The Charles River is dramatically cleaner than it was a generation ago, but it’s not fully clean. As of 2023, the river meets Massachusetts bacterial water quality standards roughly 70% of the time for swimming and nearly all of the time for boating. That’s a massive improvement from 1995, when swimming standards were met just 19% of the time and boating standards only 39%. Still, lingering pollution from sewage overflows, nutrient runoff, and legacy industrial chemicals means the river isn’t consistently safe for all types of contact.
How Clean the River Is Today
The EPA tracks water quality across the Charles River watershed using E. coli samples collected monthly at 39 sites by volunteers from the Charles River Watershed Association. The 2023 data breaks down by section:
- Upper Middle Watershed: meets standards 89.8% of the time
- Lower Middle Watershed: 88.0%
- Stop River: 83.2%
- Upper Watershed: 78.2%
- Lower Basin (Boston/Cambridge): 74.8%
- Muddy River: 63.7%
The middle stretches of the river tend to be the cleanest. The lower basin, which flows through Boston and Cambridge and is the stretch most people picture when they think of the Charles, meets bacterial standards about three-quarters of the time. The Muddy River tributary is the worst performer, failing standards more than a third of the time.
How It Got This Much Better
In 1995, the EPA launched the Charles River Initiative to tackle what was one of the most notoriously polluted urban rivers in the country. At the time, the lower Charles earned a D grade. The turnaround since then has been significant enough that in 2011, the International RiverFoundation awarded the Charles its Theiss International Riverprize, considered one of the most prestigious environmental awards for river restoration worldwide.
A major driver of the improvement has been controlling combined sewer overflows (CSOs). These are points where old sewer systems dump a mix of stormwater and raw sewage directly into the river during heavy rain. In 1987, 84 active, uncontrolled CSO outfalls discharged into Boston-area waterways including the Charles. By 2015, discharges had been eliminated from 34 of those outfalls and nearly eliminated from five more, cutting annual CSO volumes by over 2.8 billion gallons. Work continues on the remaining outfalls, with monitoring reports published as recently as 2024.
Can You Swim in the Charles?
Technically, the river meets swimming safety standards most of the time, but organized public swimming is still very limited. The City of Cambridge and the state’s Department of Conservation and Recreation have hosted supervised swim events in recent years, and there’s been growing momentum to create a permanent public swimming area. On any given summer day, though, bacterial levels can spike after rainstorms when sewer overflows activate, making the water temporarily unsafe.
The EPA’s real-time monitoring buoy, active from June through October in the lower basin, tracks conditions that influence safety. When bacteria counts are elevated or algae blooms are present, the water should be avoided entirely.
Algae Blooms and Warm-Weather Risks
Excess nutrients, particularly phosphorus from stormwater runoff, fuel algae and cyanobacteria (blue-green algae) blooms in the Charles, especially during warm months. When water temperatures rise, these blooms become more likely and can produce toxins harmful to people and pets. The EPA advises avoiding all water contact when cyanobacteria cell counts exceed 70,000 cells per milliliter, and recommends keeping dogs out of the water during any visible bloom.
Reducing phosphorus is a long-term challenge. A federal analysis found the lower Charles River receives about 40,000 kilograms of phosphorus per year but can only handle roughly 19,500 kilograms without triggering water quality problems. That means an overall reduction of about 54% is needed, with some sources like CSOs requiring cuts as steep as 96%.
Fish Advisories Still in Place
While the water itself has improved, chemicals embedded in river sediment from decades of industrial use remain a concern. The Massachusetts Department of Public Health maintains active fish consumption advisories for the entire Charles River as of 2025.
In the upper stretch, from Franklin to Natick, the hazards are mercury, chlordane, and DDT. Children under 12 and anyone who is pregnant, nursing, or may become pregnant should not eat any fish from this section. Everyone else should limit consumption to two meals per month.
In the lower stretch, from Natick to the Museum of Science dam in Boston, the contaminants shift to PCBs, pesticides, and PFAS. The advisories here are stricter: no one should eat carp from this section, and all other species should be limited to one meal every six months. Children and pregnant or nursing individuals should avoid largemouth bass entirely and limit other species to one meal every six months.
These advisories reflect legacy contamination that will take decades to fully clear, even as bacterial water quality continues to improve.
The Bottom Line on Water Quality
The Charles River is genuinely, measurably cleaner than it was 30 years ago. Boating is safe virtually all of the time. Swimming is plausible on most dry-weather summer days, though still not officially encouraged outside of organized events. The main remaining problems are post-rainstorm sewage spikes, warm-weather algae blooms fueled by phosphorus runoff, and persistent industrial chemicals in the sediment that make eating fish risky. It’s a river in the middle of a long recovery: no longer the punchline it once was, but not yet the pristine waterway its advocates are working toward.