Climate change has a direct influence on both hydrology and floodplain vegetation of water courses, which are key players in river morphodynamics. The river system response to climate change is complex and the effects of non-linear interactions between alterations in water, sediment and vegetation remain incompletely understood. Which of these components becomes dominant in shaping the river channel when climate becomes drier or wetter? To answer this question, we investigate the cross-sectional response of sand-bed rivers to climate change focusing on channel width and depth, which respond directly to changes in boundary stresses. Thanks to the exceptional availability of long time-series of daily discharge and cross-sectional profiles, the Pilcomayo River is an ideal living lab for this investigation. We constructed a two-dimensional model of the river using the open-source state-of-the-art, structured Delft3D code. The highly dynamic behaviour and quick morphological adaptation of the Pilcomayo allowed reducing the period of time covered by the simulations because the river cross-section adapts its morphology to a new value of the water discharge within hours or days, which is crucial for modelling investigations. Calibration and validation were successfully performed by comparison with historical data. We considered several scenarios representing current, dryer and wetter climates. The results show that a dryer climate reduces the river channel depth and enlarges the width. A wetter climate increases the channel depth but produces negligible widening. Vegetation, sparser with a drier climate and denser with a wetter climate, is found to control the channel width. This analysis is unique and shows which alterations can be expected in alluvial sand-bed rivers with natural vegetated banks due to climate change.

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The Pilcomayo is a very active sand-bed river located in South America, where during the last decades, quick changes in cross-sectional shape and dimensions have been observed. The total amount of sediment transported by the river is around 140 millions of tons per year, being one of the largest in the world (Martín-Vide et al., 2014). Due to hydrological regime, the flood season is from January to March, with a maximum discharge around 4600 m3/s. However, the monthly average flow during floods is 650 m3/s. During the dry season, the river is almost dry, with the minimum measured discharge being 1.5 m3/s. The bed is formed by fine sand (Fig. 1), with a median size D50 equal to 100 μm. Wash load is formed mostly by silt and clay and represents the 89% of the total amount of sediment transported by the river. The high variability of flows and the presence of a highly erodible bed made of fine sand makes the Pilcomayo a very active river with cross-sections responding very fast to the flow. @en