Headwater drainage evolution in a foreland basin system using surfaces and topographic metrics (Zadorra River, Upper Ebro, Spain)

Abstract

[EN]Foreland basins are elongate depressions that develop around the flanks of collisional mountain ranges. Rivers are key agents that shape their landscape geomorphology and evolution. Their upstream headwater areas are geomorphologically dynamic locations prone to drainage network reorganisations via headward erosion by the foreland axial river. They are complex areas to investigate due to geological complexity, the absence of fluvial deposits and long timescales over which drainage evolution occurs. In this study, we examined the Zadorra River, a headwater tributary to the Ebro River that preserves a diverse number of different geomorphological features that record long-term drainage evolution. Four key sectors with different surface configurations and topographic characteristics were identified. We have used surfaces (erosional, depositional and composite) in combination with topographic metrics (longitudinal profiles, hypsometry and drainage divide analysis) to understand the patterns and timing of fluvial incision linked to incision wave propagation and headwater drainage integration by the Mediterranean draining Ebro River. The study describes the drainage evolution of the Ebro headwaters, beginning with the transformation of internally drained basins into east- and south-directed tributaries. The paleo-Arakil River, closer to the advancing Ebro headwaters and Mediterranean divide, was integrated first through headward erosion, while the more distant proto-Zadorra River was integrated later. This spatial and temporal shift focused subsequent drainage evolution on the proto-Zadorra, whose northward erosion captured and beheaded the paleo-Arakil, forming the modern Zadorra-Ebro and Arakil-Ebro tributaries. Geological evidence and fluvial terraces indicate this headwater drainage integration spanned from the Miocene to the Quaternary and will continue northward in the future. The study highlights the geological and geomorphological complexity of foreland basin headwater regions and their interplay for drainage expansion and integration.