Riverkeeper monitoring unprecedented harmful algal bloom in Hudson River estuary
Ongoing tracking and public reporting critical to protect drinking water and community health
September 12, 2025
Riverkeeper Team
Ossining, NY — September 12, 2025: Riverkeeper and a network of partners are currently monitoring and collecting data on a large, widespread harmful algal bloom (HAB) in the Hudson River Estuary. Based on observations to date, this appears to be the most extensive HAB in the estuary in living memory.
The public is advised to avoid water contact on shorelines where a suspected HAB is present, as HABs can produce toxins that pose health risks to people, pets, and wildlife via ingestion, inhalation, or skin contact. The most common symptoms that may occur from exposure include skin rashes or stomach upset. Exposure to high concentrations of the more potent and rare toxins can damage the liver or neurological system. Dogs that swim in waters affected by HABs have died.
This HAB appears to be centered in the Hudson River near Kingston. As of this week, HAB conditions have been observed mostly in shallow areas from shorelines from the Village of Catskill and City of Hudson, south to the Town of Esopus and Town of Hyde Park, with isolated observations at other points along the river. Not all areas within this reach may be affected, or affected to the same degree, and it is unknown how long the condition may persist.
The public can help by taking photos and reporting observations to the New York State Department of Environmental Conservation.
It can be hard to distinguish harmless plants floating on the water’s surface from a HAB, and both may be present simultaneously. For more information and tips for identifying HABs, visit Riverkeeper.org and the NYS DEC’s HABs page.
Harmful algal blooms are caused by a type of bacteria, not algae. Under certain conditions — warm water temperatures, excess nutrients, and stagnant water — this cyanobacteria can multiply rapidly and form visible blooms on the water.
Climate change is creating conditions that make harmful algal blooms more likely and more severe. Warmer water temperatures, longer periods of low river flow, and more frequent droughts create an ideal environment for HAB-forming cyanobacteria to thrive. Extreme precipitation events can also increase runoff of nutrients such as phosphorus and nitrogen into the river, fueling blooms. Treated sewage, fertilizer, agricultural runoff, and insufficiently maintained septic systems are common sources of nutrient runoff.
“HABs can pose a threat to drinking water, and Riverkeeper has worked closely with the Hudson River Drinking Water Intermunicipal Council (known as the Hudson 7) and the New York State Department of Health to ensure that drinking water plants along the river are effectively treating the water to ensure safe tap water for the 100,000 people who draw their water from the Hudson,” says Riverkeeper Director of Advocacy, Policy, and Planning Dan Shapley. “The current bloom underscores the need to understand and prepare for the impacts of a changing climate on our drinking water sources.”
Riverkeeper remains committed to working with the NYS Department of Environmental Conservation, the NYS Department of Health, community partners, and the Hudson River Water Quality Monitoring Collaborative — a working group of over 100 stakeholders including academics, regulators, municipalities, watershed groups, and environmental justice groups that Riverkeeper brought together — to monitor, respond to, and ultimately reduce the impact of this harmful algal bloom.
For more about water quality and harmful algal blooms in the Hudson River watershed, see Riverkeeper’s Water Quality Portal at data.riverkeeper.org.
ABOUT RIVERKEEPER
Riverkeeper protects and restores the Hudson River, and safeguards drinking water supplies through community partnerships, science, and law. Our core programs improve water quality, restore habitat for an abundance of life, and address the impact of climate change on our waterways. Founded in 1966 as the Hudson River Fishermen’s Association, Riverkeeper became the model for more than 320 Waterkeeper organizations around the world and helped establish globally-recognized standards for waterway and watershed protection. We continue to work toward the goal of a swimmable, fishable, and drinkable Hudson River for all. Learn more, get updates, and support our work by visiting riverkeeper.org.
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