Sediment patterns and source areas within the Letaba River, Kruger National Park, South Africa

Abstract

Sediment distributions vary across a river system depending on the hydraulic processes operating at different river reaches and affect flow regimes and influence the formation of landforms. The research was conducted in the Letaba River which is sourced in the Eastern Escarpment and flows through Kruger National Park (KNP), South Africa. It flows across bedrock types including granite and granitic gneiss, and volcanic and sedimentary rocks of the Karoo Supergroup. This underlying geology has played a role in the development of the river system, geomorphology and sediment patterns. This study aims to understand sediment sources and dynamics on the KNP section of the Letaba River as it flows through the KNP, focusing on the distribution of grain sizes, heavy minerals and minor trace elements in the different storage areas within the river system, and identifying the potential sediment source areas with the use of a multivariate sediment mixing model. Field data collection involved geomorphic mapping along river reaches within KNP, and geomorphic features such as sandbars, subaqueous dunes, levees, overbank deposits and bedrock outcrops were identified. Sediment samples were collected from the main river channel and four tributaries. Results of grain size analysis based on the Folk and Ward method indicate that the majority of the sediments present on geomorphic features are coarse grained, negatively skewed and mesokurtic to leptokurtic. The presence of overbank deposits and coarse grain sizes indicates that the river geomorphology is modified by the occurrence of floods. The distribution of heavy mineral assemblages highlights the role of hydraulic processes in the distribution of minerals derived from the underlying geology. X-ray fluorescence analysis of the sediment measured a total of 18 trace elements within the sediment. The trace elements that were identified are: Zr, Ba, Cr, Sr, V, Ni, Cu, Zn, Rb, Y, Co, Pb, Th, Ga, Sc, Nb, Mo, U, and are listed in order of abundance. Results from the multivariate sediment mixing model indicate that the highest relative contribution of sediment is derived from the channel bed (55%), followed by the channel bank (28%), and the tributaries contributing the lowest proportion (17%) of the sediment. The results of this study highlight spatial patterns of morphological features, the influence of floods in the development of the river system and the identification of relative percentage contributions from potential sediment sources. This can be used to determine the role of tributary floods in sediment erosion and transport along the main river channel. This research can also be used to inform catchment management plans aimed at reducing erosion and sediment influx into the Letaba River.

Keywords: sediment, grain sizes, geomorphic features, sediment sources, multivariate sediment mixing model, Letaba River.