The18.6 year tidal cycle influence on the Couesnon river behaviour, Mont Saint-Michel Bay (France)

River meandering in fluvial environment are well documented. But few studies describe their behaviour in salt marches areas where both tidal currents and river discharge were the main acting processes (Fagherazzi et al., 2004). This study aims at a better understanding of their behaviour on sand flat surfaces, through the example of the Couesnon river that wander at the foot of the Abbey, in the inner part of the Mont-Saint-Michel Bay. Such results are useful in the actual context of coastal anthropisation, or in a local context, to help stake-holders in the project of re-establishment of the marine nature of the Mont-Saint-Michel. A large aerial or satellite image dataset from 1969 to 2007, and a DEM constructed from a LiDAR survey in February 2009, are used to observe the channel position of the river at each date. Those positions are linked to annual high tide levels, annual river discharges and wind data. The analysis of the Couesnon channel migration shows that the river changes its channel planform in a global dynamic, well correlated with the 18.6 year lunar nodal tidal cycle, called the Saros cycle. When the channel migrates to the East, the Saros cycle is on an ascendant phase. When the channel migrates to the West, the Saros cycle is on a descendant phase. Extreme East or West positions of the river channel take place during high or low phases of the Saros cycle, respectively.