Boston’s riverways had their water quality falter last year, mostly as a result of sewage overflows and heavy rains fueled by climate change.

The Charles River Watershed Association (CRWA) released its compiled 2024 data on June 17, showing that while the Muddy River has continued to improve, the Charles has actually gotten worse as ten sewer overflow routes continue to feed into the basin.

“The 2024 report card, with grades ranging from A to C+, shows vast improvements in the Charles River Watershed,” the CRWA wrote. “But, progress is largely stalled. Grades illuminate how stormwater pollution, combined sewer overflows (CSOs), and climate impacts like drought, heat, and increased precipitation continue to impede safe recreation and enjoyment of the river, at a time when residents rely on it the most. “

The lower basin, the section of the Charles River running through Boston, dropped from a B+ in 2023 to just a B in 2024, indicating that water quality was “moderately degraded.”

The scores are a combination of how often E. Coli levels are unsafe, cyanobacteria blooms and the number of CSO discharges. Discharges and cyanobacteria were the problem areas for the lower basin, where an algal bloom lasting three entire weeks at one point in 2024.

2024 also saw 24 CSO discharges, pouring a combined 48 million gallons of unfiltered water into the river. A year earlier, Boston saw heavy rains cause at least 39 overflow events, dumping a combined 72 million gallons of sewage and stormwater. Because scores are averaged over three years, 2023’s downpour still affects the current grade.

That was 136 times the amount discharged a year prior, highlighting the city’s struggles to adapt to climate change. Boston saw severe drought in 2020, followed by heavy rains the next year, followed by severe drought again a year after, then yet more heavy rain 2023 before swinging back to severe drought in 2024. Drought carries its own problems for a river system, but sewer overflows spike when the weather flips back to the opposite extreme.

It’s not all doom and gloom for the watershed. The Muddy River, which has been the focus of concerted daylighting and cleanup efforts in recent years from a coalition of governmental bodies and civic groups, saw its grade climb from C to C+.

That’s still the lowest grade in the watershed, but as the Muddy River Restoration Project continues it could rise further in the years to come. The coalition finished analyzing existing conditions in 2024, working toward a near total transformation of Charlesgate and the surrounding infrastructure.

The CRWA is also targeting the Charles’ ten extant CSO outlets for further improvement. It’s using a combination of public pressure through its Cut The Crap campaign, launched last Earth Day, and legislative ambitions in H.1046, a bill currently at the Massachusetts State House that would eliminate CSO discharges entirely by 2050.

How quickly those could come is another matter. Stefan Geller, the CRWA’s senior communications manager, told The Boston Guardian that legislative paths can be difficult to estimate and might even be preempted in the coming year by other advances.

“Timeframes are a bit harder to answer for a multitude of reasons, as things like the implementation of regulations or project proposals/permitting can vary widely in terms of timing,” he said. “It's very unlikely that CSOs will be eliminated, or stormwater runoff will be dramatically reduced before next year, but there are other forms of progress that can happen in that time, like the Environmental Protection Agency officially adopting the proposed [permitting process] or MWRA possibly committing to building a CSO storage tunnel.”