From recreation to pollution & nearly back again...
This 1923 photo shows how glamorous it used to be to swim in the Charles River (photo (c) Leslie Jones NEED PERMISSION). Within decades, the river would be closed to swimming & suffered from 3 distinct sources of pollution. With the EPA's Clean Water Act regulating industrial pollution, and work to separate sewage output, 2 of these sources have been largely curbed. The remaining task: mitigate algal blooms by addressing the root problem: lack of roots = lack of habitat for algae-eaters aka zooplankton.
Before urban development, the Charles River was a free-flowing tidal estuary. A complex habitat of wetlands and mud flats surrounded the main channel and supported a diversity of species including shellfish, migratory birds, and anadramous fish.
1892 map of Charles River Basin showing principal sources of pollution. Today, the Charles' lower basin is a typical urban waterbody. Dams maintain a near-constant water level and hardscape (aka concrete sides) covers much of the watershed. Wetlands and littoral vegetation are largely absent.