Understanding reference processes: Linkages between river flows, sediment dynamics and vegetated landforms along the Tagliamento River, Italy

The Tagliamento River, Northeast Italy, represents an important Alpine to Mediterranean braided system, where interactions between river flows, sediment dynamics and vegetated landforms can be investigated within a relatively unconfined setting. We analysed data from contemporary and historical sources, including stage records, photographs and topographic surveys. From these we identified river stages at which thresholds in surface hydrological connectivity and biogeomorphological adjustment appeared to occur, contributing to a shifting habitat mosaic. Significant adjustments in landscape elements within the active tract commence at river stages well below bankfull with return periods of a few months. Flow Pulse events with return periods from a few months to 2 years support a dynamic inundation pattern, ranging from a patchwork of isolated water bodies within a predominantly terrestrial landscape at low river stages to isolated vegetated islands within a fully connected aquatic landscape as the river approaches bankfull. Across this range, interactions between flow, sediment and vegetation lead to gradual and abrupt transitions in persistence, form and connectedness of different landscape elements. Bankfull flows (return period over 2.5 years) topple and disperse significant numbers of large trees, seeding the next generation of vegetated patches, and larger floods (return period around 10 years) induce significant turnover of established islands and floodplain surfaces. The results reported in this paper illustrate how extensive interdisciplinary research on a single river system call provide useful insights concerning the time scales and thresholds that characterize water-sediment-vegetation interactions in piedmont reaches of Alpine to Mediterranean braided systems. Anthropogenic effects on river systems are ubiquitous throughout Europe. However, systems such as the Tagliamento River that retain significant process dynamism and morphological integrity, provide a laboratory within which reference processes and process-form interactions can be investigated, understood and then incorporated into innovative restoration design on more impacted systems.