Sedimentation in low-gradient ephemeral channel systems in southern Africa

Ephemeral river systems are vital water resources in semi-arid regions, but remain poorly studied regarding recent sedimentation. Their storage function gains importance under more frequent droughts and rapid population growth. This study addresses this gap by examining the Iishana system in northern Namibia and southern Angola, a transboundary network of ephemeral channels and depressions. The aim is to characterize sediment properties and quantify sedimentation rates in channel depressions to assess their influence on water storage capacity. Sediment profiles were physically and geochemically analyzed and dated by using ²¹⁰Pb and ¹³⁷Cs radionuclides, supplemented by radiocarbon analysis. Results show predominantly fine-grained sand and silt with weak pedogenic developments. Age–depth models indicate very low accumulation rates ranging from 0.017 to 0.12 g cm-2 yr-1(Constant Flux: Constant Sedimentation) and 0.05–0.07 g cm−² yr− 1 (Constant Rate of Supply). While ¹³⁷Cs proved unsuitable due to low activity, ²¹⁰Pb provided robust chronologies. Findings indicate that natural sediment infill currently has little effect on the water storage capacity of ephemeral depressions. They highlight both the potential and limitations of radionuclide dating in southern Africa, providing insights into sediment dynamics in semi-arid, low-gradient systems and contributing to a better understanding of these systems and to improved water management strategies.