How multiple anthropic pressures may lead to unplanned channel patterns: Insights from the evolutionary trajectory of the Po River (Italy)

The aim of this work is to investigate how a sequence of anthropic pressures that occurred on a river sector can generate mutual feedback that determines unplanned channel configurations. We reconstructed the evolutionary trajectory and historical human impacts on a sector of the Po River (Italy). A training scheme was implemented from the 1930 s to the 1950 s along a multi-thread sector of the river to ensure its navigability by bent navigation structures. Other human activities were carried out from the 1960 s to the 1970 s for the exploitation of sediment and water resources, inducing a dramatic reduction in sediment availability along the Po River. Our results show an evolution from multi-thread configurations in the 1950 s to single-thread configurations at the beginning of the 21th century, accompanied by remarkable channel narrowing (-50 %). This evolution has been interpreted as follows: Riverbed lowering occurred exclusively in the main channel during the 1970 s, and the training works enhanced the disconnection and deactivation of the secondary channels that were located behind navigation structures. This localised incision of the active channel (-4 m) was determined by the sediment starvation. In the absence of navigation structures, it is likely that the channel adjustments would have been less profound. The current single thread sinuous pattern is the result of these two anthropic pressures, with the training works defined as an anthropic predisposing condition and the sediment starvation recognised as the triggering factor generating morphological modifications. The channel rearrangement that resulted from these processes can accordingly be defined as unplanned, that is, not designed but caused by diachronous impacts acting on the same river sector for different purposes. The lesson learned from the Po River suggests that anthropogenic, unplanned channel configurations can represent a common type of riverscape in densely inhabited areas significantly affecting recovery potential and future geomorphological trajectories.