The Couesnon River () is a river. It starts in the "département" of Mayenne in north-western France. It forms an estuary at Mont-Saint-Michel. Its last stretch is the border between the historical duchies of Normandy and Brittany. In the past, its course changed often between two beds. They are on the north and south of the Mont Saint-Michel. In the end, it settled to the south bed. This created ("made") the saying "The Couesnon's madness placed Mont Saint-Michel in Normandy" ("in french", ""le couesnon en sa folie mit le Mont en Normandie""), as the mount is just to the Norman side of the river's mouth, now. However, the administrative boundary which separates the two regions does not depend on the course of the river. The border is about six kilometres west of the mount.