Halfmoon Lake
Flathead County, MontanaView mapDirections
Montana DEQ publishes the water-quality measurements for Halfmoon Lake. Here is what has been measured.
- Limited data
Halfmoon Lake in Flathead County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 17.8 ft visibility.
Halfmoon 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 2023-08-21. Methodology: how these readings are compared.
What Halfmoon Lake is good for
From access and monitoring records, not editorial picks.
- Clear water17.8 ft of visibility
Swimming conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Halfmoon Lake 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 Halfmoon Lake
These are the measurements Montana DEQ publishes for Halfmoon Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Crystal clear, you can see 17.8 ft down. TSI 36.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 17.8ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | No reading The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. | |
| Chlorophyll-a (algae) | No reading 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. | 36 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about Halfmoon Lake
Answers come straight from the public measurements and survey records on this page.
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Stays near Halfmoon Lake
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Fishing Halfmoon Lake
Water Quality Trend: Worsening
Based on 12 years of monitoring (2012–2023).
Water clarity
Worsening4 years
Phosphorus
Worsening+0.5 µg/L/yr11 years
Chlorophyll-a
Stable+0.01 µg/L/yr12 years
Nearby Lakes in Flathead County
Frequently asked questions about Halfmoon Lake
Where is Halfmoon Lake?
- Halfmoon Lake is a lake in Flathead County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Halfmoon Lake?
- Halfmoon Lake in Flathead County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 17.8 ft visibility.
Is Halfmoon Lake 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
12 measurements from 1 monitoring station, 2018–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS12 samples · 2018–2023
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.