Etra Lake
Mercer County, New Jersey21 acresView mapDirections
NJDEP publishes the water-quality measurements for Etra Lake. Here is what has been measured.
- Hypereutrophic
- Impaired · Mercury
- Limited data
Etra Lake in Mercer County, New Jersey has water-quality measurements published by NJDEP, shown here as reported. Water clarity: 3.9 ft visibility. Phosphorus: 46.6 µg/L. Chlorophyll-a (algae): 35.4 µg/L. Surface area: 21 acres.
Etra Lake is a 21-acre highly nutrient-loaded hypereutrophic lake in Mercer County, New Jersey. Compared to the 14 other monitored lakes in Mercer County, Etra Lake ranks #5 for water quality.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2023-10-11. Methodology: how these readings are compared.
Swimming conditions
Readings onlyLimited data
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. Etra Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, metals (other than mercury), nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pesticides, pCBs. Check local advisories before you swim.
Whether Etra Lake 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 Etra Lake
These are the measurements NJDEP publishes for Etra Lake, 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 3.9 ft. Phosphorus 46.6 µg/L. Chlorophyll-a 35.4 µg/L. TSI 61.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 3.9ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 46.6µg/L | Fair GoodFairPoor |
| Chlorophyll-a (algae) | 35.4µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 61 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Etra Lake
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
21 acres
No bathymetric survey has been published for Etra Lake, so its depth is not documented here.
Stays via Stay22
Stays near Etra Lake
Compare lakeside cabins, hotels, and vacation rentals near Etra Lake in Mercer 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 Etra Lake are listed below.
Top-rated places to stay near Etra Lake
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Fishing Etra Lake
Reservoir Info (USACE NID)
Etra Lake is a man-made reservoir impounded by the Etra Mill Dam (completed 1930), built primarily for recreation on the Rocky Brook; earth-type dam, 11 ft tall and 300 ft long.
- Surface area
- 21 ac
- Normal storage
- 66 ac-ft
- Drainage area
- 9.1 sq mi
- Hazard class
- Low
- Owner
- Township of East Windsor
Source: USACE National Inventory of Dams, NID ID NJ00298 · Operator website · Matched by proximity (0.18 km)
EPA Impairment Status
Etra Lake 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, Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit NJ02030105100050-01 · Official waterbody report
Nearby Lakes in Mercer County
Frequently asked questions about Etra Lake
Where is Etra Lake?
- Etra Lake is a lake in Mercer County, New Jersey. Its water-quality measurements are published by NJDEP and shown on this page as reported.
What is the water quality of Etra Lake?
- Etra Lake in Mercer County, New Jersey has water-quality measurements published by NJDEP, shown here as reported. Water clarity: 3.9 ft visibility. Phosphorus: 46.6 µg/L. Chlorophyll-a (algae): 35.4 µg/L. Surface area: 21 acres.
Is Etra Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Etra Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, metals (other than mercury), nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pesticides, pCBs. Check local advisories before you swim.
Explore Etra Lake across LakeQuality
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Where this data comes from
56 measurements from 2 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_BFBM56 samples · 2022–2023
Analytical methods
- Phosphorus by Colorimetry
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
Monitoring station identifiers (2)
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