The Stop River is a low and marshy stream in Medfield, Massachusetts, and partly forming the border between Norfolk and Walpole. The river rises near Highland Lake in Walpole, flows 9.3 miles (15.0 km) northwards to join the Charles River in Medfield, and ultimately drains into Boston Harbor.The Medfield Rhododendrons reservation, operated by The Trustees of Reservations, is an important and rare stand of Rhododendron maximum along the river in Woodridge Street, Medfield.