Couesnon

The Couesnon (; ) is a river running from the département of Mayenne in north-western France, forming an estuary at Mont-Saint-Michel. It is long, and its drainage basin is . Its final stretch forms the border between the historical duchies of Normandy and Brittany. Its historically irregular course, shifting between two beds to the north and south of Mont-Saint-Michel until eventually settling on the southern one, inspired the saying Le Couesnon en sa folie mit le Mont en Normandie (“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 modern administrative boundary separating the two regions does not follow the river course; it is some six kilometres west of the Mont. thumb|upright=1.45|Harold Godwinson and his men cross flumen Cosnonis (the Cousesnon); [[Bayeux Tapestry]] thumb|left|290px|The mouth of the Couesnon, at Mont-Saint-Michel

== Geography == thumb|The muddy edges of the Couesnon are almost like quicksand to the unwary. 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 causes large sandbars within the river. On the other, the three watercourses drive the sediment back out to sea.