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The Nashua River, 60.4 km long, is a tributary of the Merrimack River in Massachusetts and New Hampshire in the United States. It is formed in eastern Worcester County, Massachusetts, at the confluence of the North Nashua River and South Nashua River, and flows generally north-northeast past Groton to join the Merrimack at Nashua, New Hampshire. The Nashua River watershed occupies a major portion of north-central Massachusetts and a much smaller portion of southern New Hampshire.
The North Nashua River rises west of Fitchburg and Westminster. It flows about 48 km generally southeast past Fitchburg, and joins the South Nashua River, shown on USGS topographic maps as the main stem of the Nashua River, about 8 km below its issuance from the Wachusett Reservoir.
