Williamson Reservoir
Chouteau County, MontanaView mapDirections
Montana DEQ publishes the water-quality measurements for Williamson Reservoir. Here is what has been measured.
- Limited data
Williamson Reservoir in Chouteau County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 410 µg/L. Chlorophyll-a (algae): 6.7 µg/L.
Williamson Reservoir is a lake in Chouteau County, Montana.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2022-09-20. Methodology: how these readings are compared.
Swimming conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Williamson Reservoir is safe to swim in today is a call for the Montana Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Williamson Reservoir
These are the measurements Montana DEQ publishes for Williamson Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
No clarity data. Phosphorus 410 µg/L. Chlorophyll-a 6.7 µg/L. TSI 70.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | No reading How deep a black-and-white disk stays visible from the surface. Higher is clearer. | |
| Total phosphorus | 410µg/L | Measured, not scored The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. |
| Chlorophyll-a (algae) | 6.7µg/L | Measured, not scored The green pigment in algae — a direct measure of how much algae is in the water. |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 70 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
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Frequently asked questions about Williamson Reservoir
Where is Williamson Reservoir?
- Williamson Reservoir is a lake in Chouteau County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Williamson Reservoir?
- Williamson Reservoir in Chouteau County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 410 µg/L. Chlorophyll-a (algae): 6.7 µg/L.
Is Williamson Reservoir safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.