Halfmoon Lake
Belknap County, New HampshireView mapDirections
NHDES publishes the water-quality measurements for Halfmoon Lake. Here is what has been measured.
- Impaired · Algal growth
- Public access
- Limited data
Halfmoon Lake in Belknap County, New Hampshire has water-quality measurements published by NHDES, shown here as reported. Water clarity: 14.2 ft visibility.
Halfmoon Lake is a lake in Belknap County, New Hampshire. Limited fish-species data available. Public access is available.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2021-07-21. Methodology: how these readings are compared.
What Halfmoon Lake is good for
From access and monitoring records, not editorial picks.
- Clear water14.2 ft of visibility
Swimming conditions
No readingBacteria advisory
Nutrient and algae readings are not a swimming clearance. Halfmoon Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, biotoxins, mercury, nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pH / acidity. Check local advisories before you swim.
Whether Halfmoon Lake is safe to swim in today is a call for the New Hampshire Department of Environmental Services 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 NHDES 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.
Good clarity, visible to about 14.2 ft. TSI 39.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 14.2ft | 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. | 39 | 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
EPA Impairment Status
Halfmoon Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Aquatic life, Fish consumption, Recreation.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit NHLAK700060402-03 · Official waterbody report
Nearby Lakes in Belknap County
Frequently asked questions about Halfmoon Lake
Where is Halfmoon Lake?
- Halfmoon Lake is a lake in Belknap County, New Hampshire. Its water-quality measurements are published by NHDES and shown on this page as reported.
What is the water quality of Halfmoon Lake?
- Halfmoon Lake in Belknap County, New Hampshire has water-quality measurements published by NHDES, shown here as reported. Water clarity: 14.2 ft visibility.
Is Halfmoon Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Halfmoon Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, biotoxins, mercury, nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pH / acidity. Check local advisories before you swim.
Can I fish at Halfmoon Lake?
- Public access to Halfmoon Lake is listed as yes on the lake record. Seasons, size and bag limits, and whether you need a license are set by New Hampshire's fish and wildlife agency, so check its current regulations before you go.
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Where this data comes from
6 measurements from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-21.
Collected and reported by
- North American Lake Management Society
NALMS6 samples · 2021
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.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database