoir
These readings exist but are not enough for an A–F grade — see why.
No clarity data. Phosphorus 410 µg/L. Chlorophyll-a 6.7 µg/L. TSI 70.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 410 µg/L | Not graded |
| Chlorophyll-a (Algae) | 6.7 µg/L | Not graded |
| Trophic State Index (TSI) | 70 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Fishing Williamson Reservoir
Graded lakes near Williamson Reservoir
Williamson Reservoir has no grade, but these lakes within a short drive do.
Nearby Lakes in Chouteau County
Frequently asked questions about Williamson Reservoir
Where is Williamson Reservoir?
- Williamson Reservoir is a lake in Chouteau 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 Williamson Reservoir?
- Williamson Reservoir in Chouteau County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Phosphorus: 410 µg/L. Chlorophyll-a (algae): 6.7 µg/L.
Is Williamson Reservoir safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
Explore Williamson Reservoir across LakeQuality
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Where this data comes from
2 measurements from 1 monitoring station, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- USGS Montana Water Science Center
USGS-MT2 samples · 2022
Analytical methods
- P, wu, WCA, persulfate CF color
- Plant pigments, sonication & 445
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.