A Look at Rocky Brook

July 1st, 2019

From extensive patches of farmland to the dense residential center of Hightstown, the Rocky Brook passes through a variety of land uses as it runs northwest from its headwaters in Millstone Township to its confluence with the Millstone River in East Windsor. Centuries worth of human impacts are still evident today in the three dams along its 10.5 mile length that form Perrinville Lake, Etra Lake and Peddie Lake, remnants of the area’s industrial past.

Chemical measurements taken by our dedicated Chemical Action Team volunteers, Tanya Dymtrow and Tom Smith, indicate recent reductions in nutrient pollution that have had a beneficial effect on overall health of the brook. Reduction in phosphates and nitrates prevent the overgrowth of aquatic plants and algae, making more dissolved oxygen available and lowering the pH.

These increases in water quality may account for the increase in the diversity of aquatic life at our biological sampling location in Rocky Brook Park, sampled by Patricia Donahue and Tom Smith, over the past two years. However, even with these improvements, the biological health of the stream is still impaired, likely due to polluted stormwater runoff entering the stream from the more densely paved areas of the watershed.

Impacts of stormwater runoff from 2018’s record rainfall is reflected in worse temperature and turbidity scores. As stormwater flows over paved surfaces, heat from the pavement is transferred to the water and runs into the stream, increasing the temperature of the water in a phenomenon known as thermal pollution. Volunteers recorded fluctuations in temperature above 31 degrees Celsius on more than three occasions in Rocky Brook, falling outside of the temperature range that fish and other organisms that live in the stream are adapted to survive.

In addition to these increases in temperature, stormwater runoff over cleared land and impervious surfaces also carries materials that cause the water to be cloudy, or turbid. This turbidity interferes with the light that enters the stream, affecting aquatic plant’s ability to produce oxygen and interfering with the breathing of aquatic animals.

The Watershed Institute is installing floating wetlands upstream from Peddie Lake. We also have plans underway in Hightstown to reduce the impact of stormwater runoff on the Rocky Brook by installing green infrastructure in Rocky Brook Park to help capture and treat runoff before it enters the brook. You can help support The Watershed Institute’s initiatives at Rocky Brook and other streams in the Millstone River and Central Delaware watersheds by becoming a member, making a one-time donation to cover the cost of sampling equipment, or taking advantage of the many volunteering opportunities available.


Many thanks to Marylin Anker, Tanya Dmytrow, Patricia Donahue and Tom Smith for monitoring Rocky Brook and its tributaries. The StreamWatch program is funded by Janssen Pharmaceuticals Inc, Colgate Palmolive Company, and the New Jersey Water Supply Authority.

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