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 a

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