About 80 years ago, due to excessive fecal contamination in Boston’s Charles River, swimming was prohibited and beaches were closed indefinitely. However, extensive cleanup efforts over the past decades have led to significant improvements and the water quality now meets swimming standards most of the time. Today, bringing public swimming back to the Charles is getting some serious consideration. To support this effort, we monitored the water quality at several potential swimming locations during the summers of 2010 and 2011. Our monitoring included daily and hourly analysis of E. coli and cyanobacteria cell counts, as well as temperature, turbidity and nutrients. The resulting database consists of about 2,000 E.coli data points. The data allowed us to quantify differences in water quality between the potential swimming locations. We also observed that best correlation between E.coli and precipitation was found by lagging the precipitation by one or two days. The hourly data can show significant variability within the day, with typically higher concentrations at night. Our cyanobacteria data reveal strong dynamics, including blooms and population crashes, the causes of which are not immediately clear. Unlike for E. coli, we don’t have a good understanding of what drives the dynamics of cyanobacteria. The history of the Charles River, including degradation and subsequent recovery, is typical of urban water systems. Bringing swimming back to the Charles would make it the first swimmable urban river in America. This paper serves as a case study that is applicable to any recovering urban river.