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Boston Harbor and Massachusetts Bay
MWRA Environmental Quality Department

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Boston Harbor

Boston Harbor encompasses nearly 50 square miles and is an estuary, where fresh water from tributary rivers mixes with the salt water of Massachusetts Bay. The Boston Harbor watershed, or drainage basin, is the area of land that receives all the streams and rainfall to the harbor. There are eight main watershed areas that drain into Boston Harbor, as described in the State of Boston Harbor Report.

Rain and snow on paved land leads to stormwater runoff that carries pollutants into the sewer system or nearby water bodies. Undeveloped land around the harbor with natural vegetation filters and cleans the water that flows through the layers of soil.

The Massachusetts Water Resources Authority monitors water quality at more than 70 locations in Boston Harbor and its tributary rivers. MWRA makes its sampling data available for download from the water quality data page.

Also available for download or printing:

How Clean Is Boston Harbor?
Bacteria in Boston Harbor
(pdf)
Reversing Eutrophication in the Harbor
Harbor beach fact sheets

Photo of sailboat in Boston Harbor
Sailboat in Boston Harbor

Boston Harbor Sampling Locations Harbor sampling locations


All Harbor and river sampling locations

Recent Harbor Temperatures (NOAA)

Download Boston Harbor sampling data

Boston Harbor's tributary rivers