Fish and their passage | Ecology of the river | Health of the river

The Charles River is fed by about 80 brooks and streams, and several major aquifers as it flows snakelike for 80 miles, starting at Echo Lake in Hopkinton, through 58 cities and towns in eastern Massachusetts before emptying into Boston Harbor. Its watershed contains 33 lakes and ponds. Despite the river's length and relatively large drainage area (308 square miles), its source is only 26 miles from its mouth, and the river drops only 350 feet from source to sea. It is the most densely populated river basin in New England.

Native Americans used the river for local transportation and fishing, and as a link in the route from southeastern Massachusetts to northern New England. Early European settlers harnessed the river for water power.

Over time, a total of 20 dams were built along the Charles River, mostly to generate power for industry. The dams slowed the flow of the Charles River, hampering the river's ability to cleanse itself with uninterrupted flow. They also flooded pastureland and cut off migratory fish from upstream reaches. In some places, the dams created new stretches of shoreline, and expanded water and land habitat. The best example is the Lakes District.  This scenic area drew thousands of boaters around the turn of the 20th century and became the premier social and recreational spot of its time.

Later industrial use left the river heavily polluted. Formerly abundant fish populations went into severe decline, and adjacent river habitats were compromised. Over the past four decades, environmental initiatives have significantly improved the water quality and ecology of the Charles. Fish have returned to the river and native plants placed along the banks together with restored wetlands have resulted in a return of local birds, small mammals, amphibians and reptiles. 74% of the Charles River is now suitable for swimming in dry weather.