Deep River is a town in Middlesex County, Connecticut, United States. The population was 4,629 at the 2010 census. The town center is also 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 Deep River, Chester, and Essex, Connecticut.Every year on the third Saturday in July, Deep River hosts the Deep River Ancient Muster, the largest one day gathering of fife and drum corps in the world.