South Mountain Lake-Deep Spot
Grafton County, New Hampshire27 acresView mapDirections
NHDES publishes the water-quality measurements for South Mountain Lake-Deep Spot. Here is what has been measured.
- Mesotrophic
- Impaired · Mercury
South Mountain Lake-Deep Spot in Grafton County, New Hampshire has water-quality measurements published by NHDES, shown here as reported. Phosphorus: 12 µg/L. Chlorophyll-a (algae): 5.1 µg/L. Surface area: 27 acres.
South Mountain Lake-Deep Spot is a 27-acre mesotrophic lake in Grafton County, New Hampshire. Compared to the 26 other monitored lakes in Grafton County, South Mountain Lake-Deep Spot ranks #22 for water quality.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2025-08-05. Methodology: how these readings are compared.
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. South Mountain Lake-Deep Spot is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, pH / acidity. Check local advisories before you swim.
Whether South Mountain Lake-Deep Spot 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 South Mountain Lake-Deep Spot
These are the measurements NHDES publishes for South Mountain Lake-Deep Spot, 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 12 µg/L. Chlorophyll-a 5.1 µg/L. TSI 43.
| 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 | 12µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 5.1µg/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. | 43 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about South Mountain Lake-Deep Spot
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
27 acres
No bathymetric survey has been published for South Mountain Lake-Deep Spot, so its depth is not documented here.
Stays via Stay22
Stays near South Mountain Lake-Deep Spot
Compare lakeside cabins, hotels, and vacation rentals near South Mountain Lake-Deep Spot in Grafton County, New Hampshire — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near South Mountain Lake-Deep Spot
Prices for Oct 9–11, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Follow South Mountain Lake-Deep Spot’s readings
Email me when new measurements post for this lake. Updates stay free.
Or email
Fishing South Mountain Lake-Deep Spot
Reservoir Info (USACE NID)
South Mountain Lake-Deep Spot is a man-made reservoir impounded by the Upper Mountain Lake Dam (completed 1963), built primarily for recreation on the Waterman Brook; earth-type dam, 31 ft tall and 660 ft long.
- Surface area
- 27 ac
- Normal storage
- 325 ac-ft
- Max storage
- 499 ac-ft
- Drainage area
- 3.4 sq mi
- Hazard class
- Low
- Owner
- MOUNTAIN LAKES VILLAGE DISTRICT
Source: USACE National Inventory of Dams, NID ID NH00190 · Operator website · Matched by proximity (0.11 km)
EPA Impairment Status
South Mountain Lake-Deep Spot 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.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit NHLAK801030505-04 · Official waterbody report
Nearby Lakes in Grafton County
Frequently asked questions about South Mountain Lake-Deep Spot
Where is South Mountain Lake-Deep Spot?
- South Mountain Lake-Deep Spot is a lake in Grafton County, New Hampshire. Its water-quality measurements are published by NHDES and shown on this page as reported.
What is the water quality of South Mountain Lake-Deep Spot?
- South Mountain Lake-Deep Spot in Grafton County, New Hampshire has water-quality measurements published by NHDES, shown here as reported. Phosphorus: 12 µg/L. Chlorophyll-a (algae): 5.1 µg/L. Surface area: 27 acres.
Is South Mountain Lake-Deep Spot safe to swim in?
- Nutrient and algae readings are not a swimming clearance. South Mountain Lake-Deep Spot is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, pH / acidity. Check local advisories before you swim.
Explore South Mountain Lake-Deep Spot across LakeQuality
More lakes in New Hampshire
Other Lakes in New Hampshire
Mesotrophic Lakes in New Hampshire
Where this data comes from
13 measurements from 1 monitoring station, 2021–2025. Retrieved from the EPA Water Quality Portal on 2026-08-21.
Collected and reported by
- New Hampshire Department Of Environmental Services (BEACH)
1111330013 samples · 2021–2025
Analytical methods
- 10-115-01-1 F ~ LACHAT; Total Phosphorus- Manual persulfate digest
- CHLOROPHYLL A-B-C DETERMINATION (MODIFIED)
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