The Coginchaug River is a tributary to the Mattabesset and Connecticut rivers. It flows north from Durham, Connecticut through Middlefield, and meets the Mattabesset in Middletown just short of the Connecticut River.

The name, Coginchaug, comes from the 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.