South Mountain Lake-Deep Spot
South Mountain Lake-Deep Spot in Grafton County, New Hampshire has a water quality grade of B (Good). Phosphorus: 12 µg/L. Chlorophyll-a (algae): 5.1 µg/L. Surface area: 27 acres.
- Area
- 27 acres
- Type
- Mesotrophic
South Mountain Lake-Deep Spot is a 27-acre mesotrophic lake in Grafton County, New Hampshire. Water quality is graded B based on 1 sampling years through 2025. 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. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLow algae
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Good for swimming, clear water with low algae levels
Water Quality Grade: B (Good)
No clarity data. Phosphorus level: 12 µg/L. Chlorophyll-a: 5.1 µg/L. Trophic State Index: 43.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 12 µg/L | Good |
| Chlorophyll-a (Algae) | 5.1 µg/L | Not graded |
| Trophic State Index (TSI) | 43 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay 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
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Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 27 acres |
No bathymetric survey has been published for South Mountain Lake-Deep Spot, so its depth is not documented here.
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
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit NHLAK801030505-04 · Official waterbody report · Matched by proximity (0.25 km)
County Ranking
Ranked #22 of 27 lakes in Grafton County
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. LakeQuality grades it B for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for South Mountain Lake-Deep Spot?
- South Mountain Lake-Deep Spot in Grafton County, New Hampshire has a water quality grade of B (Good). 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?
- Good for swimming, clear water with low algae levels. South Mountain Lake-Deep Spot has a water quality grade of B (Good). Algae concentration (chlorophyll-a) is 5.1 µg/L. Always check local health department advisories before swimming.
Explore South Mountain Lake-Deep Spot across LakeGrade
More New Hampshire Lake Rankings
Top 25 by overall water quality score
Top 25 by Secchi water clarity
Grade A/B + low algae filter
Panfish-rich, public access
Full state directory
Other Grade B 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)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database