It flows northeast from Hopkinton to Boston along a highly meandering route, that doubles back on itself several times and travels through 23 cities and towns before reaching the Atlantic Ocean.

[2] The Charles River is fed by approximately 80 streams and several major aquifers as it flows 80 miles (129 km),[3] starting at Teresa Road just north[4] of Echo Lake (42°12′54″N 71°30′52″W / 42.215°N 71.514444°W / 42.215; -71.514444) in Hopkinton, passing through 23 cities and towns in eastern Massachusetts before emptying into Boston Harbor.

The river is busy, apart from the winter months, with rowing, sculling, canoeing, kayaking, paddleboarding, dragonboating, and sailing, both recreational and competitive.

[9] The major boathouses, starting up stream near Watertown, are Community Rowing, Inc., housing competitive, recreational, and learning programs along with the Boston College Crew; Northeastern University's Henderson; Cambridge Boat Club; Newell, home of Harvard Men's Rowing; Weld, home of Harvard Women's Rowing; Riverside Boat Club; Boston University's DeWolfe; Massachusetts Institute of Technology's Pierce; and, in the Lower Basin, Union Boat Club.

[15] After two decades of water quality improvement efforts spearheaded by the Environmental Protection Agency,[16] on July 13, 2013, swimming for the general public was officially permitted for the first time in more than 50 years.

With catches from the Charles from Natick to Boston the public is advised not to eat carp, and for non-pregnant, non-nursing adults, to limit large mouth bass consumption to no more than twice a month.

[21] Still another explanation is that Quinobequin was a descriptive term for any long body of water for Eastern Algonquin peoples, which European explorers and settlers interpreted as a local proper name.

[21] As native populations were driven out by European settlers, the Charles River became an early center for hydropower and manufacturing in North America.

Waltham was the site of the first fully integrated textile factory in America, built by Francis Cabot Lowell in 1814,[22] and by the 19th century the Charles River was one of the most industrialized areas in the United States.

The arsenal was declared a Super Fund site, and after its closure by the government it had to be cleaned at significant expense before it could be safely used again for other purposes.

Likewise, the many factories and mills along the banks of the Charles supported a buoyant economy in their time but left a legacy of massive pollution.

Their work led to the design initiatives of noted landscape architects Charles Eliot and Arthur Shurcliff, both of whom had apprenticed with Frederick Law Olmsted and Guy Lowell.

The new dam, completed in 1910, stabilized the water level from Boston to Watertown, eliminating the existing mud flats, and a narrow embankment was built between Leverett Circle and Charlesgate.

As sewage, industrial wastewater and urban runoff flowed freely into the river from the surrounding city, the Charles River became well known for its high level of pollutants, gaining such notoriety that by 1955, Bernard DeVoto wrote in Harper's Magazine that the Charles was "foul and noisome, polluted by offal and industrious wastes, scummy with oil, unlikely to be mistaken for water.

"[24] Fish kills and submerged vehicles were a common sight, along with toxic chemical plumes that colored parts of the river pink and orange.

[27] In 1978, a new Charles River Dam was constructed downstream from the Science Museum site to keep salt water out of the basin.

[33] 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.

[35] A study[36] published in the Journal of the American Water Resources Association in April 2008 and completed by researchers at Northeastern University, found high concentrations of E. coli bacteria in the Charles River after a long period of no rain.

[40] Both the annual race[41] and the Conservancy event have been held in deep water with swimmers jumping in off a dock, to avoid the toxic sediments on the bottom of the river that still make beach swimming dangerous.

American Shad used to be one of the most common species in the river until the 1800s when population numbers decreased because of new dams and poor water quality.

With improved water quality and partial dam breaches created in modern times, the CRWA, along with the Massachusetts Division of Marine Fisheries and the U.S.