Surface water

The Concord River basin covers about 398 mi2 of northeastern Massachusetts. The basin is heavily forested (about 71 percent of the land area), and contains many wetlands, lakes, and ponds. There are a total of 121 lakes and ponds, 75 of which have an area of 10 acres or more. Three lakes, all in the tributary Sudbury River basin, are over 500 acres in size. They include Whitehall Reservoir in Hopkinton (601 acres), Lake Cochituate in Framingham, Natick, and Wayland (594 acres), and Sudbury Reservoir in Marlborough and Southborough (1,292 acres).

The Concord River begins at the junction of the Assabet and Sudbury Rivers in Concord, and flows about 16 mi northward to the city of Lowell, where it joins the Merrimack River. The main channel is about 47 mi long from the drainage divide in Westborough to its confluence with the Merrimack River in Lowell. The relatively low stream gradient of the river, about 5 ft/mi, causes it to be generally slow moving. The Assabet River begins in wetlands in Westborough and flows generally northward and northeastward to its confluence with the Sudbury River in Concord. The river is about 30 mi long, and, except at dams, has an average channel slope of about 6 ft/mi.

The Sudbury River also begins in wetlands in Westborough before flowing eastward to Ashland, then northward and northeastward to its confluence with the Assabet River. In its lower reaches, the Sudbury River has a lower stream gradient than the Assabet River. The lowest stream gradient, about 1 ft/mi, occurs along an approximately 12-mi reach of the Sudbury River that flows through a national wildlife refuge in the towns of Sudbury, Wayland, Lincoln, and Concord. After joining the Assabet River in Concord to become the Concord River, the stream gradient continues to be very low until it reaches the Tabot Dam in Billerica.

The upper 75 mi2 of the Sudbury River basin are part of the MWRA's water-supply system. The reservoirs in this area, including Sudbury Reservoir, were created to supply water during emergencies and high-demand periods. This part of the basin is also used to convey water from the Wachusett Reservoir in the adjacent Nashua River basin through aqueducts for use in the Concord River basin or in Greater Boston.