Impact of dam construction on river banks evolution and sediment dynamics. A case study from the Po River (Italy).

Rivers draining densely populated landscapes are extremely impacted by modern human engineering: armored beds, artificial levees and dams modified natural fluvial dynamics, and consequently, the evolution of alluvial plains, deltas and coastal environments. Dams, in particular, segmented the longitudinal continuity of the river and reduced (or even interrupted) the export of sediment toward the sea. Here we investigate the impact of the Isola Serafini dam on the upstream portion of the Po River (Italy) influenced by backwater, by using an integrated approach of aerial and satellite images, longitudinal cross-sections, grain size analysis, backscatter data and multibeam bathymetry. The analysis of aerial photographs, acquired every 10 yr since the dam construction in 1960, and of longitudinal cross-sections, allows understanding how the river adjusts its profile in response to the backwater and quantifying areas of net river banks erosion and deposition in meanders. The drowning of the reaches influenced by backwater reduced the progradation of point bars and promoted the deposition of fine grained sediments, as highlighted by grain size analysis on surficial sediment sampled across and along the river course. Calibrated back-scatter data with grain-size distributions of two selected meanders, under the backwater effect and beyond, show how sands are progressively replaced by fine-grained sediments in the meander belt and in the river axis, mainly reflecting the reduction of flow velocity, inferred also by river bed roughness. The understanding of river and sediment dynamics under the influence of backwater due to dam construction is useful when studying pristine systems in which natural backwater affects their evolution, as in the case of the formation of standing water bodies during the drowning of an incised valley.