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

Abstract

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.

Life cycle of gullies: a susceptibility assessment in the Southern Main Ethiopian Rift

“…The four catchments (i.e. Shafe, Basso, Sile, and Elgo) were selected as being representative of the small and steep catchments draining the western shoulder of the rift, as they display large gradients in elevation, lithology, land use land cover (LULC) and tectonic influence as well as various levels of land degradation (Belayneh et al, 2022).…”

“…The topography of the area is characterized by plateaus to the west, transitioning into an eastward-dipping slope marked by sharp topographic escarpments (Belayneh et al, 2022). The elevation ranges from ~1100 to ~3300 m a.s.l.…”

“…are not yet affected by the base level fall and experience little or no river incision (Belayneh et al, 2022). The rejuvenation of the landscape caused the occurrence of large landslides; these landslides are old and show very few signs of activity (Belayneh et al, 2022).…”

“…More than 7300 permanent gullies were mapped across the four river catchments using a combination of Google Earth imagery interpretation and field surveys (Belayneh et al, 2022) (Figure 1). The gullies were classified based on their level of activity, i.e., active, partially active, and inactive (Figure 2).…”

“…Modelling both the spatial distribution of the likely initiation point of the active gullies and the position of the active/inactive gully heads will help to constrain the dynamics of initiation, expansion, and stabilization of gullies. We focus on river catchments in the Southern Main Ethiopian Rift, an area characterized by severe land degradation (Schϋtt & Thiemann, 2006;Belayneh et al, 2022;Endale et al, 2022;Tilahun et al, 2022).…”