The water quality of the Charles River is often at its worst after a large rainfall because of pollutants carried by runoff, and sewage overflows. For 2011, the EPA reported that the Charles met state bacterial standards for boating and swimming 96% and 89% of the time on dry days, and 74% and 35% of the time on wet days, respectively. Overall boatability and swimability of 82% and 54% in 2011 compare with 39% and 19% in 1995. In June 2018, the Environmental Protection Agency graded the river's 2017 bacterial water quality "A−".