The City of Cambridge’s sewer and drain collection system consists of separated and combined catchments. During significant rain storms, combined sewer systems reach their capacity and fill up with a mixture of sanitary waste and rain water. During extended periods of heavy rain, 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 instead of surcharging into the streets.

Both the Charles River and Alewife Brook receive discharges from CSOs. There are seven permitted CSOs on the Charles River: two owned by the 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 sewer separation and stormwater management program. The long term plans for CSO control will reduce the bacteria and other pollutants associated with CSO discharges to both the Alewife Brook and the Charles River.

Because of increased health risks, public health officials recommend avoiding contact with CSO contaminated waters 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 and pose associated health risks.

A Storm Fact Sheet is available at the Massachusetts Department of Public Health web site and a Home Care Guide is available at the Massachusetts Department of Environmental Protection web site.

For information on water quality in the Alewife + Mystic Rivers, consult the Real-Time Water Quality Monitoring page or the Mystic River Watershed Association. The Mystic River web site is designed to collect remote data every 15 minutes and water quality samples daily within the Mystic River Watershed. 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.