The Mystic River is a 3.4 miles (5.5 km) estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 in the southeast corner of the U. S. state of Connecticut

Connecticut

Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

. Its main tributary is Whitford Brook. It empties into Fishers Island Sound, dividing the village of Mystic

Mystic, Connecticut

Mystic is a village and census-designated place in New London County, Connecticut, in the United States. The population was 4,001 at the 2000 census. A historic locality, Mystic has no independent government because it is not a legally recognized municipality in the state of Connecticut...

 between the towns of Groton

Groton, Connecticut

Groton is a town located on the Thames River in New London County, Connecticut, United States. The population was 39,907 at the 2000 census....

 and Stonington

Stonington, Connecticut

The Town of Stonington is located in New London County, Connecticut, in the state's southeastern corner. It includes the borough of Stonington, the villages of Pawcatuck, Lords Point, Wequetequock, the eastern halves of the villages of Mystic and Old Mystic...

. Much of the river is tidal. The location of three large, 19th century shipbuilding firms, Mystic River is now the home of the Mystic Seaport

Mystic Seaport

Mystic Seaport, the Museum of America and the Sea, in Mystic, Connecticut, is notable both for its collection of sailing ships and boats, and for the re-creation of crafts and fabric of an entire 19th century seafaring village...

maritime museum.