The Couesnon (; br, Kouenon) is a river running from the ''

département In the administrative divisions of France, the department (french: département, ) is one of the three levels of government under the national level (" territorial collectivities"), between the administrative regions and the communes. Ninety ...

'' of

Mayenne Mayenne () is a landlocked department in northwest France named after the river Mayenne. Mayenne is part of the administrative region of Pays de la Loire and is surrounded by the departments of Manche, Orne, Sarthe, Maine-et-Loire, and Ill ...

in north-western France, forming an

estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

at Mont Saint-Michel. It is long, and its drainage basin is . Its final stretch forms the border between the historical duchies of

Normandy Normandy (; french: link=no, Normandie ; nrf, Normaundie, Nouormandie ; from Old French , plural of ''Normant'', originally from the word for "northman" in several Scandinavian languages) is a geographical and cultural region in Northwestern ...

and

Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo: ''Bertaèyn'' ) is a peninsula, historical country and cultural area in the west of modern France, covering the western part of what was known as Armorica during the period ...

. 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 and the Sélune form part of the complex water system of the bay of Mont Saint-Michel. On one side, the

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

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 Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface flo ...

, to reduce the erosion of its banks. In 1969, a

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...

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. During

spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...

s, a

tidal bore Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (serv ...

— a wave caused by the incoming tide — can be seen working its way up the river. The Couesnon flows through the following departments and towns: *

Ille-et-Vilaine Ille-et-Vilaine (; br, Il-ha-Gwilen) is a department of France, located in the region of Brittany in the northwest of the country. It is named after the two rivers of the Ille and the Vilaine. It had a population of 1,079,498 in 2019.

Fougères Fougères (; br, Felger; Gallo: ''Foujerr'') is a commune and a sub-prefecture of the Ille-et-Vilaine department in the region of Brittany in northwestern France. As of 2017, Fougères had 20,418 inhabitants. The Fougères area comprises appr ...

, Antrain *

Manche Manche (, ) is a coastal French département in Normandy, on the English Channel, which is known as ''La Manche'', literally "the sleeve", in French. It had a population of 495,045 in 2019.Pontorson Pontorson () is a commune in the Manche department in north-western France. On 1 January 2016, the former communes of Macey and Vessey were merged into Pontorson. Geography Pontorson is situated about 10 kilometres from the Mont Saint-Miche ...

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 Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

is moderate with a

Secchi disc The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line, and lowered slowly down ...

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