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If the Hudson’s looking a little more like split-pea soup than sparkling blue this fall, you’re not imagining it. A roughly 60-mile stretch of the river—from Beacon up past Germantown—has been blanketed with a thick, green scum scientists say marks the Hudson’s worst algal bloom in nearly 40 years.
Typically, these blooms happen in still water (think lakes, ponds or maybe your neglected backyard fountain), but rarely in a deep tidal river that mixes with the ocean twice daily. This year, however, a perfect storm of record heat, drought and sewage overflow created ideal conditions for cyanobacteria (aka blue-green algae) to thrive. According to the National Weather Service, the Hudson Valley saw its warmest September since 2018, averaging nearly 71 degrees Fahrenheit. With rain at just 60% of normal levels, the river’s flow slowed, leaving less water to flush out pollutants.
“It is unusual for rivers in general,” Stuart Findlay, senior scientist at the Cary Institute, told Gothamist. “One year out of five you may get any evidence at all, much less of this extent.” Shannon Roback, science director at Riverkeeper, put it more bluntly: “What we are seeing right now is definitely not normal. This is much more widespread and severe than what we’ve seen in the past on the Hudson.”
The culprit behind the neon sludge is Microcystis, a toxin-producing bacteria that can be dangerous to humans and pets. The CDC warns that exposure can cause everything from skin irritation and nausea to liver and nerve damage. Dogs are especially at risk, so keep Fido far from the water’s edge.
While the bloom is finally dissipating with cooler weather, scientists warn this may be a preview of the Hudson’s future. “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,” Roback explained.
The New York State Department of Environmental Conservation has confirmed the bloom is dying off, but concentrations remain unusually high for a river system. For now, consider kayaking and swimming off the itinerary north of Beacon—unless you’re into lime-green water and a side of toxins.