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

département In the administrative divisions of France, the department (, ) is one of the three levels of government under the national level (" territorial collectivities"), between the administrative regions and the communes. There are a total of 101 ...

'' 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 Il ...

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 enviro ...

at Mont-Saint-Michel. It is long, and its

drainage basin A drainage basin is an area of land in which all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...

is . Its final stretch forms the border between the historical duchies of

Normandy Normandy (; or ) is a geographical and cultural region in northwestern Europe, roughly coextensive with the historical Duchy of Normandy. Normandy comprises Normandy (administrative region), mainland Normandy (a part of France) and insular N ...

and

Brittany Brittany ( ) is a peninsula, historical country and cultural area in the north-west of modern France, covering the western part of what was known as Armorica in Roman Gaul. It became an Kingdom of Brittany, independent kingdom and then a Duch ...

. 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. 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 causes 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 ...

, 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, aqua ...

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 T ...

linking Mont-Saint-Michel to the mainland, have caused the buildup of mudflats. To prevent the Mont front 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 ...

s, a

tidal bore A tidal bore, often simply given as bore in context, is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay, reversing the direction of the river or bay's cu ...

— 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 Ille-et-Vilaine (; Gallo language, Gallo: ''Ill-e-Vilaenn'', ) is a departments of France, department of France, located in the regions of France, region of Brittany (administrative region), Brittany in the northwest of the country. It is named a ...

Fougères Fougères (; ; Gallo language, Gallo: ''Foujerr'') is a Communes of France, commune and a Subprefectures in France, sub-prefecture of the Ille-et-Vilaine Departments of France, department, located in Brittany (administrative region), Brittany, no ...

, Antrain *

Manche Manche (, ; Norman language, Norman: ) is a coastal Departments of France, French ''département'' in Normandy (administrative region), Normandy on the English Channel, which is known as , literally "the sleeve", in French. Manche is bordered by ...

: 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 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 both water clarity and wa ...

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 dow ...

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