Abstract
Coarse sediment transport – defined as including gravel and larger clasts – in mountain channels is of enormous importance due to its consequences for stream ecology, available water storage, and natural hazards scenarios. Near-natural sediment regimes are now known to be crucial to provide the hydromorphological conditions that support dynamic aquatic ecosystems and to achieve or to maintain a good ecological status in rivers. However, coarse sediment transport processes are often in conflict with hazard mitigation as well as hydropower production, as these commonly involve structures which disconnect longitudinal and/or lateral sediment fluxes. In fact, starting from the late 19th century – but massively since the 1950s – Alpine rivers have been equipped with structures such as check-dams, retention basins and bank protections to prevent bed and bank erosion. In addition, many hydropower reservoirs have been built mostly in the period 1930s-1960, and intense gravel mining was carried out in Alpine rivers in the second half of the 20th century. Bed incision, channel narrowing and associated morphological variations have been documented for the 20th century in the main Alpine rivers, and mostly to have occurred since the 1950s (Comiti, 2012; Hohensinner et al., 2020).
While the direct link between human pressures and channel changes in Alpine rivers is evident, the quantitative knowledge of coarse sediments trapped and/or removed from channel beds and the relative role of flood-control measures, of hydropower reservoir and of gravel mining is very often unknown. Such a lack of quantitative understanding of past and present alterations in sediment fluxes may hinder the effectiveness of restoration (or better rehabilitation) measures, as those relying on augmenting or favoring coarse sediment transport. This work provides quantitative estimates of coarse sediment fluxes and of their alterations in rivers of South Tyrol – a paradigmatic region of the Eastern Alps in terms of the variety of geological and climatic conditions as well as in terms of a long history of sediment management – and proposes the linkages between alterations and observed channel changes.