A tenfold increase in the Orange River mean Holocene mud flux: implications for soil erosion in South Africa

Soil erosion poses a major threat to sustainable agriculture in southern Africa but is difficult to quantify. One measure of soil erosion is the sediment flux of rivers. The Orange River is the principal source of sediment to the western margin of South Africa with an estimated mean mud flux over the last 11 500 years (the Holocene epoch) of 5.1 (3.2—7.4) million metric tons/year (Mt/yr). A total of 43 gigatons (Gt; 10¹⁵ g) representing 72% of the Holocene mud flux has accumulated on the shelf in the Orange River prodelta and mudbelt, a clayey fine-silt deposit focused on the inner to middle shelf. Only 8% (5 Gt) of the mud flux occurs in Holocene calcareous ooze on the slope. Comparison of the clay to mud ratio of offshore deposits with Orange River suspended sediment and catchment soils indicates that 20% (11 Gt) of the Holocene mud flux has been lost as clay beyond the margin. The Orange River mud flux prior to the building of large dams (1930—1969) is ten times greater than the mean Holocene mud flux and is reconciled with estimates of soil erosion within the catchment. A tenfold increase in the Orange River mud flux implies up to a hundredfold increase in total soil erosion depending on the extent of mud storage over periods of decades to centuries within the catchment. Erosion has shifted from areas of high relief and rainfall of the Drakensberg escarpment during the Holocene to intensely cultivated lands of low relief having moderate to high rainfall in the eastern catchment and to a lesser extent, grazing areas of the southern Orange River catchment.