Surface water

The Charles River and its tributaries drain about 321 mi2 west and southwest of Boston. Although the lower part of the basin is heavily urbanized, about 72 percent of the area above the USGS gage at Dover is covered by forests (Wandle, 1984a, p. 33). The middle and upper parts of the basin also contain many wetlands and small lakes and ponds. Above Watertown, the basin contains 139 lakes and ponds, 65 of which have an area of 10 acres or more. Only one lake, Cambridge Reservoir in Lexington, Lincoln, and Waltham (549 acres), is larger than 500 acres.

The Charles River begins in Hopkinton near Echo Lake, about 30 mi southwest of Boston, and meanders about 80 mi to its mouth at Boston Harbor. The river flows through extensive wetlands in its middle reaches, and stream gradients throughout the basin are relatively low. From the headwaters area in Hopkinton to the USGS streamflow-gaging station at Dover, a distance of about 41 river mi, the channel slope of the Charles River is about 4.5 ft/mi.

Below the Watertown Dam, the river was influenced by tides before the beginning of this century. In 1908, a dam was built just over a mile above the mouth of the river to prevent tidal flooding of the lowlands and to create a pool with a constant water level. This dam was replaced by the Charles River Dam at a nearby downstream site in 1978. The river contains many dams and associated ponds, many of which were mill ponds. The largest former mill pond on the Charles River is the "lakes district" in Waltham, Newton, and Weston, which was created about 1812 by increasing the height of a dam in Waltham.

Today, the flow is affected by releases of water from Stony Brook and Norumbega Reservoirs, diversions from the Sudbury River basin, diversions to Mother Brook, and diversions to Wellesley, Needham, and Dedham for municipal supplies. The largest diversions from the basin are through Mother Brook and to the Stony Brook Reservoir system at the town line between Waltham and Weston, which supplies water to the city of Cambridge. By law, water from the Sudbury Aqueduct can be used to augment the low flow of the Charles River.