The Assabet River begins at the Assabet Reservoir (Mill Pond) in Westborough and flows northeast to its confluence with the Sudbury River in Concord at Egg Rock. The Lower Assabet segment runs from just upstream of Elizabeth Brook in Stow to the confluence with the Sudbury River in Concord. It runs through the mill towns of Maynard and the former Damondale (now in West Concord) and contains two mill dams. The lower 4.4 miles of this segment—from just below the Damonmill dam (now breached) in Concord to the confluence—together with contiguous sections of the Sudbury and Concord rivers, was federally designated in 1999 as a Wild & Scenic River, reflecting its natural beauty, recreational opportunities, and free-flowing condition. The river borders the Assabet River National Wildlife Refuge.
