Deep River is a town in Middlesex County, Connecticut, United States. The town is part of the Lower Connecticut River Valley Planning Region. The population was 4,415 at the 2020 census. The town center is designated by the U.S. Census Bureau as a census-designated place. Deep River is part of what the locals call the "Tri-town Area", made u…Deep River is a town in Middlesex County, Connecticut, United States. The town is part of the Lower Connecticut River Valley Planning Region. The population was 4,415 at the 2020 census. The town center is designated by the U.S. Census Bureau as a census-designated place. Deep River is part of what the locals call the "Tri-town Area", made up of the towns of Deep River, Chester, and Essex.