Update as of August 3, 2026: The Massachusetts Department of Public Health (DPH) has completed a visual assessment of the Charles River and confirm the existence of benthic cyanobacteria mats around Millennium Park, and additional chunks of benthic cyanobacteria floating on the water surface. As a result of these confirmed observations, the DPH recommends the current advisory remain in place until further notice. The DPH will continue to monitor conditions in the river and provide updates, including future testing results, when available.
Update as of July 23, 2026: The Massachusetts Department of Public Health (DPH) shared results from its July 22 sampling at Nahanton Park and Millennium Park that showed the following:
- Microcystins not detected in either sample
- Cylindrospermopsin detected in both samples, but at concentrations below the DPH advisory of 14 ppb
The DPH further explained that because heavy rain likely caused a temporary drop in toxin levels and benthic algal mats remain in place, the current cyanobacteria advisory will remain in place. The DPH will continue to monitor conditions in the area and provide updates.
A full overview of findings and information about the advisory was shared by the Health Director at last night's Select Board meeting.
Previously shared on July 21, 2026: The Massachusetts Department of Public Health (DPH) has issued a precautionary cyanoHAB (cyanobacteria harmful algal bloom) advisory for the Charles River between Kendrick St./Nahanton St. (Needham/Newton) and Bridge St./Spring St. (Dedham/West Roxbury). Signage will be posted at all access points along this stretch, including Millennium Park and Cutler State Park.
Staff from the DPH are awaiting test results to confirm the presence of cyanobacteria exceeding DPH guidelines for recreational waterbodies in Massachusetts.
Recommendations for People and Pets to Avoid Contract With Water:
- Do not swim, boat, or jet ski in the river
- Do not eat fish or swallow river water
- Keep pets and animals away from the water
- Rinse off after any contact with river water
- Avoid water contact for both people and pets
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