Long-term River Trajectories to Enhance Restoration Efficiency and Sustainability on the Upper Rhine: an Interdisciplinary Study (Rohrschollen Island, France)

While the history of a fluvial hydrosystem can provide essential knowledge on present functioning, historical context remains rarely considered in river restoration. Here we show the relevance of an interdisciplinary historical study for improving restoration within the framework of a European LIFE+ project on the French side of the Upper-Rhine (Rohrschollen Island). Planimetric evolution combined with historical high flow data enabled to reconstruct pre-disturbance hydromorphological functioning and major changes that occurred on the reach. A deposition frequency assessment combining vertical evolution of the Rhine thalweg, chronology of deposits in the floodplain and a hydrological model revealed that the period of vertical incision in the main channel corresponded to high rates of narrowing and lateral channel filling. The analysis of filling processes by Passega diagram and IRLS dating highlight that periods of engineering works were closely related to fine sediment deposition, which present also concomitant heavy metal accumulation. In fact, current fluvial forms, processes and sediment chemistry around the Rohrschollen Island directly reflect the disturbances that occurred during past correction works, and up to today. Our results underscore the advantage of combining functional restoration with detailed knowledge of past-trajectory to: (i) understand the functioning of the hydrosystem prior anthropogenic disturbances, (ii) characterize the human-driven morphodynamic adjustments during the two last centuries, (iii) characterize physio-chemical sediment properties to trace anthropogenic activities and evaluate the potential impact of the restoration on pollutant remobilization, (iv) deduce post-restoration evolution and (v) evaluate efficiency and sustainability of the restoration effects. We anticipate our approach to expand the toolbox of decision-makers and help orientating functional restoration actions in the future.