The Couesnon River is a short (90 km, 56 mile long) 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 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 one mile west of the Mont.
The Couesnon, the Sée and the Sélune form part of the complex water system of the bay of Mont Saint-Michel. On one side, the tide brings large quantities of sediment which cause large sandbars within the river. On the other, the three watercourses drive the sediment back out to sea.
In the 20th century, the Couesnon was turned into a canal, to reduce the erosion of its banks. In 1969, a dam was built. These modifications, and the causeway linking Mont Saint-Michel to the mainland, have caused the buildup of mudflats. To prevent the Mont becoming too connected to the mainland, plans are in place to replace the causeway with a bridge, and to modify the dam so that the sediment can once again be driven out to sea.