THE GRADIENT OF THE OKAVANGO FAN, BOTSWANA, AND ITS SEDIMENTOLOGICAL AND TECTONIC IMPLICATIONS

The Okavango alluvial fan of northern Botswana supports the largest we tland in southern Africa (ca 25,000 km(2)). It is situated in a tecton ically active extension of the East African Rift system. Only limited topographic information is available in the area because of its remote ness. The water surface gradient down the axis of the wetland was meas ured using a differential Global Positioning System. Two discrete grad ients occur: 1:5570 on the upper, confined flood plain, and 1:3400 on the lower, unconfined alluvial fan, with a slight downstream steepenin g of gradient on the fan. The change in gradient is a consequence of t he loss of confinement of the seasonal flood water. Historical records indicate that distributary channels are able to meander at both gradi ents. Gross fluvial characteristics may mainly depend on bedload. The regional gradient on the fan represents a balance between clastic sedi mentation on the proximal fan and chemical sedimentation on the distal fan. Although a distributary channel system occurs down the central a xis of the fan, it is presently poorly developed, because of low sedim ent load, with much of the water dispersal occuring by overland flow, forming extensive wetlands. The regional gradients show local, small s cale perturbations and satellite images and aerial photographs indicat e that these are associated with lineaments representing active faults . Changes in channel character are associated with these faults and em phasise the sensitivity of the fluvial system to subtle changes in gra dient. (C) 1997 Elsevier Science Limited.