Charles River cyanobacteria advisory in effect for Needham to West Roxbury

Boston’s Public Health Commission is warning residents about a potential cyanobacteria (blue-green algae) bloom on the Charles River. In an advisory last updated July 21, 2026, officials say the concern is affecting the river between Kendrick Street in Needham and Spring Street in West Roxbury, including Millennium Park and Cutler State Park.

What residents should do now

Out of caution while state lab testing is underway, Boston is urging people and pets to avoid contact with the water and any areas of algae concentration, even on shore. That means:

• Avoid direct contact with the water and impacted shoreline (including swimming and other water activities).
• Do not allow pets to swim in or drink the water.
• Rinse off right away if you, your child, or your pet comes in contact with the water or shoreline algae.

The advisory includes a map with precise location at the bottom of the release.

Why cyanobacteria is a concern for people and pets

Boston says cyanobacteria can produce toxins that may make people and pets sick. Officials note:

• For humans: the primary concern is ingesting water while swimming. Direct skin contact and inhaling water droplets are secondary concerns.
• For pets: the primary concern is ingesting water containing cyanobacteria or scum that has washed ashore or gotten onto their skin or fur.

Contact can cause skin and eye irritation, and inhalation can cause respiratory irritation (which can worsen pre-existing respiratory conditions). Ingestion can cause acute gastrointestinal symptoms, such as vomiting and diarrhea. In more serious cases, officials say the toxins can affect the functioning of the liver, kidneys, and/or neurological systems, and severe cases can result in death.

What to do if your pet is exposed

Boston advises calling a veterinarian immediately if a pet was around an algae bloom and shows symptoms that present soon after exposure, such as vomiting, staggering, drooling, or convulsions.

How long the advisory could last (and when it can be lifted)

Boston notes algae blooms are common in summer and may last for weeks or disappear quickly. The Massachusetts Department of Public Health will continue monitoring cyanobacteria levels, and will conduct sampling for lab analysis once the bloom has dissipated.

To lift the advisory, Boston says DPH requires two rounds of samples collected a week apart that are below 70,000 cells/milliliter of water.

Where to check for updates

For the latest status of the Charles River advisory, check the city’s update page on Boston.gov. Boston also points residents to a Massachusetts mass.gov list that tracks bodies of water with algae bloom advisories as updates are posted.

Quick reminder for river walks, running, and families

If you run, walk, or spend time near this stretch of the Charles River, treat the water and algae areas there as off-limits for contact until officials say the advisory has been lifted. Keep kids supervised near the shoreline, and make sure pets can’t drink or lick algae or scum on shore.

Sources

• Boston.gov — Health Advisory: Officials investigating Charles River cyanobacteria algae bloom
• Mass.gov — Cyanobacterial Harmful Algal Blooms (CyanoHABs) & Water
• NBC Boston — Charles River water algae warning/danger coverage