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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 Boston Harbor with natural vegetation acts as a filter to clean water that flows through its many layers of soil.

The Massachusetts Water Resources Authority monitors water quality at more than 70 locations in Boston Harbor and its tributary rivers. Long-term Harbor monitoring helps to identify changes in water quality over time.

MWRA makes its sampling data available for download from the water quality data page.


Photo of sailboat in Boston Harbor
Sailboat in Boston Harbor

Boston Harbor Sampling Locations Harbor sampling locations


See all Harbor and river sampling locations

Recent Harbor Temperatures (NOAA)

Download Boston Harbor sampling data

Boston Harbor's tributary rivers