A width-based approach to estimating historical changes in coarse sediment fluxes at river reach and network scales

Knowledge about historical changes in sediment fluxes in most coarse-bedded rivers worldwide is extremely limited. In consideration of this deficiency, we developed a width-based approach to estimating multi-decade changes in coarse sediment fluxes occurring at reaches of the Po River and 21 of its tributaries in northern Italy. The estimation was based on temporal variations in the reach-averaged width of the river's active channel, and such width was expressed through a dimensionless index of coarse bed material load (I-q). The index was determined in two periods: 1954-1998 and 1998-2020. Statistically significant relationships were found between temporal variations in I-q occurring in reaches of the Po River and at key locations of each specific reach (i.e., upstream reaches and tributaries). Such evidence of coherent changes in sediment transfer through space and time led us to conclude that I-q variations can be regarded as a reliable proxy for historical changes in sediment transport in a river reach. The application of the approach to the investigation of the Po River catchment provided new insights into the historical changes characterizing coarse sediment fluxes along the river and its major tributaries. From 1954 to 1998, an average decrease in coarse sediment fluxes of about -20% and -30% occurred along the river and the terminal sectors of its tributaries, respectively. The estimations showed that coarse sediment fluxes exhibited a slightly lower decrease in the last two decades, with sediment flux recovery occurring only in some tributaries. The results suggest that a profound change in sediment dynamics and fluxes has occurred, and is likely still ongoing, in the Po River system, despite the decrease in human disturbances (e.g., in-channel sediment mining) in more recent times.