Sediment source areas and the role of floods in sediment mixing on the Letaba River, South Africa

Abstract

ABSTRACT The morphodynamics of rivers in semiarid environments tend to be controlled by episodic flood events, but the precise nature of sedimentary processes and sediment source areas within catchments affected by episodic floods remains not well understood. This study presents geomorphological, sedimentary and mineralogical data from different river reaches along the semiarid Letaba River within Kruger National Park, northeast South Africa. Geomorphological and sedimentary data were used to evaluate river morphodynamic behaviour and identify the major controls on landform development. Mineralogical data (heavy minerals and trace elements) were examined from surface sediment samples from different river reaches to determine the potential source areas for sediment reworking within the catchment (channel bank, channel bed, tributaries) for flood-transported sediments. Results of this analysis on downstream reaches show catchment-averaged sediment contributions from the channel bed of 42.5%, tributaries of 35%, and channel bank of 22.5%. These sediment contributions likely arise as a result of the different flow conditions experienced in different locations of the river system during floods. However, there is significant variability between samples from individual reaches, likely related to valley width and bedrock vs sediment availability. The contributions of river sediments from different sources are conceptually developed through a model of sediment source variability at different hydrodynamic stages. This highlights the role of floods in reactivating non-perennial tributaries and adding new sediment into the main channel. Measurements of perennial channel storage, therefore, do not give the whole picture of total sediment availability or activation during floods.