The Couesnon 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 in its madness placed the Mont in Normandy" , 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.
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.
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