THE ZAMBEZI RIVER - TECTONISM, CLIMATIC-CHANGE AND DRAINAGE EVOLUTION

The longitudinal profile of the Zambezi River forms two concave-upwards sections, with their boundary at the Victoria Falls. This form has been ascribed to the process of pediplantation and the Victoria Falls identified as one of several knick points that have traversed the river since the breakup of Gondwanaland. An alternative model explains the river's long profile by suggesting that the Upper and Middle Zambezi evolved as entirely separate river systems, which only joined together in comparatively recent times. The alluvial sequence of the Middle Zambezi is described and interpreted in terms of the latter hypothesis. The river capture event caused a change in grade and is marked by a deposit believed to record a cataclysmic flood. Capture is inferred to have resulted from overtopping of the lake that formed the end sink of the proto-Upper Zambezi, rather than from headward erosion of the proto-Middle Zambezi. This event is dated, from archaeological evidence, to the peak of the last interglacial, at the end of the Middle Pleistocene. Subsequent drainage diversions between the Middle Zambezi and the Kalahari are interpreted as the product of rifting of the Chobe Graben and aggradation of the Chobe Swamps. River capture by overtopping implies high rainfall over Central Southern Africa at the peak of the last interglacial, which is contrary to predictions that Africa's rain belts then lay north of their modern mean positions. This anomaly is resolved by postulating a southern polar warming episode at that time and agrees with suggestions that the last interglacial was marked by large scale ablation of the West Antarctic Ice Sheet. © 1990.