How multiple anthropic pressures may produce unplanned channel patterns: A case study from a lowland sector of the Po River (Italy)

Lowland rivers have been extensively affected by anthropic pressures in many regions of the Earth. The aim of this work is to investigate how a sequence of multiple anthropic pressures that took place on a single river sector can generate mutual feedbacks determining unplanned channel configurations. To enlighten this topic, we reconstructed the last century evolutionary trajectory and the historical human impacts that affected a 121 km length sector of the Po River (Northern Italy).

Two main groups of anthropic pressures acted on the study sector. An extensive training scheme was implemented from the 1930s to the 1950s along a multi-thread sector of the Po River to ensure its permanent navigability by bent navigation structures placed within the active channel, which concentrated the water flow during low discharge conditions into a single sinuous channel. Then, other human activities were carried out for exploiting sediment (in-channel mining) and water (dams’ construction) resources, inducing a dramatic reduction of sediment availability and fluxes along the river. Such activities had a peak of intensity from the 1960s to the 1970s.

Our results show an evolution from predominantly anabranching or wandering patterns in the 1950s to a single-thread configuration at the beginning of the 21th century, accompanied by remarkable narrowing (about -50%) of the active channel. This evolution has been interpreted as follow:  Riverbed lowering occurred exclusively in the main channel in the 1970s and the training works enhanced the disconnection and deactivation of the secondary channels that were located behind and protected by the navigation structures. This localized incision of about -4 m of the active channel was determined by the profound sediment starvation caused by in-channel mining and dams’ construction. In the absence of the navigation structures, it is likely that the channel adjustments would have been less profound, with more homogeneous and less intense riverbed incision along the entire active channel, partial maintenance of activity in the secondary channels and, therefore, potential preservation of the multi-tread patterns characterizing the river sector in the first half of the 20th century.

The current single thread sinuous pattern of the study sector is then the result of these two specific anthropic pressures that acted –and interacted– during the 20th century in this portion of the river. The morphological modifications that led to this result can then be defined as unplanned, that is, not designed but fortuitously caused by multiple diachronous impacts acting on the same river sector for different human 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. A comprehensive understanding of riverine sedimentary, hydraulic and geomorphological processes represents a crucial aspect for properly managing lowland fluvial systems in the Anthropocene, also with a view to restoration strategies on highly impacted riverscapes.