Boston Harbor and Massachusetts Bay
MWRA Environmental Quality Department

Tributary Rivers - Water Clarity


MWRA tests for water clarity by measuring Secchi depth in the rivers. Generally, the tributary rivers are not as clear as the harbor because they have higher concentrations of particulates and algae. The water clarity in the rivers has not changed significantly since monitoring began in 1992.

The Charles River has an interesting pattern of water clarity: the water is more clear in the urbanized basin than it is upstream. As the river slows and widens through Cambridge and Boston, the rate of flow decreases, allowing solids to settle out. As a result, Secchi depths increase downstream. (See Spatial Trend in Charles River Water Clarity.)


Chlorophyll concentrations are typically higher in the rivers than in the harbor, because nutrient levels are higher in rivers. Also, the residence time of water is greater in the dammed portions of the rivers than in the harbor. Both these factors allow more algae to build up. Algae levels are at their highest in the rivers and the harbor in the summertime. The Mystic and Charles rivers have the highest amounts of algae; the Neponset the least. Since river chlorophyll monitoring began in 1992, there have been no significant trends in algal blooms over time.

Rivers have higher chlorophyll levels than the harbor, but both ecosystems are subject to dramatic seasonal fluctuations. Chlorophyll levels are high in the summer when the light is intense, and low in the dimmer winter months. River chlorophyll is measured downstream near the river mouth but before mixing with saltwater. Both the Mystic and Charles are very eutrophied, with high average chlorophyll due to poor flushing and high nutrient inputs. The Neponset has the lowest chlorophyll of all the rivers, probably due to a relatively natural river flow and lower nutrient inputs. (See larger image)