The
Couesnon River 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. However, the administrative boundary separating the two regions does not depend on the course of the river, and is about six kilometers west of the Mont.