Long-term drainage system evolution in the Wabe Shebele River basin (SE Ethiopia - SW Somalia)

Abstract

Large river systems play an important role in Earth dynamics since they exert an influence on geological, geomorphological, and geochemical processes. On the other hand, these landforms occur in a variety of topographic and plate tectonic settings and tend to persist for 107–108 yr, resisting variations in environmental conditions. Both modern and ancient big rivers configurations suggest that the persistence of such features is much longer on passive margins with long-lasting continental tilting and long-term rainfall without interference from continental glaciation, desertification, and volcanism. The literature on the geological evolution of river systems concentrates almost entirely on large-scale river basins. Very little research has been done to understand the evolution of smaller river networks that have a much lower persistence because they are more sensitive to climatic and tectonic changes. In this work we focused on the Wabe Shebele River basin (SE Ethiopia, SW Somalia) that drains the eastern slope of the Horn of Africa. It is a medium-scale drainage system (drainage area of ~105 km2) developed on a long-lived regional slope to the SE inherited from the Early Mesozoic and influenced by tectonic structures relative to a passive continental margin and slightly affected by volcanism. By taking into consideration the present topography (swath profiles, filtered topography, slope, local relief), the river network (river longitudinal profiles, channel gradient), and the ancient landforms present in the region, we demonstrate that the Wabe Shebele River basin, despite its medium scale, is a long-lived landform persisting at least since the Oligocene. The results show that even smaller river systems can have a long-term history in favorable tectonic and topographic conditions.