Multidecadal Evolution Of The Turbidity Maximum Zone In A Macrotidal River Under Climate And Anthropogenic Pressures

Climate and human pressures can influence the evolution of estuarine sediment dynamics concurrently, but the understanding and quantification of their cause-effect relationships are still challenging due to the occurrence of complex hydro-morpho-sedimentary feedbacks. The Garonne Tidal River (GTR, upper Gironde Estuary, France) is a clear example of a system stressed by both anthropogenic and climate change, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. To understand the relative effect of each hydrological and geomorphological pressure on the turbidity maximum zone (TMZ), the sediment dynamics in the GTR over the last six decades was evaluated using the width-averaged idealized iFlow model. Model results show a gradual increase in tidal amplitude and currents over the decades that has led to the upstream shift of the landward sediment-transport capacity components (external M-4 tide, spatial settling lag, and tidal return flow). The upstream displacement of the TMZ between the 1950s and the 2010s was estimated to be at least 19 km, of which about three fourth was induced by geomorphological changes and one fourth by hydrological changes. Concerning the geomorphological changes, the natural evolution of the lower Gironde morphology was the main pressure inducing the displacement of the TMZ in the GTR. Anthropogenic and natural changes in morphology and bed roughness in the GTR itself also contributed to this evolution. The natural geomorphological changes were, in turn, probably promoted by the evolution of sediment dynamics, so this study reveals the closed circle that governs the intensification of the TMZ.Plain Language Summary According to local managers, the Garonne Tidal River has experienced an increase of sediment concentration between the 1960s and the 2010s. During this period, the tidal river was subject to decreasing river discharges, morphological changes controlled by natural boundary conditions, and gravel extraction. To test this hypothesis and to better understand the influence of climate and human pressures on the estuarine sediment dynamics, this study evaluates the evolution of sediment-transport patterns in the Garonne Tidal River over the last six decades. In the absence of historical observations of sediment concentration, we applied the idealized model iFlow to different scenarios of river discharge and morphology. Model results show a gradual increase in the landward transport of sediment over the decades. The natural evolution of the morphology was the main pressure inducing the upstream shift of the maximum concentrations. The model uses simplified descriptions of physical mechanisms, allowing for a systematic analysis of the underlying physical processes that contributed to the multidecadal evolution of sediment transport.