Variazioni morfologiche degli alvei dei principali corsi d'acqua veneto-friulani negli ultimi 200 anni

The streams of the Veneto and Friuli Venezia Giulia Regions, as many other Italian streams, have undergone remarkable channel adjustments in the last decades, which have been induced by a range of human interventions. The paper, which reports the state of the art about five streams (Brenta River, Piave River, Cellina Torrent, Tagliamento River and Torre Torrent), aims to compare the evolutionary trends of such streams and the factors that have driven to channel changes in the medium and short periods (last 200 years). The selected streams are relatively large (their drainage basins range between 446 km2 and 3899 km2) and have very wide gravel channels, often with a braided morphology, in the study reaches. Planform changes, in particular channel width variations, have been analysed using maps and aerial photographs (9 to 12 different maps/photos were available for each stream). Bed-level changes have been investigated through cross-section comparisons and field surveys. The river channels have shown very similar evolutionary trends and magnitude of adjustments. Channel changes were not notable over the 19th century, whereas the channels have been in an unstable condition in the following period. Narrowing and incision have been the dominant processes during the 20th century, up to the 1980s-1990s. Channel width reduction has been of 50-70 %, referring to the width in the first half of the 19th century, and bed-level lowering of 2-3 m on average, but locally up to 8-9 m. A different evolutionary trend, except for the Torre Torrent, has occurred in the last 15-20 years, since channel widening and aggradation have been the most common processes. The magnitude of changes during the last phase has been generally much smaller in comparison with those of the previous phases of adjustment. The gravel mining and channelization works have been the main causes of channel changes, in particular as for the main phase of adjustment that took place from the 1950s to the 1980s/1990s. The dams (except for the Cellina Torrent), torrent control works, and reforestation have probably had a smaller effect on channel dynamics than mining and channelization. As pointed out in previous studies, it is confirmed that human interventions have dramatically altered the sediment regime, whereas a significant change of channel-forming discharges has been documented in just one case (Cellina).