SEDIMENT CHARACTERIZATION OF TWO DRY RIVERS IN THE KALAHARI REGION OF SOUTH AFRICA

The Molopo and the Nossob Rivers are ephemeral rivers in the Kalahari Desert in South Africa located near the intersection of Botswana, Namibia, and South Africa. Both rivers receive extremely low rainfall (mean annual is 150 to 350 mm for the region) and the Molopo is associated with a carbonate rich duricrust. Both riverbeds have some degree of vegetation. Duricrust development in the Kalahari and elsewhere has been linked to the geomorphology of the region, microfauna, and climatic conditions; however little grain size data are available for duricrusts specifically. This study improves the database by analyzing river sediments, including the duricrust, for grain size to characterize the river systems.

Forty-seven samples were collected at 5 m intervals along a transect in each riverbed. The Molopo River was sampled by first removing the duricrust and then sampling the underlying sediments. Dilute hydrochloric acid was added to remove any carbonate. The samples were then wet sieved through a 63 mm sieve using sodium hexametaphosphate to assist in defloculation of clay particles. Weight percent sand is reported. The fine-grained sediments were analyzed on a Sedigraph 5100.

Our results present the mean, mode, and median grain sizes of the Nossob River (NR), the Molopo River with duricrust (MRC), and the Molopo River without duricrust (MR). The NR maintains a consistently lower percentage of coarse silt than the MR, and also has a greater percentage (> 50 %) of clays, suggesting either the NR receives greater degree of transport of fine-grained sediments, or less removal through winnowing, probably related to the vegetative cover present in the study area. The MR and MRC cumulative frequency data reveal only minor differences in grain size distributions between the duricrust and the underlying MR sediments. However, there is a greater variation per sample in mean, mode, and median grain size in the MR samples than in the MRC. The coarse-grained sediments (>63 mm) in both river systems accounted for less than 45% of the bulk sediment in the MR, MRC and NR samples, with the majority significantly lower.