South Reservoir
Middlesex County, Massachusetts80 acresView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
- Mesotrophic
- Impaired · Algal growth
- #1 of 11 in the county
- 42 ft max depth
- Limited data
South Reservoir in Middlesex County, Massachusetts has a water quality grade of A (Excellent). Chlorophyll-a (algae): 2.5 µg/L. Surface area: 80 acres.
- Max depth
- ~42 ft est.
- Area
- 80 acres
- Type
- Mesotrophic
- Chlorophyll-a
- 2.5 µg/L
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Watch South Reservoir’s letter
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South Reservoir is a 80-acre mesotrophic lake in Middlesex County, Massachusetts with an estimated average depth of about 42 feet. Water quality is graded A based on 1 sampling years through 2017. Compared to the 10 other monitored lakes in Middlesex County, South Reservoir ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, MassDEP, last sampled 2017-09-14. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyLimited data
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.
The A is nutrients and algae, not a swimming clearance. South Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, nutrients (phosphorus, nitrogen), oxygen depletion (low DO). Check local advisories before you swim.
Water Quality Grade: A (Excellent)
No clarity data. Chlorophyll-a 2.5 µg/L. TSI 40. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | 2.5 µg/L | Good |
| Trophic State Index (TSI) | 40 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~42 ft est. |
| Average Depth | ~15 ft est. |
| Surface Area | 80 acres |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Massachusetts has not published bathymetry for South Reservoir.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing South Reservoir
Reservoir Info (USACE NID)
South Reservoir is a man-made reservoir impounded by the South Reservoir Dam (completed 1882), built primarily for water supply on the Meetinghouse Brook; earth-type dam, 45 ft tall and 1,000 ft long.
- Surface area
- 80 ac
- Normal storage
- 1,614 ac-ft
- Max storage
- 2,040 ac-ft
- Drainage area
- 0.77 sq mi
- Hazard class
- High
- Owner
- Town of Winchester
Source: USACE National Inventory of Dams, NID ID MA00453 · Operator website
EPA Impairment Status
South Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit MA71045 · Official waterbody report · Matched by proximity (2.33 km)
County Ranking
Ranked #1 of 11 lakes in Middlesex County
Nearby Lakes in Middlesex County
Frequently asked questions about South Reservoir
Where is South Reservoir?
- South Reservoir is a lake in Middlesex County, Massachusetts. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for South Reservoir?
- South Reservoir in Middlesex County, Massachusetts has a water quality grade of A (Excellent). Chlorophyll-a (algae): 2.5 µg/L. Surface area: 80 acres.
Is South Reservoir safe to swim in?
- The A is nutrients and algae, not a swimming clearance. South Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, nutrients (phosphorus, nitrogen), oxygen depletion (low DO). Check local advisories before you swim.
How deep is South Reservoir?
- No published bathymetric survey covers South Reservoir, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 42 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 80 acres.
Explore South Reservoir across LakeQuality
Species activity outlook by water temperature and season
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Where this data comes from
3 measurements from 1 monitoring station, 2017. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Massachusetts Department of Environmental Protection (MassDEP)
MASSDEP3 samples · 2017
Analytical methods
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
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
Lake size & watershed from HydroLAKES (HydroSHEDS)