The Couesnon (; br, Kouenon) is a river running from the '' département'' of Mayenne in north-western France, forming an estuary at

Mont Saint-Michel Mont-Saint-Michel (; Norman: ''Mont Saint Miché''; ) is a tidal island and mainland commune in Normandy, France. The island lies approximately off the country's north-western coast, at the mouth of the Couesnon River near Avranches and is ...

. It is long, and its drainage basin is . Its final stretch forms the border between the historical

duchies A duchy, also called a dukedom, is a medieval country, territory, fief, or domain ruled by a duke or duchess, a ruler hierarchically second to the king or queen in Western European tradition. There once existed an important difference between " ...

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" ("''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. BayeuxTapestryScene17

Geography

The Couesnon, the

Sée The Sée is a 79 km long river in the Manche department, Normandy, France, beginning near Sourdeval. It empties into the bay of Mont Saint-Michel (part of the English Channel) in Avranches, close to the mouth of the Sélune The Sélune is ...

and the

Sélune The Sélune is an 85 km long river in the Manche department, Normandy, France, beginning near Saint-Cyr-du-Bailleul. It empties into the bay of Mont Saint-Michel (part of the English Channel) near Avranches, close to the mouth of the Sée ...

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 A causeway is a track, road or railway on the upper point of an embankment across "a low, or wet place, or piece of water". It can be constructed of earth, masonry, wood, or concrete. One of the earliest known wooden causeways is the Sweet Tra ...

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. During spring tides, a tidal bore — a wave caused by the incoming tide — can be seen working its way up the river. The Couesnon flows through the following

departments Department may refer to: * Departmentalization, division of a larger organization into parts with specific responsibility Government and military *Department (administrative division), a geographical and administrative division within a country, ...

and towns: * Ille-et-Vilaine: Fougères,

Antrain Antrain (; br, Entraven; Gallo: ''Antrein'') is a former commune in the Ille-et-Vilaine department in the Brittany in northwestern France. On 1 January 2019, it was merged into the new commune Val-Couesnon. Château de Bonnefontaine dates to the ...

* Manche: Pontorson

Hydrology and water quality

Summer flow is typically in the range of . the pH has been measured at 8.26 (Hogan, 2006) and summer water temperature as 19 degrees Celsius in the area of the mouth. Water turbidity is moderate with a Secchi disc reading of 14 centimetres. Electrical conductivity at the mouth is high even at the verge of the tidal influence.

References

* Hogan, C Michael, ''Water quality of freshwater bodies in France'', Lumina Tech. Press, Aberdeen (2006)

External links

The public works project
{{Authority control Rivers of France Rivers of Mayenne Rivers of Brittany Rivers of Pays de la Loire Rivers of Ille-et-Vilaine 0Couesnon