The Coginchaug River in Connecticut, with a watershed including 39 sq mi of forests, pastures, farmland, industrial, and commercial areas, is the main tributary of the Mattabesset River. It is 16.1 mi long, and the river flows northwards from a point approximately 1.8 mi south of the Durham line in Guilford, Connecticut  into Durham and then Middlefield, meeting the Mattabesset in Middletown, about upstream of the Connecticut River. The name "Coginchaug" comes from a local Native American name for the Durham area and it was the original name for the town. It has been said to mean "The Great Swamp", and is a reference to the meadows found in the central part of town.