From concave‐up to concave‐down: river profile transitions driven by lowland avulsions in the Río Colorado terminus system, Bolivia.

River avulsion is a key process in shaping landscapes, rapidly shifting channels and altering surface processes and sedimentary patterns, particularly in lowland alluvial environments. While avulsion mechanisms are well understood in humid and deltaic regions, many dryland rivers exhibit straight or concave‐down long profiles in their lower reaches, with the mechanisms behind these transformations remaining unclear. In this study, the long profiles of rivers in the Río Colorado terminus system near Salar de Uyuni, Bolivia, are investigated using high‐resolution satellite imagery, in situ measurements and flume experiments. The results reveal that nodal avulsions in upstream sections are typically preceded by multiple local avulsions in the downstream reaches. Analyses of 10 river profiles, each over 10 km in length, show that three profiles transition from concave‐up upstream to concave‐down downstream through nodal avulsion, while the remaining seven maintain relatively straight profiles due to local avulsion. Slope analysis indicates a sharp increase in the final 4 km towards the river termini, associated with a decrease in sinuosity. A novel model is proposed in which rivers prograde basinward through cut‐and‐fill processes, with the landward migration of avulsion locations driven by in‐channel aggradation and overbank flooding. This process divides the original concave‐up long profiles into concave‐up upstream and concave‐down downstream segments, with avulsions occurring primarily at the upstream end of the concave‐down section as the profile straightens. The flume experiments corroborate this model, demonstrating that avulsion locations in dryland river terminus systems tend to migrate landwards, driven by profile straightening and sediment redistribution. These findings offer new insights into the evolution of alluvial rivers in semi‐arid and arid endorheic basins, with broader implications for understanding the formation of both terrestrial river systems and ancient fluvial landscapes on Mars. [ABSTRACT FROM AUTHOR]