The Mystic River basin, located in Middlesex and Suffolk Counties, contains all or part of 21 towns and cities, including the cities of Boston, Cambridge, Revere, Malden, Melrose, Chelsea, Everett, Woburn, Medford, and Somerville. Water demands exceed the capacity of the few small aquifers and streams in this basin and many towns and cities are provided with public-water supplies by the MWRA.

The Stony Brook and Hobbs Brook Reservoirs in the Charles River basin, which supply the city of Cambridge, are also major sources of imported water. The following description of the water resources of the Mystic River basin is based on Hydrologic Investigations Atlas 589 (Delaney and Gay, 1980).