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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority

MWRA Combined Sewer Overflow (CSO) Notifications

A combined sewer overflow (CSO) occurs when a large storm overwhelms the sewage system causing rainwater to mix with wastewater and discharge to a nearby water body. This relief measure prevents sewage backups into homes and businesses. MWRA owns combined sewer outfalls that discharge a mixture of stormwater and sewage to the Charles and Mystic Rivers, Alewife Brook, and Boston Harbor. When a CSO occurs at an MWRA-owned outfall, there is a notification displayed on the map below.

The cities of Boston, Cambridge, Chelsea and Somerville manage their own combined sewer outfalls, including CSO reporting. These communities must be contacted directly for information regarding their CSO discharges. To see where all CSO outfalls are located, download this map of MWRA and community CSO sites.

Public health officials recommend avoiding contact with water bodies during rainstorms and for 48 hours afterwards, as there may be increased health risks due to bacteria or other pollutants associated with urban stormwater runoff and CSO discharges. Discharges consist of partially treated or untreated sewage and waste. More Information on Combined Sewer Overflows and how to prevent them.

Subscribe to MWRA CSO discharge notifications

Select CSOs to display:

MWRA CSO Discharge

Discharges from MWRA CSOs and Storage Facilities
Treated discharge
(last 2 days)
Untreated discharge
(last 2 days)
No Recent Treated discharge No Recent Untreated discharge
Community CSOs - More information available from:

    Boston Water & Sewer Commission     Cambridge        Chelsea        Somerville

Data are preliminary and subject to change. MWRA publishes final data in an Annual Report.

Mouse over each map symbol below for the CSO number and other information. Click on the dot to go to the community page with information about their CSOs.

Mouse over each map symbol below for the CSO number and other information. To refresh the web page for the latest CSO event information, press Ctrl +F5 in Windows or Shift + ⌘ + R in Mac. For mobile devices, clear the browser cache first and then refresh.


The CSO discharge table below provides additional details. Discharge duration and volume are estimates. Data for CSOs are preliminary and subject to change.

* Review of metering data indicates that no activation occurred.
Event rainfall is from the National Weather Service gauge at Logan Airport, which may vary substantially from local rainfall conditions.
Volumes for MWR205 include all flow treated by the Somerville Marginal treatment facility, some of which may also be included in the volume reported for SOM007A/MWR205A if it discharged.

Download list of previous MWRA CSO discharges 2020 to present (Excel)

Subscribe to MWRA CSO discharge notifications

More Information on Combined Sewer Overflows

Community CSO information
Cambridge CSO Activations
Somerville CSO Activations