The Vaia Event: Primary Impacts of the Storm and Subsequent Evolution of the Malgonera Stream (Dolomites)

Large infrequent disturbances (LIDs) increasingly affect mountain basins, but secondary impacts are often disregarded although their understanding would provide advice to river managers. The following work aims at investigating primary and secondary impacts of the Vaia storm on the channel morphology and large wood (LW) load of a reach of Malgonera Stream (6,025 m2). To achieve the objectives, remote sensing and field data were exploited. Remote sensing data were used to compute two DoDs: the first investigated the Vaia impact (2010–2019) while the second the subsequent evolution (2020–2021). LW field data, combined with remote sensing surveys, were collected to compute the LW load entailed by Vaia and the subsequent fluctuations (2020–2021). The net sediment volume after the event was −2,025 m3. The area of erosion was 2,659 m2, while the deposition covered 1,222 m2. The recent DoD (2020–2021) featured a net sediment volume of 15.79 m3, showing irrelevant geomorphic changes. After the Vaia storm, 96.3 m3ha−1 of LW was detected inside the study area. Around 83 logs per ha were classified as single elements while the remaining as jams’ components. Between 2020 and 2021, the LW load increased up to 102 m3ha−1, for a total of 485 elements per ha due to cantilever failures of unstable banks. This work (i) underlines the capability of LIDs to rearrange the morphology of mountain streams, (ii) providing first evidence on secondary processes of LW recruitment along unstable mountain streams and (iii) highlighting how changes in the LW load are detached from morphological changes.