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D&R Canal State Park Warns Folks to Keep Away from a Harmful Algal Bloom on the Millstone River

By Barbara A. Preston | July 23, 2022

Canal towpath walkers may have noticed the Millstone River has turned green from Rocky Hill to Manville, including Griggstown and Blackwells Mills along the way. The river also smells horrible.

The D&R Canal State Park issued a code red warning on July 21, notifying the public to steer clear of the Millstone River. A Harmful Algal Bloom (HAB), which comes with a high risk of adverse health effects for people, pets, and wildlife, has been confirmed.

The green Millstone River where it meets the clear and healthy Bedens Brook in Montgomery Township. Photo from the D&R Canal State Park.

Impact on Recreation Activities along the D&R State Park:

▪ Avoid contact with the river water.

▪ Do not drink the water (especially dogs).

▪ Do not allow animals to eat dried algae, or groom themselves after coming into contact with the water.

▪ Do not canoe, kayak, or boat in the Millstone River.

▪ Use caution while fishing; wear gloves and/or wash your hands after touching the water.

▪ Do not eat the fish.

▪ No swimming.

▪ People, pets and livestock that come into contact with a bloom should rinse off with fresh water as soon as possible.

Land-use activities such as biking, running, or walking on the towpath/rail-trail is permitted.

The Harmful Algal Bloom (HAB) has high levels of potent toxins.

HAB has NOT been found in the D&R Canal. Both the Millstone River and the parallel canal provide drinking water to large portions of central New Jersey, as well as providing recreational uses as well.

Operating since 1931, the Raritan-Millstone Water Treatment Plant, the largest in New Jersey American Water’s system, is located at the confluence of the Raritan and Millstone Rivers. It provides water to more than 1.2 to 1.5 million people in seven counties in central New Jersey, according to

The NJ Department of Environmental Protection is monitoring the water, and will advise the state park officials and the water company on how to move forward.

What is this green stuff?

Harmful Algal Blooms (HABs) are not caused by true algae but rather by cyanobacteria that in many ways resemble and behave like algae. These cyanobacteria naturally occur in fresh water and can proliferate to unhealthful levels in sunlight and hot weather, forming dense mats resembling pea soup or spilled paint.

The confirmed toxins present in the Millstone River, as tested at the Route 518 bridge in Rocky Hill in late July, are microcystins.

“These potent toxins are known to be toxic to fish, mammals (including humans), and birds through skin contact, ingestion, and possibly inhalation,” according to a study by Cindy P. Driscoll of the Maryland Department of Natural Resources.

The NJ DEP warns that human exposure can cause a range of health effects, including rashes, allergy-like reactions, flu-like symptoms, gastroenteritis, respiratory irritation, skin rashes and eye irritation.

Microcystin poisoning has also been linked to the mortality and illness of great blue herons.

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When will the algal bloom go away?

The duration of a Harmful Algal Bloom is unpredictable, and may be influenced by availability of nutrients and weather conditions, according to the DEP’s Division of Water Monitoring and Standards.

The Watershed Institute in Pennington is helping to clear up a recent algal bloom in the Mercer County Park Systems’ Rosedale Lake. The lake was closed to public use in 2021 from early July through mid-November.

“For several summers, many New Jersey waterways — including Rosedale Lake — have been closed to swimming, fishing, and other recreation because of the rise of harmful algal blooms (HABs) caused by excessive pesticides, fertilizers, and other pollutants flowing into the water from tributaries and nearby lands. One pound of phosphorous can stimulate the growth of 1,100 pounds of algae in a lake or pond,” according to

Last year, the staff and scientists at the Watershed Institute dropped the first of 20 floating wetlands into the waters in an innovative approach to halt toxic algae blooms and improve water quality.

Instead of using chemicals, the large floating wetlands were planted with fast-growing grasses, swamp milkweed, and cardinal flowers. These hardy, long-rooted plants will soak up nitrogen, phosphorus and other nutrients that stimulate the growth of blue-green algae, posing health threats for people, pets, and other animals, according to the institute’s website. The Watershed is monitoring the lake regularly to determine the effectiveness of the floating wetlands.

Other conventional methods to prevent algal blooms include aeration, chemical/biological additives, or ultrasonic technology. The treatment solutions that are available on the market, they all have their cons.

“For example, algaecides offer quick results but damage the entire water ecosystem,” according

It appears that preventing the algal bloom in the first place is the way to go. The Watershed and the NJ DEP have plenty of suggestions that communities can take to protect the waterways from pollution. ■

The NJ DEP’s HAB website, at contains general information about cyanobacterial HABs, what to do if people or pets are exposed, links to the EPA and U.S. Centers for Disease Control’s HABs websites.

Also, visit for more information on algal blooms.

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