Flagging Program - Live Water Quality Alerts

Water Quality Today

Red flags are being flown at some boathouses due to a Combined Sewer Overflow (CSO). A discharge at CAM005 discharged on 7/25/2025 beginning at 3:05pm and ending at 3:20pm, potentially affecting the Charles River downstream of the Eliot Bridge across from Mount Auburn Hospital. 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

CRWA’s water quality models are predicting blue flags at all other boathouses, based only on E. coli bacteria predictions.

( Last updated: July 26, 2025 at 8:00 AM )

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.