Surface water

The Nashua River basin covers about 443 mi2 of north-central Massachusetts. About 75 percent of the basin is forested and about 4 percent contains lakes. There are a total of 161 lakes and ponds in the basin, of which 94 have an area of 10 acres or more. Only one lake, the Wachusett Reservoir in Boylston, Clinton, and West Boylston is larger than 500 acres in size. This reservoir, which covers about 4,135 acres, is an important part of the MWRA's water-supply system.

The Nashua River begins at the junction of the North Nashua and the South Nashua Rivers in Lancaster, in the southeastern part of the basin, and flows in a northeasterly direction into New Hampshire, where it joins the Merrimack River at the city of Nashua. The main channel of the river is located along the eastern side of the basin, and western tributaries are much longer than those that enter the river from the east.

The length of the main channel from the drainage divide to the USGS streamflow-gaging station in East Pepperell, several miles south of the State border with New Hampshire, is about 50 mi, with an average channel slope of about 10 ft/mi.

The North Nashua River begins in Fitchburg and flows about 23 mi in a southeasterly direction over about a dozen dams and through Fitchburg and Leominster before joining the South Branch Nashua River in Lancaster. The North Nashua River has a gradient of about 41 ft/mi. The South Nashua River begins at the outlet of Wachusett Reservoir and flows northward about 5 mi, with an average channel slope of about 12 ft/mi, before joining the North Nashua River.

The flow of the Nashua River and its many tributaries is affected by regulation or diversions or both. Some of the runoff in the headwaters region of the Nashua River basin is diverted into reservoirs for use by the city of Worcester, and most of the flow in the South Nashua River basin upstream from Wachusett Dam is diverted out of the basin for municipal supplies in the Greater Boston area and Worcester area.