Abstract

The Orange River is one of the World’s most turbid; delivering 60 million tons of sediment each year to the western margin of South Africa. Much of this sediment is believed to be from soil erosion, an increasing environmental threat to sustainability in southern Africa. This study focuses on the upper reaches of the Orange River above the Caledon River confluence, because it is here that high rainfall and topographic relief of the Drakensberg Mountains produce most of the Orange River’s suspended load. Comparison of grain size, mineralogy and geochemistry of the suspended sediment load with catchment bedrock soils provides an estimate of the source of the suspended sediment. Major and trace element ratios indicate that the suspended sediment load is primarily derived from Karoo (upper Beaufort and Stormberg groups) sedimentary rocks rather than Drakensberg basalt. The Caledon River carries the largest fine-mud suspended load primarily from the erosion of Karoo sedimentary rock soils. The organic carbon content of the suspended load ranges from 1.0 to 1.3 weight % with δ13C values that range from −19.7 to −16.9‰ PDB. The δ13C values of the organic fraction of soils is highly variable (−21.5 to −12.7‰ PDB) and reflect the mix of C3 and C4 vegetation in the catchment area.

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