Couesnon

The Couesnon (French pronunciation: [kwenɔ̃]; Breton: Kouenon) is a river running from the département of Mayenne in north-western France, forming an estuary at Mont Saint-Michel.

However, the modern administrative boundary separating the two regions does not follow the river course; it is some six kilometers west of the Mont.

The Couesnon, the Sée and the Sélune form part of the complex water system of the bay of Mont Saint-Michel.

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.

the pH has been measured at 8.26 (Hogan, 2006) and summer water temperature as 19 degrees Celsius in the area of the mouth.

Harold Godwinson and his men cross flumen Cosnonis (the Cousesnon); Bayeux Tapestry
The mouth of the Couesnon, at Mont Saint-Michel
The muddy edges of the Couesnon are almost like quicksand to the unwary.
The causeway connecting Mont Saint-Michel to the mainland, responsible for the massive buildup of mud and sediment in the area