Effects of River Infrastructures on the Floodplain Sedimentary Environment in the Rhône River

River infrastructures such as dikes, groynes, and dams are ubiquitous on most large rivers, and although their consequences on the riverbed morphology have often been studied, the effect they might have on the river floodplain and margins remains largely unknown. By investigating the structure and composition of floodplain sediments in three areas along the Rhône River that were extensively engineered during the last 150 years, this paper aims to understand whether river infrastructures might systematically induce a change in sedimentation patterns in the river margins. A total of fifteen sediment cores were sampled in three distinct reaches of the Rhône valley downstream of Lyon. They were thoroughly analyzed in terms of grain size, using heatmap representations and end-member analysis. Six metallic elements (Zn, Cr, Pb, Cu, Ni, Cd) were systematically quantified. In six out of the fifteen cores, total organic carbon (TOC), organic contaminants (PCBs) concentrations, and 137Cs activity were also assessed. A sharp change in grain size distribution is consistently identified in the sediment cores of all three study reaches. The sediments above this change are fine (D50 < 100 μm), poorly classified and homogenous. They also show a relative increase in contamination when compared with deeper sediments. We interpret this change in sediment characteristics as the consequence of an abrupt decrease in connectivity between the floodplain and the river channel, likely due to the implementation of navigation infrastructures in the channel in the second half of the nineteenth century. In one case, the dating of the sediment cores allows linking the grain size change to the implementation of navigation infrastructures in the channel. In the other study areas, the effect of engineering seems delayed in time due to local variability. The implementation of river infrastructures resulted in a loss of connectivity between the floodplain and the channel. This was reflected as the homogenization of the floodplain sedimentary environment of the Rhône River all along its course from Lyon to the sea. A similar phenomenon might be present in most engineered rivers across Europe.