Methodological aspects of an integrated approach on postglacial floodplains deposits: interests for the reconstruction of the fluvial dynamic of the Loire River during the Holocene

In this methodological paper, the authors want to show the interest of crossing planform analysis with the study of relief anomalies and sedimentological record in order to describe both spatial and temporal evolution on a Loire river's floodplain section at plurimillenial scale. Using a set of georeferenced then digitalized maps and photographs, planform changes are quantified at centennial scale with morphometric parameters. This approach of active channel's evolution is then extended to the whole alluvial plain. Relative chronology of the main steps of floodplain's construction is deduced from surface topographic anomalies. In the aim of reducing river's slope effects, a reference plan based on channel water is subtracted from the LiDAR DEM (Digital Elevation Model). The result of such an operation is a representation of the floodplain's relative altitude, which leads to enlighten several classes of abandoned channels and their active belts. These seqences, where the sedimentological record has embedded the floodplain's history, are then described on cores and trenches at pertinent points. Geometry of the deposits is refined using tomographic electrical resistivity. A systematic use of radiocarbon dating gives absolute ages to the main steps of alluvial plain's construction. The results obtained are then synthetized and reveal the main transformations of the fluvial system during the last past millennia. The changes are mainly driven by Holocene climatic variations like the LIA (Little Ice Age), last major degradation that had an important role in the genesis of current European alluvial landscapes. In sum, the integrated approach allows to describe rhythm, intensity and response time at the Holocene valley scale, and can be used whatever the river considered.