Combined Sewer Overflows

The City of Cambridge's sewer and drain collection system consists of separated and combined catchments. During significant rain storms combined sewer systems may fill up beyond their capacity with a mixture of sanitary waste and rain water. A combined sewer overflow (CSO) acts like a relief valve allowing sewerage to discharge into waterways instead of backing up into homes and businesses, and into the streets.


Combined Sewer Overflow Diagram


View map showing all outfalls and contributing catchment areas in Cambridge.

What is being done about CSO's

Both the Charles River and Alewife Brook receive discharges from CSOs. There are seven permitted CSOs on the Charles River: two owned by the Massachusetts Water Resources Authority (MWRA) and five owned by the City of Cambridge. There are eight permitted CSOs on the Alewife Brook: one owned by the MWRA, one owned by the City of Somerville, and six owned by the City of Cambridge. Cambridge is working to reduce and eliminate CSO discharges through a series of sewer separation and stormwater management programs outlined in the MWRA's Long Term CSO Control Plan.  The City of Cambridge is working with the MWRA on projects to reduce CSO's to the Charles River and Alewife Brook.  CSO discharges have already been reduced by 98% since 1988 to the Charles River and 48% since 1987 to the Alewife Brook.  The CSO work is in part guided by the Boston Harbor Court Case and the Lower Charles River Basin and Alewife Brook/Upper Mystic River Basin water quality CSO Variances.

Important Health Information

During large storms CSO's are released to the Charles River and Alewife Brook.  Because of increased health risks associated with CSO discharges, public health officials recommend avoiding contact with waterbodies during rainstorms and for at least 48 hours following a CSO release, and wearing protective clothing such as gloves and boots if contact is unavoidable. Contact with floodwaters should also be avoided as they may contain similar contaminants.

Related links:


For information on water quality in the Alewife and Mystic Rivers, please see the Mystic River Watershed Association website. For information on water quality within the Charles River Basin, consult the Charles River Watershed Association water quality monitoring web site. The CRWA runs a daily flagging program in the lower reaches of the Charles River during the summer months, and a monthly testing program year-round throughout the Charles River watershed.




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Dept of Public Works

147 Hampshire St
Cambridge MA 02139


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Monday: 8:30am-8pm
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