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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority


Boston Harbor's Tributary Rivers

The Massachusetts Water Resources Authority monitors water quality at more than 70 locations in Boston Harbor and its tributary rivers. Boston Harbor is an estuary--a marine ecosystem where fresh water from the rivers enters the ocean. The largest rivers draining into Boston Harbor are the Charles, Neponset, and Mystic. Since 1989, MWRA has regularly monitored water quality in these rivers, measuring bacteria, algae, water clarity, dissolved oxygen, nutrients, and suspended solids.

See comparison of clarity in Boston Harbor and Rivers

Download river monitoring data.


The Charles meanders nearly 80 miles from Echo Lake in Hopkinton to Boston Harbor. The river's mouth, originally a tidal estuarine saltmarsh, has been dammed since the early 1900s. The dam controls river flow and flooding, and fills the Charles River basin, a popular urban recreational area. However, the river's ecosystem is degraded by the dam, as it prevents tidal flushing, trapping pollutants in the basin.

Charles River water quality

Charles River page


The Mystic flows from the Mystic Lakes in Winchester and Arlington, east through Medford, somerville, and Chelsea to Boston Harbor.

Only five miles long, much of the river winds through dense residential and industrial areas, and is impacted by pollution from surrounding development. The Mystic is dammed at its mouth.

Mystic River water quality

Mystic River page


The Neponset River travels 28 miles from Foxboro to Boston Harbor. The Neponset estuary is the harbor's largest saltmarsh, and serves as important wildlife habitat and a natural filter of pollutants.

The estuary is designated as an Area of Critical Environmental Concern (ACEC), one of three such areas in the Neponset watershed.

Neponset River water quality

Neponset River page
Photo: Neponset River Watershed Association

More about Boston Harbor