Water Quality Today
Red flags are being flown at some boathouses due to a Combined Sewer Overflow (CSO). A discharge at CAM005 discharged on October 8, 2025 starting at 9:00AM, potentially affecting areas downstream of Eliot Bridge. Blue flags will be restored 48 hours from the end time. Public health officials recommend avoiding contact with the river during rainstorms and for 48 hours afterward, as there may be increased health risks due to bacteria or other pollutants discharged by CSOs. For more information see:
https://www.cambridgema.gov/Departments/publicworks/combinedseweroverflowreporting
In accordance with the MA Dept. of Public Health (DPH), a cyanobacteria (blue-green algae) advisory has been posted in the Charles River downstream of the River Street Bridge (a.k.a. the Cambridge Street Bridge). Red flags are being flown at the Charles River Yacht Club, Union Boat Club, Paddle Boston at Kendall Square, the Charlesgate Yacht Club, and the Riverside Boat Club. Public health officials recommend that people and pets avoid contact in areas of algae concentration and rinse thoroughly in the event of contact.
For additional information on cyanobacteria blooms, we encourage you to visit the following websites:
https://www.mass.gov/lists/algae-information
https://www.mass.gov/guides/cyanobacterial-harmful-algal-blooms-cyanohabs-water
https://www.cdc.gov/habs/index.html
https://www.epa.gov/cyanohabs
Current Cyanobacteria Bloom Advisories - https://www.mass.gov/alerts/harmful-cyanobacterial-bloom-advisories-in-massachusetts
At this time, it is recommended that you avoid contact with the water. People should rinse off with fresh water immediately if they or their pet comes into contact with the water. If you believe you or your pet is experiencing adverse health effects, contact your doctor or veterinarian immediately. Blue-green algae is extremely toxic to pets.
CRWA’s water quality models are predicting blue flags at all boathouses, based only on E. coli bacteria predictions.
( Last updated: October 8, 2025 at 2:00 PM )
What Do the Flags Mean?
- Blue flags indicate suitable boating conditions.
- Red flags indicate potential health risks. Consider
avoiding areas where red flags are posted or make sure you wash after boating nearby.
About CRWA's Water Quality Models
CRWA uses 3 different indicators to determine flag colors at each boathouse.
- If any Combined Sewer Overflow (CSO) discharges took place in the past 48 hours in the Lower
Basin, red flags will be flown downstream of the CSO discharge point.
- If a cyanobacteria (blue-green algae) bloom is present in the vicinity, red flags will be flown
at the affected boathouses for the duration of the bloom. Cyanobacteria monitoring is conducted at least every two
weeks while the notification program is active and CRWA coordinates closely with the MA Department of Public Health
who recommends bloom advisories.
- If CRWA's water quality models predict E. coli bacteria levels that exceed the State's
recreational water quality standard threshold value, red flags will be flown at the affected boathouses. CRWA estimates E. coli
bacteria levels using models that take into account rainfall and river
flow. CRWA collects water samples weekly to verify the model forecasts.
Stay Updated
The best way to keep up with the CRWA flagging updates is to sign
up for email alerts. Email communications from the CRWA are the official source for status updates relating to
the Charles River.
You can also follow the CRWA Flagging Program on Twitter to get
automated updates from this website.
