Upper Assabet River

The Assabet River begins at the outlet of 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. At times of low flow during the summer, 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, and warmwater fishing are popular, but there are relatively few hiking trails.

The Upper Assabet River received an Overall grade of C+. Major improvements in wastewater treatment have resulted in reduced phosphorus pollution. The lower phosphorus concentration in the water column has reduced floating biomass but not eliminated it; the Floating Biomass indicator received a grade of C. The Water Quality grade of C was influenced by an upward trend in release of nitrates; the Nitrates indicator received a grade of D-.