How clean are the Charles, Neponset, and Mystic Rivers?

A new government report dished out grades for most of the waterways in greater Boston.

Kayakers paddle in the Charles River near the Longfellow Bridge. David Lyon

Is that river next to my house actually as clean as it looks? Is the creek running through my hometown safe for my child to wade in on a hot July day? As residents of the greater Boston area get outside this summer, questions like this may be on the minds of many looking for a respite from the heat. A wide-ranging new report from the U.S. Environmental Protection Agency looks to provide some answers. 

The Three Rivers Report Card, released annually each summer, pulls data from the Charles, Mystic, and Neponset Rivers to assign grades based on how safe the water is to swim or boat in. This year, most of the waterways surveyed earned middling grades, but some outliers remain. Overall, the grades show significant improvement from previous decades, but new officials are warning of obstacles related to stormwater pollution, combined sewer overflow (CSO) events, and climate change. 


The grades are based on E. Coli concentrations, as well as combined sewer overflow events and the presence of toxic cyanobacteria. The grades are also reflective of a three-year rolling average. 

Charles River

The Charles River Watershed had some of the highest and lowest grades handed out. The upper middle watershed and lower middle watershed of the Charles both earned As, while the Muddy River Tributary earned a C-. 

A large number of CSO events hampered the grades of the Charles and its tributaries, according to the report. Normally, a combined sewer system collects rainwater runoff, domestic sewage, and industrial wastewater into a single pipe. This wastewater is then transported to a plant for treatment before being discharged into a body of water. But occasionally, the amount of wastewater can exceed the capacity of a sewer system or treatment plant, causing a CSO event. Untreated wastewater then discharges directly into nearby rivers, lakes, and other bodies of water, according to the EPA. These can be very dangerous, exposing people to pollutants and bacteria. 

Last summer, the Charles River Watershed experienced 53 CSO events, discharging 126 million gallons of sewage and stormwater in the river, according to the report. The jump in CSO events dropped the lower basin’s grade from a B to a B-. 


“We hope these grades will spur members of the public to join us in demanding that local, state, and federal government leaders commit to eliminating CSOs, reducing polluted stormwater runoff, and investing in nature-based solutions to return the Charles to a fishable, swimmable river, as the Clean Water Act envisioned,” said Executive Director of the Charles River Watershed Association Emily Norton in a statement. 

Neponset River

The waterways associated with the Neponset River garnered a mixed bag of grades. The river’s mainstem, where most recreation takes place, earned a B. Crackrock Pond, Turner’s Pond, and Ganawatte Farm Pond all earned A+ ratings. On the other hand, Unquity Brook and Meadow Brook were labeled with an F.

Polluted stormwater runoff is the biggest challenge facing the Neponset. This can lead to the growth of invasive species and harmful algal blooms, according to the report. On average, water quality grades drop 22 percent in wet weather, causing many area of the Neponset to be unsafe for recreation. 

“Polluted stormwater runoff from streets continues to be a huge problem. We are working to educate residents and upgrade stormwater infrastructure systems to reduce pollution and prepare for climate change,” said Ian Cooke,  executive director of the Neponset River Watershed Association, in a statement. 

Mystic River

The Mystic River Watershed had some bright spots, but many low points. Upper Mystic Lake earned an A+, and most of the main parts of the river got either B+ or B- grades. But vital tributaries like Winn’s Brook in Belmont, Alewife Brook in Cambridge and Arlington, and Mill Creek in Chelsea showed clear signs of frequent contamination.


Alewife Brook, in particular, is a place where infrastructure improvements should be prioritized, according to the report. Stormwater from the urban communities of Cambridge, Somerville, Arlington, and Belmont all flow into Alewife Brook. It is also the site of frequent CSO events. 

“Alewife Brook is significantly degraded, and its waters routinely pose public health hazards to residents living nearby, at a time when we need clean rivers and healthy urban parks the most,” the Mystic River Watershed Association said in the report. 

Lower grades spurred by climate change could be on the horizon in future reports. All three rivers and their tributaries are experiencing the effects of climate change: heavier downpours, extreme heat, severe storms, and more frequent drought, according to the report. 

In these highly urbanized areas, the consequences of extreme weather are more apparent. 

“As our climate continues to change, the challenges polluted stormwater runoff and combined-sewer overflows pose for our rivers will only worsen,” the report reads. 


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