Water Quality in Rocky Brook

June 21st, 2017

With its bucolic beginnings amidst the sprawling agriculture of Millstone Township, Rocky Brook flows northwest through increasing urban development in Hightstown and commercial sections of East Windsor.

Along its 10 ½ mile run, Rocky Brook is dammed to form Perrineville Lake, Etra Lake, and then Peddie Lake before flowing into the Millstone River near Route 130. This series of lakes are remnants of an 18th century industrial past, now used primarily for recreation.

The brook’s headwaters region is characterized by just 4% impervious cover, increasing to almost 18% downstream. Three monitoring sites help to characterize the differences in water quality between the upstream and downstream stretches. NJDEP monitoring has uncovered very few water quality issues in the site furthest upstream. Even phosphorus, the most common water pollutant in our watershed, was shown to be a nonissue in this area. These conditions change, however, as more stormwater runoff and nonpoint source pollution is introduced to the brook as it flows downstream.

Macroinvertebrate populations in Rocky Brook have been assessed by StreamWatch volunteers for just over one year. Very low in diversity, the samples are overrun with scuds and lacking in species sensitive to pollution. Phosphorus contamination may be to blame for poor biological health, while also depleting oxygen from the water. StreamWatch volunteers have not measured dissolved oxygen levels below the state standard of 4 milligrams of oxygen per liter of stream water, though measurements have drawn near to this low level on several occasions. pH has also dropped below the state threshold of 6.5 units in just over 5% of Rocky Brook samples.

StreamWatch bacteria monitoring last year showed a marked improvement in E. coli levels in Peddie Lake over previous years. Volunteers have just begun monitoring the lake for bacteria this year, so we hope to continue to see improvements in pathogen concentrations.


Many thanks to Tom Smith, Tanya Dmytrow, Greg Cantrell, Joan Kaspin, Pete Klapsogeorge, and Walter Sullivan for monitoring Rocky Brook and its tributaries. The StreamWatch program is funded by Janssen Pharmaceuticals Inc, RBC Blue Water Project, Colgate Palmolive Company, Dodge Foundation, and the New Jersey Water Supply Authority.

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