Lake Winnisquam-Outlet
Belknap County, New Hampshire4,264 acresView mapDirections
NHDES publishes the water-quality measurements for Lake Winnisquam-Outlet. Here is what has been measured.
- Oligotrophic
- Impaired · Mercury
Lake Winnisquam-Outlet in Belknap County, New Hampshire has water-quality measurements published by NHDES, shown here as reported. Phosphorus: 9.8 µg/L. Surface area: 4264 acres.
Lake Winnisquam-Outlet is a 4,264-acre cold, clear oligotrophic lake in Belknap County, New Hampshire. Compared to the 26 other monitored lakes in Belknap County, Lake Winnisquam-Outlet ranks #8 for water quality.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2025-08-19. Methodology: how these readings are compared.
What Lake Winnisquam-Outlet is good for
From access and monitoring records, not editorial picks.
- Big water4,264 acres
Swimming conditions
Readings onlyLow algae
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Lake Winnisquam-Outlet is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury, nuisance exotic species. Check local advisories before you swim.
Whether Lake Winnisquam-Outlet 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 Lake Winnisquam-Outlet
These are the measurements NHDES publishes for Lake Winnisquam-Outlet, 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 9.8 µg/L. TSI 37.
| 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 | 9.8µg/L | Good GoodFairPoor |
| 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. | 37 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about Lake Winnisquam-Outlet
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
4,264 acres
No bathymetric survey has been published for Lake Winnisquam-Outlet, so its depth is not documented here.
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Fishing Lake Winnisquam-Outlet
Reservoir Info (USACE NID)
Lake Winnisquam-Outlet is a man-made reservoir impounded by the Lochmere (completed 1910), built primarily for hydroelectric on the Winnipesaukee River; masonry-type dam, 11 ft tall and 223 ft long.
- Surface area
- 4,264 ac
- Normal storage
- 20,800 ac-ft
- Max storage
- 33,280 ac-ft
- Drainage area
- 428 sq mi
- Hazard class
- Low
All listed purposes: Hydroelectric;Fire Protection, Stock, Or Small Fish Pond;Recreation.
Source: USACE National Inventory of Dams, NID ID NH00015 · Operator website · Matched by proximity (0.16 km)
EPA Impairment Status
Lake Winnisquam-Outlet is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
Designated uses not supporting: Aquatic life, Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit NHLAK700020203-01 · Official waterbody report
Nearby Lakes in Belknap County
Frequently asked questions about Lake Winnisquam-Outlet
Where is Lake Winnisquam-Outlet?
- Lake Winnisquam-Outlet 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 Lake Winnisquam-Outlet?
- Lake Winnisquam-Outlet in Belknap County, New Hampshire has water-quality measurements published by NHDES, shown here as reported. Phosphorus: 9.8 µg/L. Surface area: 4264 acres.
Is Lake Winnisquam-Outlet safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Lake Winnisquam-Outlet is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury, nuisance exotic species. Check local advisories before you swim.
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Where this data comes from
29 measurements from 1 monitoring station, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-21.
Collected and reported by
- New Hampshire Department Of Environmental Services (BEACH)
1111330029 samples · 2020–2025
Analytical methods
- Phosphorus by Colorimetry
- 10-115-01-1 F ~ LACHAT; Total Phosphorus- Manual persulfate digest
- Turbidity by Nephelometry
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