Drainage network dynamics and knickpoint evolution in the Ebro and Duero basins: From endorheism to exorheism

The study of fluvial network rearrangement provides a key to understand past and future landscape evolution. Large perturbations of hydrographic basins such as the change from endorheism to exorheism have repercussions in the steady or disequilibrium state of the basins and their drainage divides. Such transitions from internal to external drainage imply a major lowering of the geomorphological base level causing a major retreating knickpoint wave that separates the upstream low-relief area (inherited from the endorheic period) from the downstream incised area. Subsequently, the water divide migrates to reach an equilibrium in which erosion rates at both sides of the divide are similar. Previous fluvial analyses suggest that both the Duero and Ebro drainage networks, the two largest catchments in Iberia, experienced a change from endorheism to exorheism sometime between the Late Miocene and the Pliocene. Fluvial capture evidence argues for a Pliocene westward migration of the Ebro-Duero divide implying an area decrease for the Duero fluvial network (victim) in favor of the Ebro (aggressor). We used river profiles, knickpoint distribution and Chi-map calculation to understand the different degree of erosion of the Duero and the Ebro catchments and the dynamics of their drainage network. The results show an equilibrated Ebro drainage network in contrast with a disequilibrium in the erosional state of the Duero drainage, which remains reorganizing and adapting to the newly-imposed Atlantic base level. We identified at least two knickpoint wave trains in the Duero drainage resulting from the onset of exorheism: a fast-propagating wave through the cover and a low-propagating wave affecting the bedrock. Field evidence and topographic analysis suggest a westward migration of the Ebro-Duero divide, resulting in an ongoing headward erosion of the Ebro against the Duero catchment. Chi analysis provides the degree of disequilibrium of the drainage network indicating a large-scale aggressor role for the Duero and a victim role for the Ebro. We interpreted this seeming contradiction as the result of a different time scale perspective: local divide observations indicate a victim Duero in the short-term, whereas basin-scale dynamics support a victim role for the Ebro fluvial network in the long-term (multi-million-year time-scales). (C) 2018 Elsevier B.V. All rights reserved.