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Updated Combined Sewer Overflow (CSO) Control Plan

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In accordance with the Variances for Combined Sewer Overflow (CSO) Discharges to the Charles River Basin and to the Alewife Brook/Upper Mystic River Basin, the City of Cambridge (City) is developing an update to the Massachusetts Water Resources Authority’s (MWRA) CSO Long-Term Control Plan (LTCP) for the CSO outfalls that the City owns and operates. Through its NPDES CSO Permit, the City is authorized to discharge flows from eleven (11) CSOs. Since the permit was issued in 2009, two (2) of the CSOs have been permanently closed, while two (2) more are temporarily closed pending hydraulic evaluations along the Charles River. Today, four (4) and three (3) active CSO outfalls remain open along the Alewife Brook and Charles River, respectively.

Cambridge is working with and coordinating with the activities of the MWRA and the City of Somerville as they are also developing Updated CSO Control Plans for the variance waters where they own CSOs. For formation on the City of Somerville and the MWRA’s CSO Control Plan efforts visit:

City of Somerville: https://www.somervillema.gov/cso
Massachusetts Water Resources Authority: https://www.mwra.com/harbor/html/cso_variance.htm

The recorded video presentation from the public meeting on December 15, 2022 is available here.

Public Meeting - December 15, 2022 at 6:00 PM

The slides for the public meeting presentation are now available under the "Documents" tab,  Additional information to supplement and support the public meeting presentation is also available and entitled Technical Information in support of the Development of Future Typical Year of Rainfall  and is also available under the "Documents" tab. Attendees are encouraged to review these documents prior to the meeting.

The Cities of Cambridge and Somerville and the Massachusetts Water Resources Authority are working together to improve our sewers to reduce pollution in our rivers and plan for the future.  Join us virtually to learn how climate change predictions are being used to prepare our infrastructure for future storms, and what that means for the future of our sewer and stormwater systems.

Come have a voice in how we can improve our waters and get involved in the CSO Control Planning process. Register online for this Zoom meeting at https://tinyurl.com/CSOmeeting2 

This will be the second joint public meeting to update and seek feedback from the public about combined sewer overflows (CSOs).


The City of Cambridge, the City of Somerville, and the Massachusetts Water Resources Authority are collaborating on CSO Control plans to protect and improve our water resources. The first joint public meeting to inform about combined sewers, combined sewer overflows (CSOs), and CSO controls took place virtually on June 29, 2022. If you could not attend, the recoding of the meeting is now available at this link: https://youtu.be/OWT9VTY_Iu8? and a copy of the presentation can be found at this link: https://youtu.be/q_5TJ85FA9Y

The variances currently extend through August 2024, allowing time for the Cities of Cambridge and Somerville and the MWRA to evaluate further CSO controls to reduce the CSO impacts to the Charles River Basin and the Alewife Brook/Upper Mystic River. The Current Regulatory Schedule:

  • Draft Plan June 2023
  • Final Plan December 2023

Cambridge, the City of Somerville and the MWRA have requested an extension to the regulatory deadlines.


Catherine Daly Woodbury – Senior Project Manager



Page was posted on 9/1/2022 3:30 PM
Page was last modified on 5/15/2023 9:01 AM
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