CHARACTERIZATION OF FLUVIAL ISLANDS ALONG THREE DIFFERENT GRAVEL-BED RIVERS OF NORTH-EASTERN ITALY

River islands are defined as discrete areas of woodland vegetation located in the riverbed and surrounded by either water-filled channels or exposed gravels, exhibiting some stability and remaining exposed during bank-full flows. Islands are very important from both morphological and ecological points of view, representing the most natural condition of a fluvial system and are strongly influenced by human impacts. This study aims at analyzing the morphological and vegetation characteristics of three different typologies of islands (pioneer, young and stable) in three distinct rivers in the NE of Italy, affected by different intensities of human pressure. The study was conducted on several sub-reaches of the Piave, Brenta and Tagliamento rivers. The first is a gravel-bed river, which suffered intense and multiple human impacts, especially due to dam building and in-channel gravel mining. The same alterations can also be observed in the Brenta river, which also presents bank protections, hydropower schemes and water diversions. On the other hand, the Tagliamento river is a gravel-bed river characterized by a high level of naturality and very low human pressures. The analyses were conducted using aerial photographs and LiDAR data acquired in 2010 in order to define and distinguish the three different island typologies and to obtain a characterization of ground and vegetation features. The results suggest that the fluvial islands lie at different elevations and this fact implies a different resistance capacity during flood events. Pioneer islands and young islands lie at lower elevations than stable islands causing a lower capacity to survive during considerable flood events, in fact in most cases those islands typologies were removed by ordinary floods. Stable islands lie at higher elevations and only intense and infrequent flood events (RI > 10-15 years) are able to determine considerable erosions. Regarding the characteristics of vegetation, we can observe a strong distinction between the three typologies. Stable islands always exhibit the greatest vegetation height and the presence of these plants, sometimes higher than 30 m, contributes to increase the resistance and the stability of these components of fluvial systems.