Improvements to Boston’s three largest rivers are either slow or have stalled, with climate change being one of the main causes.
The rating, from the U.S. Environmental Protection Agency’s annual report, includes the Charles, Mystic and Neponset rivers. Each summer, the EPA assigns grades to sections of the rivers and their tributaries so the public can track improvements in water quality.
This year’s ratings were similar to last year’s. Most river sections received grades of A and B, meaning the water met most swimming and boating standards. The report noted slight improvements in water quality in the Neponset and slight deteriorations in the Charles and Mystic.
Similar to the Seine in Paris, where polluted water led to the delay of the 2024 Olympic Games, Boston’s rivers are struggling with pollution from heavy rainfall that has released raw sewage into the waterways.
Another problem is stormwater runoff, which carries oil, gasoline, dog poop and garbage from streets, roofs and parking lots directly into rivers and streams. Stormwater also carries nutrients such as phosphorus, which can lead to toxic cyanobacterial blooms that are dangerous to people and pets.
The problems are exacerbated by climate change, which increases the frequency and intensity of rainstorms.
Charles River
According to Emily Norton, executive director of the Charles River Watershed Association, there has been a “stall” in cleanup of the Charles for the second year in a row.
Increased rainfall, sewer overflows and stormwater runoff continued to pollute the river. In 2023, nearly 50 inches of rain caused more than 72 million gallons of sewage to be discharged into the Charles through “combined sewer overflows” (CSOs). This occurs when stormwater and sewer systems combine and are designed to overflow into rivers and bays during heavy rain.
As a result, no section of the Charles received a better grade than the previous year, and two areas saw their ratings decline: the lower middle basin from Newton to Waltham fell from A to A-, and the Lower Basin area near downtown Boston fell from B to B-. The Muddy River, which flows into the Charles through Brookline, received the worst grade in the basin, a C.
Mystical River
Water quality in the Mystic River basin ranged from A+ in Upper Mystic Lake to F in Mill Creek in Arlington, a tributary of the Mystic. Other tributaries also received poor overall ratings, including the Aberjona River, the Malden River and Alewife Brook. Belle Isle Inlet, in Winthrop the grade fell from B- to C+.
The low levels are due to ongoing wastewater pollution caused by illegal connections, cross-contamination between sewer and stormwater pipes, and sewer overflows.
Alewife Brook, for example, regularly receives the largest volume of untreated combined sewer overflow in the Greater Boston area: more than 26 million gallons in 2023. The entire Mystic River basin received over 200 million gallons of combined sewer overflow last year.
Neponset River
The Neponset offered a bright spot on the report card, with a slight overall improvement over the last two years and four ponds receiving an A+ grade. The main section of the Neponset, where most recreational activities take place, received A and B grades.
The differences were much greater among the Neponset’s tributaries. Some of the tributaries flowing through urban areas in Milton and Norwood continued to receive low ratings. Meadow Brook in Norwood received the lowest rating of F. Areas with more conservation land, such as Spring Brook in Walpole and Steep Hill Brook in Stoughton, generally received higher ratings.
