Charles (river, Massachusetts), river in eastern Massachusetts, separating the cities of Boston and Cambridge. Rising near the state border with Rhode Island, the river follows a winding northeastern course of 76 km (47 mi), flowing into Boston Bay between downtown Boston and the Charlestown section. The Charles River joins the Mystic River, flowing from the north, to form inner Boston Harbor. Near the harbor the river has been dammed to form the Charles River Basin, and a system of locks controls the water flow. Every October hundreds of rowing crews compete on the river in the Head of the Charles regatta.