Press Release: 7/31/2025

Cyanobacteria Bloom Reported in Lower Basin of Charles River

FOR IMMEDIATE RELEASE: July 31, 2025

Contact: Stefan Geller, Charles River Watershed Association

Phone: 781-572-9341

Email: sgeller@crwa.org

 

BOSTON, MA – The Massachusetts Department of Public Health (DPH) this afternoon confirmed a significant cyanobacteria bloom in the lower basin of the Charles River and recommended a public health advisory.

 

Cyanobacteria, or blue-green algae, are naturally occurring microorganisms in rivers, lakes and ponds, and during periods of rapid growth can release harmful toxins. Direct contact can cause skin rashes, and inhaling airborne toxins can affect respiratory systems, especially for those with pre-existing conditions like asthma. Small children and pets are especially susceptible to the effects of cyanotoxins.

 

Due to these severe health risks, DPH has deemed the area unsafe for people and pets and has advised the public not to swim or swallow water in the area, to keep their pets away, and to rinse off after having contact with the water.

 

DPH staff identified the bloom downstream of Massachusetts Avenue (a.k.a. the Harvard Bridge) based on visual assessments of the river conducted throughout the week, which was corroborated by sampling and microscope analysis conducted by the Charles River Watershed Association (CRWA). CRWA performs biweekly cyanobacterial cell counts at two representative locations in the lower basin and alerts DPH when cell counts approach regulatory thresholds. 

 

This is the second cyanobacteria bloom in the Charles River in 10 months; DPH confirmed a cyanobacteria bloom downstream of the Longfellow Bridge in September 2024, which at that time was the river’s first major bloom in four years.

 

Cyanobacteria grow rapidly when bodies of water experience high phosphorus levels, hotter temperatures and stagnant conditions. In the Charles River, cyanobacterial blooms are a symptom of excess stormwater pollution and degraded ecology. Current stormwater regulations mandate an over 50% reduction in nutrient loading from watershed communities by the year 2038. While progress has been made, this bloom highlights the work to be done and the importance of addressing municipal stormwater runoff. 

 

For ongoing water quality updates, please visit CRWA’s flagging webpage. 

 

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Charles River Watershed Association’s mission is to protect, restore and enhance the Charles River and its watershed through science, advocacy, and the law. CRWA develops science-based strategies to increase resilience, protect public health, and promote environmental equity as we confront a changing climate.