Middlesex Reservoir
Union County, New Jersey87 acresView mapDirections
NJDEP publishes the water-quality measurements for Middlesex Reservoir. Here is what has been measured.
- Eutrophic
- Impaired · Impaired biota (cause unknown)
Middlesex Reservoir in Union County, New Jersey has water-quality measurements published by NJDEP, shown here as reported. Water clarity: 3.6 ft visibility. Phosphorus: 173 µg/L. Chlorophyll-a (algae): 11.3 µg/L. Surface area: 87 acres.
Middlesex Reservoir is a 87-acre productive, nutrient-rich eutrophic lake in Union County, New Jersey. Compared to the 1 other monitored lakes in Union County, Middlesex Reservoir ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2023-11-06. Methodology: how these readings are compared.
Swimming conditions
Readings onlyBacteria advisory
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. Middlesex Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for impaired biota (cause unknown), metals (other than mercury), nutrients (phosphorus, nitrogen), pathogens (E. coli, fecal). Check local advisories before you swim.
Whether Middlesex Reservoir is safe to swim in today is a call for the New Jersey Department of Environmental Protection and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Middlesex Reservoir
These are the measurements NJDEP publishes for Middlesex Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Murky, only visible to about 3.6 ft. Phosphorus 173 µg/L. Chlorophyll-a 11.3 µg/L. TSI 64.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 3.6ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 173µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 11.3µg/L | Fair GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 64 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Middlesex Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
87 acres
No bathymetric survey has been published for Middlesex Reservoir, so its depth is not documented here.
Stays via Stay22
Stays near Middlesex Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Middlesex Reservoir in Union County, New Jersey — 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. The water-quality readings cover nutrients and algae, not lodging. Cabins and rentals around Middlesex Reservoir are listed below.
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Fishing Middlesex Reservoir
Reservoir Info (USACE NID)
Middlesex Reservoir is a man-made reservoir impounded by the Robinson's Branch Reservoir Dam (completed 1907), built primarily for water supply on the Robinsons Branch; earth-type dam, 27 ft tall and 425 ft long.
- Surface area
- 87 ac
- Normal storage
- 608 ac-ft
- Max storage
- 1,269 ac-ft
- Drainage area
- 19.1 sq mi
- Hazard class
- High
- Owner
- County of Union
Source: USACE National Inventory of Dams, NID ID NJ00377 · Operator website · Matched by proximity (0.91 km)
EPA Impairment Status
Middlesex Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Drinking water, Aquatic life, Recreation.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit NJ02030104050080-01 · Official waterbody report
Nearby Lakes in Union County
Frequently asked questions about Middlesex Reservoir
Where is Middlesex Reservoir?
- Middlesex Reservoir is a lake in Union County, New Jersey. Its water-quality measurements are published by NJDEP and shown on this page as reported.
What is the water quality of Middlesex Reservoir?
- Middlesex Reservoir in Union County, New Jersey has water-quality measurements published by NJDEP, shown here as reported. Water clarity: 3.6 ft visibility. Phosphorus: 173 µg/L. Chlorophyll-a (algae): 11.3 µg/L. Surface area: 87 acres.
Is Middlesex Reservoir safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Middlesex Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for impaired biota (cause unknown), metals (other than mercury), nutrients (phosphorus, nitrogen), pathogens (E. coli, fecal). Check local advisories before you swim.
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Where this data comes from
109 measurements from 4 monitoring stations, 2022–2023. Retrieved from the EPA Water Quality Portal on 2026-08-21.
Collected and reported by
- NJDEP Bureau of Freshwater and Biological Monitoring
NJDEP_BFBM109 samples · 2022–2023
Analytical methods
- Phosphorus by Colorimetry
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
Monitoring station identifiers (4)
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