High rates of sediment yield in the southern Ethiopian highlands and the role of alluvial fans in regulating sediment delivery to Rift Valley Lakes

• Discharge and sediment load of two Ethiopian mountain rivers was measured. • Extremely high sediment yield values between 1887 and 19517 t/km 2 /a were obtained. • Alluvial fans trap 60% of sediment load, reducing delivery to Rift Valley lakes. Here, we evaluate the suspended sediment yield and its spatial variability along the fluvial system of two tropical mountainous catchments (Elgo and Shafe) which drain into Lake Abaya and Lake Chamo in the southern Ethiopian Rift Valley. We quantified the sediment yield from continuous discharge measurements at six gauging stations during the rainy seasons of three consecutive years from April 2018 to December 2020, and using season-specific sediment rating curves developed based on 1938 suspended sediment samples. The average area-specific sediment yield (SSY) ranges from 1,887 to 19,517t km −2 a −1 for catchments ranging between 46 and 298 km 2 , which is very high by world standards. Whilst SSY increases with catchment area in the highlands itself, an important decrease in SSY is observed in the lowland areas. The large alluvial fan complexes of Elgo and Shafe trap ∼ 40 % and 60 % of the sediment delivered to the apex of the fans, respectively. The alluvial fans thus significantly reduce the amount of sediment delivered from the eroding highlands to the Rift Valley lakes and act as important sediment sink in the catchment’s sediment budget.