Surface water

The Shawsheen River basin covers about 78 mi2 of northeastern Massachusetts. The basin is characterized by low, rounded hills and many wetlands. It contains a total of 18 lakes and ponds, nine of which have an area of 10 acres or more. The largest lake is Fosters Pond in Andover and Wilmington, which is 135 acres.

The Shawsheen River begins in Bedford, flows northeastward for about 25 mi, and is joined by many small tributaries before entering the Merrimack River in Lawrence. In its upstream half, the river flows in a well-defined channel that meanders over a 200- to 600-ft-wide grassy flood plain. In its downstream half, the river flows through a gently curving pool and riffle channel that is crossed by several dams.

The river has a low gradient and flows through wetlands for nearly one-half of its length. From the headwaters area to the USGS streamflow-gaging station near Wilmington, a distance of about 11 river mi, the channel slope of the Shawsheen River is about 5 ft/mi.

In 1974, about 85 percent of the municipalities in the Shawsheen River basin obtained all or part of their water from sources outside of the basin. The town of Burlington began diverting water from the Shawsheen River near Wilmington in 1973, and at that time was permitted to divert water from January through June at rates that varied from 3.1 to 12.5 ft3/s, depending on streamflow available.