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Climate change could make algal blooms more common in Hudson River, scientists say
By
Rosemary Misdary
Published Oct 17, 2025
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Riverkeeper
By
Rosemary Misdary
Published Oct 17, 2025
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A thick layer of pea soup-colored scum has plagued a roughly 60-mile stretch of the Hudson River from Beacon to just north of Germantown this fall.
Algal blooms, which are usually found in stagnant waters such as ponds and lakes, are uncommon for a deep river that flows in two directions and mixes with ocean water several times per day. But warmer waters, drought and sewer overflow have contributed to what scientists call the Hudson's worst outbreak of cyanobacteria in the last 40 years. While recent cooler temperatures are helping to dissipate the bacteria, experts predict these blooms will worsen as the weather gets hotter and drier.
“It is unusual for rivers in general,” said Stuart Findlay, senior scientist at the research nonprofit Cary Institute. “One year out of five you may get any evidence at all, much less of this extent.”
The region had its warmest September on record since 2018, with an average temperature of nearly 71 degrees Fahrenheit, according to the National Weather Service. The federal drought monitor indicated New York state experienced drought-like conditions during the summer, when the city got only 8.7 inches of rain — around 60% of the average precipitation — making it the driest summer since 1999. Findlay said this contributed to less robust flow on the river, where rain can act as a flushing agent.
”As climate change progresses, this is something we expect in the future because we know that drought can make a harmful algal bloom more likely, and we know that the hotter temperatures make the water more habitable for the bacteria," Shannon Roback, science director at the nonprofit Riverkeeper, said.
Algal blooms often appear in late summer, when there tends to be less rain and water temperatures are around 77 degrees or higher. Once the bacteria spreads on the water’s surface, there’s no method to remove it except to wait for cooler temperatures.
”Part of our challenge is this did not happen last year, did not happen the year before,” Findlay said. “The sort of trigger or catalyst that led this to such a significant bloom this year, but not in previous years, is a small difference in temperature, small difference in flow rate.”
Sewage containing nitrogen and phosphorus entering the Hudson River is also contributing to the bloom.
There are no algal blooms in New York City because of the salt content in that portion of the river. Findlay and Roback said much of the sewage comes from upstate municipalities along the Hudson. Even communities that have modernized waste water treatment don’t remove nitrogen and phosphorus in their facilities because the process is very costly, according to Findlay. Agricultural and urban runoff also contributes to the problem.
”Especially during rain events, sewage likely enters the river without any treatment if there's a combined sewer system, which there are many [of] along the river,” Roback said.
The New York state Department of Environmental Conservation investigated the 60-mile stretch of the Hudson River at the end of last month. The preliminary findings were lower concentrations of cyanobacteria, which indicates the algal bloom is dying out.
Over the past decade, the agency has observed increased frequency of algal blooms across the state in flowing waterways such as the Hudson. The initial monitoring data indicates the bacteria concentrations are a low risk to public health, but the department still advises people to avoid areas showing signs of algal bloom.
Findlay said the concentrations may be low for a stagnant pond, but for a tidally mixed river like the Hudson, they are high and certainly contain toxins. In late September, Dutchess County and Saugerties issued health advisories concerning the algal blooms.
Officials say the public should avoid areas of the river with algae.
New York State Department of Environmental Conservation
The largest patch is centered around Kingston, located near major drinking sources for towns like Poughkeepsie. Findlay said this hasn’t affected drinking water in towns that depend on the Hudson River, because pollutants like phosphorus and nitrogen — main contributors to algal blooms — are filtered out before they hit faucets.
According to the U.S. Centers for Disease Control and Prevention, physical contact, inhalation and ingestion of cyanobacteria is hazardous to animals and humans, especially children. The risks include minor symptoms, such as skin irritation and gastroenteritis, as well as more severe outcomes, like liver and nerve damage. In some cases, exposure can cause death, particularly in pets. Consuming contaminated fish is also dangerous, and the algal blooms can cause the sudden mass death of fish in the area.
“People shouldn't be swimming in it, their dogs shouldn't be swimming in it,” Findlay said. “It would pretty significantly impede the use of the river for contact recreation, which is everything from swimming to kayaking.”
The blue-green algae is a known public health hazard. In a region that has seen a resurgence in nature and recreational activities such as swimming and kayaking, public health officials are warning residents to stay out of the water.
”What we are seeing right now is definitely not normal,” Roback said. “This is much more widespread and severe than what we've seen in the past on the Hudson.”
The environmental department updated its water-quality standards for all waterways around New York City this week. The update reclassifies the Hudson River along Manhattan for designated swimming, which establishes a legal mandate to reduce pollution.
Riverkeeper, Cary Institute and state environmental and health officials continue to monitor and collect samples of the algal bloom. Scientists are waiting to analyze satellite images to measure the extent of the bloom, but the closure of the U.S. Geological Survey amid the ongoing federal government shutdown has delayed the process.
Scientists expect to release a detailed report before the end of the year. Residents can report any sightings of algal blooms to the environmental department. The agency maintains a map of current statewide blooms.
Tagged
Health and Science
climate change
environment
hudson river
Rosemary Misdary
Rosemary is health & science reporter. Got a tip? Email [email protected] or Signal 646-544-9524.
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