The Assabet River begins at the Assabet Reservoir (Mill Pond) in Westborough and flows northeast to its confluence with the Sudbury River in Concord. The Upper Assabet segment runs from the Assabet Reservoir to just upstream of Elizabeth Brook in Stow. This segment runs through several mill towns and contains five mill dams and two flood control dams. During dry summers, most of the water that flows in the Upper Assabet comes from the three municipal wastewater treatment plants upstream. There are extensive protected lands along the river, and wildlife viewing, boating, bicycling, and warm-water fishing are popular.
