Morphological changes of rivers of south tyrol after the little ice age
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This thesis investigates the channel changes of Alpine rivers from the end of the Little Ice Age (1850s) to the 1950s, with the aim to determine the possible role of climatic variations occurred in this period before the onset of anthropic pressures (i.e. dams, check-dams, bank protections and gravel mining). Moreover, within a basin, connectivity plays a major role for sediment availability, transport and storage which affect the stream geomorphic change. The assessment of channel change was conducted on 17 river catchments of South Tyrol (Northern Italy), both glaciated and unglaciated, while the sediment connectivity analysis was performed on a sub-set of 6 basins. A multitemporal GIS analysis approach was adopted to assess the morphological changes (in terms of channel width and pattern) from three different sources: i) Austrian cadastral map (1858), ii) maps from the Italian Institute of Military Geography (1917-1925), iii) two aerial photo sets taken in 1945 and 1954. The assessment of sediment connectivity was carried out by the Connectivity Index, a topographic-based index which aims to identify the different sediment pathways and assess the connectivity of different parts of a catchment. The analysed river network (a total of 480 km) was subdivided into 162 morphologically homogeneous reaches (76 confined, 81 partly-confined and 5 unconfined). The statistical relationships among morphological changes and reach- and basin-scale factors were analysed by means of both univariate and multivariate methods. Overall, the analysed rivers varied their morphological pattern mostly exhibiting a shift from multi-thread/transitional to single-thread patterns, but unchanged planform type were also common. Variations in channel width varied substantially among the analysed rivers, which featured both narrowing (slightly prevailing) and widening (the least common), as well as many cases of very limited changes. The results of connectivity index analysis show that the reach-scale statistical relationship between the average connectivity index and the respective width variation is very weak and statistically not significant, although in some cases tentative trends are apparent. Channel width variations appeared statistically, although weakly, related to some morphometric variables, and significant differences emerge comparing glaciated vs unglaciated basins. Climate-related variations (glacier dynamics and channel disturbance frequency) and connectivity distribution are argued to be the dominant factors that affected channel variations. This thesis work helps us to better understand the general trends observed at the regional scale and allow us to put forward conclusions on the channel width changes occurred before the widespread, direct human modifications of river networks that took place after the 1950s in most of the Alps.
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