Couesnon

The Couesnon River () is a river. It starts in the "département" of Mayenne in north-western France. It forms an estuary at Mont-Saint-Michel. Its last stretch is the border between the historical duchies of Normandy and Brittany. In the past, its course changed often between two beds. They are on the north and south of the Mont Saint-Michel. In the end, it settled to the south bed. This created ("made") the saying "The Couesnon's madness placed Mont Saint-Michel in Normandy" ("in french", "le couesnon en sa folie mit le Mont en Normandie"), as the mount is just to the Norman side of the river's mouth, now. However, the administrative boundary which separates the two regions does not depend on the course of the river. The border is about six kilometres west of the mount.
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 brings lots of sediment. This causes large sandbars within the river. On the other side, the three rivers bring the sediment back to sea.
In the 20th century, the Couesnon was made into a canal. This to reduced the erosion of its banks. In 1969, a dam was built. These changes, and the causeway linking Mont Saint-Michel to the mainland, have made mudflats. The French built a bridge to replace the causeway. It was finished in 2013. This helped to remove the silt.
During spring tides, a tidal bore — a wave caused by the tide rising — can be seen going up the river.
The Couesnon flows through the following departments and towns:
Hydrology and water quality.
In the summer, the water flow is typically in the range of . The pH has been measured at 8.26 (Hogan, 2006). The temperature of the water is 19 degrees Celsius in summer, 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.