The River Stour ( or ) is a river in East Anglia, England. It is 47 miles (76 km) long and forms most of the county boundary between Suffolk to the north, and Essex to the south. It rises in eastern Cambridgeshire, passes to the east of Haverhill, through Cavendish, Bures, Sudbury, Nayland, Stratford St Mary, Dedham and flows through the Dedham Vale Area of Outstanding Natural Beauty. It becomes tidal just before Manningtree in Essex and joins the North Sea at Harwich.