1500 ans d’histoire fluviale dans la vallée du Gage (bassin supérieur de la Loire, sud-est du Massif central, France) : approche pluridisciplinaire = 1500 years of fluvial history in the Gage Valley (Upper Loire basin, south-eastern Massif Central, France): pluridisciplinary approach

A multi-proxy study (litho-stratigraphical survey, magnetic susceptibility, organic matter content, palaeoecology) has been conducted on organic and detrital deposits from the Gage Valley (Upper Loire Basin). This research aims at accurately describe the evolution of this little floodplain over the last 1,500 years, in a medium mountain range context (Ardèche Uplands, France). From the 6th-7th to the 13th centuries AD, basal organic deposits argue for a period of stability with reduced hydro-sedimentary inflows. The macrofossil study indicates the establishment of a marshy floodplain with swamp forests. Palaeoecological and archaeological evidence underlines a major anthropization phase since this period (settlements and agro-pastoral landscape). An aggradation phase characterized by increasing discharges is identified during the first half of the 15th century. It matches with the beginning of the Little Ice Age (LIA 1). This step has resulted in a quick rise of the floodplain ground-level due to the deposit of coarse material. These alluvial deposits are overlapped by a new channel, probably during the 16th century. The infilling of this channel by a sandy-silty to a gravelly sediment began at the beginning of the 17th century (LIA 2). Thus, the current floodplain of the Gage River derives probably from a high-connectivity between upstream and downstream sections, before the installation of the current channel of the Gage River (end of the 19th and 20th century). This chronological reconstruction of the recent evolutions of the Gage floodplain is highly informative as regards climate, morphodynamic and landscape changes during the LIA, on the south-eastern Massif central compared to the Rhone Valley for instance