An extraordinary event changed the (morphological) appearance of a famous Alpine stream

ABSTRACT: Extraordinary meteorological events can drive rapid and remarkable changes in riverine environments by supplying high and sudden amounts of water, sediments, and large wood (LW). Compound forces such as windthrow can dramatically exacerbate the impact of these disturbances, especially during long events that can change the connectivity and coupling of sources of sediments and large wood in rivers. This contribution investigates the impact of the Vaia storm (27-30 October, 2018) on the Rio Cordon basin, (Eastern Italian Alps). The occurrence of heavy and prolonged rainfall, summed to high wind speed, generated widespread hillslope instabilities all over the basin causing a huge input of LW into the main channel. More than 100 standing trees (height up to 34 m, diameter up to 62 cm) fell down along 300 m of channel, changing the morphological setting of the stream. Apart from the remarkable amount of LW input in the stream, most of it virtually immobile given the size of the logs, the major effect is related with the geomorphic settings and sediment transport. In fact, LW is already causing flow diversion and incipient avulsions. Several log-steps are already present in the chan nel, retaining sediment and decreasing the channel slope. In this context, the assessment of LW presence and its interaction with the sediments appears of fundamental importance to better comprehend the next dynamics along the channel. The novelty of the presence of LW will offer an important study case for next steps forward into the LW-sediment interaction analysis.