Water quality

Water quantity is easy to monitor with river gauges. The USGS has plenty of these online.

But how do we know about water quality? Water quality monitoring is work and this resource from Hudson River Park gives some insight. After explaining the work it also reports that "Hudson River Park’s water quality supported indirect contact for recreation 286 out of the past 366 days."

But for a large watershed, it might not just be a single point or a single level of accomplishment. Massachusetts's Charles River Watershed Association uses a report card system for reporting on 39 sampling sites throughout their watershed. 

Since it is continuously changing, Orange County, Florida maintains a Water Quality Dashboard. An amazing interface, but not something that every community has enough data to populate.