Effects of sediment connectivity changes on channel evolutionary trajectory: the case study of the Taro and Ceno rivers in the Northern Apennines (Italy)

The post-1950s evolutionary trajectories of the Taro and Ceno rivers (Northern Apennines, Italy) were investigated, focusing on the correlation between channel adjustments and sediment alterations. The specific aims were to: i) highlight similarities and dissimilarities between the evolutionary trajectories of the two rivers; ii) assess the role of sediment connectivity decrease caused by land use changes along the hillslopes, and iii) evaluate the impact of gravel mining. Changes in active channel width and bed level were quantified using multi-temporal orthophotos (1954-2020) and topographical cross-sections. Land use dynamics were reconstructed through multi-temporal land cover maps, and changes in structural sediment connectivity were assessed by applying the Index of Connectivity (IC). Evolution of landslide-prone areas was analysed to estimate changes in sediment supply, and the influence of anthropogenic activities was also considered through mapping gravel mining areas and in-channel works. Results showed progressive afforestation, after the 1950s in the Ceno and after the 1970s in the Taro basin, with decreases in landslide-prone areas and IC. Hillslope stabilization caused a decrease in sediment supply to both channels, with subsequent channel narrowing and bed incision (-50 % and-3.3 m in the Taro and-38 % and-2.5 m in the Ceno, respectively). More intense morphological changes in the Taro, especially between 1954 and 1976, were linked to the intense gravel mining activity, completely absent along the Ceno. The combined effects of catchment-scale afforestation and reach-scale gravel extraction therefore played a key role in shaping the dissimilarities in the evolutionary trajectories between the two rivers.