The Massachusetts Department of Public Health issued a public health advisory Thursday, Aug. 20, for a cyanobacteria bloom in the Charles River Lower Basin, warning residents across the metro area to stay out of the water downstream of the Harvard Bridge.
Newton residents who kayak, walk dogs or run along the Lower Basin Esplanade should take note: the advisory covers a stretch of river that draws recreators from across the region, and it marks the third straight summer the Charles has been hit with a bloom, according to the Charles River Watershed Association.
CRWA River Science Manager Marielena Lima detected the problem during surveillance on Wednesday, Aug. 19, observing "very, very green" water at two monitoring sites in the Cambridge area, she told GBH News on Aug. 21. Cell counts confirmed the bloom had crossed DPH's regulatory threshold.
"It is becoming more of a consistent, recurring event each summer," Lima said.
DPH conducted its own site inspection on Aug. 20 to visually confirm the bloom before recommending the advisory. The Boston Public Health Commission followed with its own alert on Friday, Aug. 21.
What residents should know
The advisory covers all water downstream of the Harvard Bridge (also called the Mass Ave Bridge). DPH advises the public not to swim or swallow water in the affected area, to keep pets away and to rinse off after any contact.
Cyanobacteria, sometimes called blue-green algae, can release toxins during rapid growth. Direct contact can cause skin rashes, and inhaling airborne toxins can affect respiratory systems, particularly for people with asthma. Small children and dogs face the highest risk.
Paul Shoemaker, director of the Environmental and Occupational Health Division at the Boston Public Health Commission, said on Aug. 21 that the toxins "can cause gastrointestinal upset, nausea, vomiting, diarrhea" and can be "very serious, particularly for small children and pets."
Canine deaths from cyanobacteria ingestion have been documented in Massachusetts, NBC Boston reported. In July 2026, a West Roxbury resident told GBH News her dog died of liver failure two days after swimming in the Charles at Millennium Park.
Pet owners should call a veterinarian immediately if a dog shows vomiting, staggering, drooling or convulsions after being near the water.
When will the advisory lift?
The advisory cannot be lifted until two rounds of samples collected a week apart fall below 70,000 cells per milliliter of water. No timeline has been announced.
Last year's bloom started in late July 2025 and expanded by Oct. 7, 2025, to all areas downstream of the Cambridge Street/River Street bridge, lasting at least 10 weeks.
How to stay informed
CRWA's Flagging Program posts live water quality alerts through color-coded flags at 13 locations from Watertown to Boston. Red flags fly for the duration of a bloom. Residents can sign up for email alerts at crwa.org/flagging-program.
DPH maintains an updated list of all bodies of water with algae bloom advisories at mass.gov/algae-blooms. The Boston Public Health Commission's environmental health division can be reached at 617-534-5965.
