An outbreak of toxic cyanobacteria is plaguing the Charles River just a week before the waterway hosts the world’s largest three-day rowing competition.

The 2025 Head of the Charles Regatta takes place in Boston and Cambridge from Oct. 17 to 19. The iconic event will feature roughly 12,000 rowers and draw thousands of spectators along the Charles River.

According to a Mass.gov emergency alert , the water downstream of the Cambridge Street/River Street bridge and Back Bay Fens has been reported harmful as of Oct. 6.

“With climate change bringing increased precipitation and extreme heat, cyanobacteria blooms are becoming more frequent and long-lasting,” Stefan Geller, of the Charles River Watershed Association, told the Boston Globe . “The Charles River has had cyanobacteria bloo

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