The Couesnon River (French pronunciation: ​[kwenɔ̃]) is a river running from the département of Mayenne in north-western France, forming an estuary at Mont Saint-Michel. Its final stretch forms the border between the historical duchies of Normandy and Brittany. Its historically irregular course, alternating between two beds on the north and south of the Mont Saint-Michel but eventually definitely settling to the south bed, inspired the saying "The Couesnon's madness placed Mont Saint-Michel in Normandy" ("le couesnon en sa folie mit le Mont en Normandie"), as the Mont is just to the Norman side of the river's current mouth.