The lower Loire river floodplain formation: a multi-scaled approach to characterize landforms, processes and drivers evolution on the long-term (last 10 milleniums) (Val d'Authion), France

International audienceIn the current context of global change, the knowledge of floodplain formation in response to past climate and societal changes is a solution for a better perception of the current and future dynamics. This research deals with floodplains formation, particularly with fluvial landforms, processes and drivers evolution on the long-term, during the Holocene. The case study is the Lower Loire River floodplain, a major hydrosystem of mid latitudes Western Europe, where current flood risks issues are significant and where the river embankment began during the Middle Ages [Research program: AGES (Ancient Geomorphological Evolutions)]. Fluvial landforms and sedimentary records studied are archived in this wide floodplain (section width 10Km, length 70Km). Hydrosedimentary flows come from a watershed (> 100 000Km2) extending partly on Hercynian massifs (Massif Central and Massif Armoricain) and a meso-cenozoic sedimentary basin (Bassin de Paris). Past fluvial dynamics, processes and temporalities of floodplain formation were reconstructed through an integrated approach [geomatics (LiDAR DEM), morphostratigraphy (core-drillings), sedimentology, geophysics (resistivity methods), geochronology and archaeology]. Within the Lower Loire River floodplain, Holocene fluvial landforms and alluvial deposits are nested in Weichselian formations and vertically stacked. During the last 10 millenniums, many Weichselian inheritances and the relatively low specific stream power of the river explain the low lateral mobility of the active channel. The Loire River built a significant natural levee and induced a vertical aggradation of the distal plain. Fluvial dynamics and sedimentary environments of the Lower Loire River evolved according to multi-millennium and multi-centennial scales. Responses of the river to external factors (climate, anthropogenic impacts, eustatism) and autocyclic evolution are discussed. Several episodes of the floodplain formation are characterized and are correlated with Holocene drivers evolution. Current fluvial dynamics inherit this floodplain formation on the long-term