Landslides and Gullies Interact as Sources of Lake Sediments in a Rifting Context: Insights from a Highly Degraded Mountain Environment

Like many other lakes in the world, the interconnected Abaya and Chamo lakes in the Southern Main Ethiopian Rift are affected by rapid sediment accumulation. Although land degradation is a well-known issue in this part of the African continent, the main sediment sources, their spatial distribution and interaction in the Abaya–Chamo lakes’ basin have not yet been documented. Here, we present a systematic inventory, characterization, and spatial analysis of landslides and gullies as concentrated sediment sources, for four representative river catchments impacted by landscape rejuvenation. Using Google Earth imagery and field surveys, we mapped with high accuracy a total of 7336 gullies and 430 landslides. Recent landslides observed during the last decade were few, small and shallow, and appear to have played a minor role in the current sediment dynamics. Large landslides are old and inactive. Although they do not contribute to the current sediment budget, they contribute indirectly to landscape dynamics by favoring the occurrence of gullies. Overall, large percentages of severe to extremely degraded areas of gully erosion are located in rejuvenated landscapes, especially at the level of the old landslides. Many active gullies are connected to the river network, as such acting as the source of sediment. Our analysis is a step towards understanding the nature and control of anthropic activities on sediment production in the region. We also highlight the importance of considering the interactions between sediment sources and the connectivity of the geomorphological system.