Abstract
This work analyzes the river morphology evolution over 70 years in three river reaches of South Tyrol. Mareit, Ahr and the Eisack rivers underwent the typical degradation trajectory of the 20th century: a strong human pressure during the 1960 - 1980 period, characterized by a general floodplain and riverbed narrowing, dramatic reduction of sediment supply and followed by a massive river engineering interventions. In the last 15 years, the three analyzed reaches have been subject to river restoration actions. In order to assess the effects of restoration actions, pre- and post-restoration conditions were analyzed through photointerpretation of aerial imagery and analysis of morphological units surfaces through the application of the Geomorphic Units Survey and Classification System, GUS (Rinaldi et al., 2015). A set of aerial pictures from 1945, 1954, 1985, 2011, 2014 and 2017 was analyzed by a GIS in order to interpret and classify river morphologies. Moreover, two digital elevation models (DEM) generated from airborne topographic LiDAR (in 2010) and bathymetric LiDAR (in 2016) were consulted in doubtful situations. Total surface of each geomorphic units was calculated for every analyzed year and compared to assess the historical modification trajectory of the three study reaches. An overall increase in morphological quality after restoration works has been observed; however, as time passes from restoration actions the Geomorphic Units Richness Index (GUSI-R) and the Geomorphic Units Density Index (GUSI-D) slightly decrease due to a natural readjustment of the river to a new equilibrium condition characterized by weakness of sediment transport (especially coarse) and altered flow regime. Results outlines how river restoration actions needs to be projected in a watershed context, analyzing the limiting factors that characterize the system and promoting extensive action at the watershed scale that aim to improve the overall quality of the river environment.