Anthropogenic activities since the end of the Little Ice Age: a critical factor driving fluvial changes on the Isonzo River (Italy, Slovenia)

Abstract

The Isonzo is a 140‑km long river that draws its source in the Julian Alps in Slovenia and joins the Gulf of Trieste in the Northern Adriatic, Italy. Its catchment area (~3400 km2) consists of mid‑altitude mountains (70%), a piedmont (22%), and a coastal plain (8%) influenced by Mediterranean climatic conditions. The river is a high‑energy system due to its pronounced hydraulical gradients and with frequent damage to man‑made structures. The objective of this study is to determine the recent functioning of the Isonzo River by (i) a diachronic (from the Little Ice Age), plan‑form analysis of the alluvial plain, using old maps, topographic maps of the IGM and aerial photographs, and (ii) an analysis of channel incision and sediment transport, using data derived from national and regional water services and field observations. Over the last 200 years, the Isonzo River shows a clear tendency to active‑channel narrowing between 15 km and 2 km upstream of the river‑mouth. This phenomenon was accompanied by an incision of the active channel, estimated to be between 0.5 m and 1.5 m at the most vulnerable sites. In the context of climate change since ca. 1860, the narrowing/incision of the active channel increased from the 1880s to 1920s, and prticularly since 1960, because of human impacts (channelisation, embankments, dams) in the catchment, and despite the increase of flood frequencies. The recent hydro‑morpho‑sedimentary functioning of the Isonzo shows that the river is still in a transitional phase towards a physical equilibrium that changes after every human intervention in the active channel.