Detailed chronology of mid-altitude fluvial system response to changing climate and societies at the end of the Little Ice Age (Southwestern Alps and Cévennes, France)

Over a historical timescale, landscapes have been strongly affected by fluctuations in climate and by the impact of human societies. This study examines the historical evolution of mid-altitude fluvial systems in the Western Alps and Cévennes (SE Massif Central) in the context of marked climate and anthropogenic change at the end of the Little Ice Age (late 19th century). This work contributes to the reconstruction of river paleodynamics by investigating the nature and chronology of geomorphological changes in upper river basins. In order to produce a detailed reconstruction of hydromorphological changes, we combined two approaches: the study of historical archives and the use of bioindicators (the dating of terraces using dendrochronology and of silt benches using lichenometry in order to reconstruct the evolution of the river channels). The 19th century is a particularly propitious period for the study of surface dynamics because archives have preserved a wealth of qualitative and quantitative data concerning rivers (economic statistics, meteorological and hydrological observations, illustrative documents, descriptions by contemporary observers). It is also a period for which reliable dating methods are available for detrital deposits in rivers. The period provides new information on how the transition between the Little Ice Age and current climate conditions affected the evolution of drainage basins and stream channels by highlighting a succession of phases in the erosive process (e.g., the preparatory role of the drought of 1830–1838 in the erosion crisis of 1855–1870) and refining the chronology of events (very early onset of riverbed incision). The results highlight the effect of climate (small hydroclimatic fluctuations), amplified by strong anthropization, on the rhythm of landscape change and on the relative stabilization of the landscape at the end of the 19th century. In addition, the synchronization of phenomena on the two sides of the Rhone Valley shows that these changes took place on a regional scale.