How does the morphology of natural rivers evolve? The wild Vjosa river in the last five decades

Near natural rivers corridors represent unique dynamic ecosystems, providing fundamental ecosystem services. Hence, research is needed to better understand their functionalities, and references for river response to natural drivers. River systems with larger catchments offer more opportunities for regulation and exploitment, and therefore large free flowing rivers are particularly rare. In continental and Mediterranean Europe only very few large free flowing rivers can still be found. Among them, the Vjosa/Aoos River in Albania and Greece has been recognized as a key large fluvial corridor, a model ecosystem of European significance. In the Balkans, as for other developing and emerging regions, anthropic effects on river systems are not widespread yet, and linked to rapidly occurring social and economic development. There, river systems are facing rapid human exploitation (e.g. sediment mining) with environmental impact assessments often lacking fundamental hydro-morphological and ecological baseline data. We develop a multitemporal analysis of channel changes in the unconfined braided reach of the Vjosa and of the possible causes. To this aim we quantify human pressures - sediment mining, hydropower plants, land use changes - and climate variability, integrating locally collected data with publicly available remote sensing information. The active river corridor experienced halving the maximum and mean width and possibly incision in the absence of relevant upstream human stressors. The most plausible explanation is associated with observed reduction of flood volumes and durations between 1960 and 1990. A first key outcome of our work is the possible range of variability of the active corridor geometry of a wild river, providing an important knowledge base for their management and conservation, especially in the light of possible future infrastructures development and of the challenges in accessing suitable data in many regions of the world. The second important finding of this study is the possibility offered by the Vjosa of isolating the role of hydrological change as the major driver of multi-decadal channel adjustment. Our findings in one of the last large, near natural rivers in Europe can be also used to support river restoration planning in regulated rivers in similar geographical regions.