Remote sensing to evaluate long-term changes in fluvial biomorphology

Nowadays, rivers are affected by increasing human pressure, which might eventually result in a loss of their natural characteristics. These changes are further enhanced by a variation in the hydrological regime, mostly connected with climate change, characterized by more frequent and extreme drought and flood events. The present work focused on a 40 km-long reach of the most anthropized large river in Italy, the Po River, which crosses the northern part of the country through metropoles, industrial areas and intensive agricultural fields. The investigation combined satellite images (Landsat dataset) with hydrological records, aiming to shed greater light on changes in river planform and riparian vegetation coverage over summer (June-August) in the period 1984-2023. Satellite data were handled in Google Earth Engine, analyzing common indexes like the Modified Normalized Difference Water Index (MNDWI), used to identify how wetted river planform morphology changed over time, and the Normalized Difference Vegetation Index (NDVI), considered as a proxy of vegetation coverage variations. Variations in river hydrology were investigated in multiple gauging stations, considering the annual and the seasonal minimum water levels, the discharge duration curve, the stage-discharge relationship, and the yearly annual water volume. The comparison of subsequent satellite images pointed out a process of oversimplification of the river planform morphology, accompanied by an increase in riparian vegetation coverage over time. After an initial period where exposed bars tended to be barer and potentially stable over time, in the most recent decade the vegetation has been able to colonize the exposed land more, eventually stabilizing the planform morphology. This result suggests that prolonged dry periods, which are more common in recent years, might impact large rivers like the Po, poten tially enhancing the development of vegetation on exposed land, and eventually resulting in a less dynamic planform morphology. Even if derived from a very local case study, these results are in line with similar investigations performed worldwide, which pointed out a general decrease in river sinuosity, accompanied by an increasing presence of vegetation along banks and bars.