The Muddy River’s water quality improved by less than one percentage point this year, and the tributary remains the most polluted section of the Charles River.

The Charles River Watershed Association (CRWA) has released its annual river water quality report card, which averages water sample data over a three-year period to determine which sections of the river meet safe boating and swimming bacteria levels.

The Muddy River scored a grade of C, the same as it received last year, which is significantly worse than the rest of the Charles. It has improved from a D- in 2019, when the CRWA started issuing the report.

“The Muddy River, the most polluted tributary, gets a C,” said Elizabeth Norton, executive director of CRWA, in a speech at an event announcing the report card in August. “That’s the one area we test that, literally each year, we’re not seeing progress.”

The tributary is so polluted because it is in a highly developed area when compared to the rest of the Charles, said Marielena Lima, the river science program manager for the CRWA. Since surfaces like concrete or asphalt don’t absorb excess rainfall, all the water drains to the tributary, which can cause sewage overflows.

“There’s a lot of impervious surface, and so any rain that we have will automatically funnel into the Muddy River,” Lima said. “The other big thing is the hydrology of the Muddy River. It’s very man-made, very altered for us to work around the river. That does have impacts on how the water flows and the water quality.”

Lima said the hydrology had recently improved, due to a project by the U.S. Army Corps of Engineers completed in November. The $38 million project dredged the tributary to increase how much rainfall it could absorb. However, much work remains, as the river is still fed by polluted urban rainfall and, in part, underground sewers.

The lower basin of the Charles River, which includes the section that meets Back Bay and Beacon Hill, scored a B- on the report card. This is slightly worse than its score of B last year, which experts attribute to the increased amount of rainfall.

“All of the sites actually went down in grades slightly,” Lima said. “That’s because this three-year average has two wetter years. The trend with bacteria levels is that, when you have rainfall, that allows stormwater runoff pollution to go into the river. Usually when it rains, if you take a sample within 24 hours, you’ll see a higher bacteria level.”

The lower basin’s grade, Norton said, means it is swimmable a little over half the time. However, she noted, the section’s average grade does not reflect the variation in water quality from one place to another. Some locations have substantially improved, despite the overall score.

“There are actually parts of the Charles in the Lower Basin that our data show, near the Longfellow Bridge, that are swimmable virtually all the time,” Norton said. “We think it's time to start having that conversation about opening up areas for swimming.”