THE OKAVANGO DELTA

Abstract

SUMMARY The Okavango Delta is a large, low gradient (ca. 1:3400), alluvial fan situated in a graben structure which is an extension of the East African Rift system. The catchment is situated in sub-tropical central Angola (rainfall ca. 1000 mm/a), and is underlain by Kalahari sand. Okavango River water therefore has a very low dissolved solid content (ca. 40 ppm), and a very low suspended load. The main clastic sediment load consists of eroded Kalahari sand, which is transported as bedload. Base flow in the river sustains about 4000 km2 of permanent swamps in the Panhandle and upper fan. The latter are flanked by seasonal swamps, which become inundated during advance of the seasonal flood wave. This flood wave takes four months to traverse the 250 km length of the fan. The extent of seasonal flooding is variable and can exceed 12 000 km2. The fan is situated in a semiarid region (rainfall 513 mm/a), in which evapotranspiration exceeds rainfall by a factor of three. Only 15% of inflow plus rainfall leaves the fan as surface flow, the rest being lost to the atmosphere. All of the clastic sediment load and most of the solute load is deposited on the fan. In the Panhandle region, bedload deposition occurs on point bars in a meandering river system, and produces alluvial ridges. Periodic avulsions give rise to anastomosis of the channel. Sedimentation patterns on the fan at the lower end of the Panhandle are closely tied to ecosystem function. Channel margins in the permanent swamps consist of peat, stabilised by papyrus, and are permeable. Water leaks from the channels, causing channel bed aggradation. Loss of water and aggradation in primary channels leads to channel failure, and the destruction of the accumulated peat by fire. Leaked water flows along hippo trails in the backswamps, giving rise to secondary distributary channel systems. Constant change in primary channel location creates a mosaic of wetland habitats in different successional stages, from open water to near climax aquatic grassland habitats. Little clastic sediment reaches the seasonal swamps, and the main sedimentary process is by precipitation of solutes, primarily as a consequence of transpiration. Aquatic plants induce saturation in silica in interstitial water in the root zone. Trees, which grow exclusively on islands, are responsible for intense transpirational water loss which induces saturation in calcite, the precipitation of which results in vertical expansion and hence island growth. Groundwater beneath these islands eventually becomes saline, resulting in species zonation. Islands are initiated by termites, or result from topographic inversion associated with channel failure. The resulting undulating topography creates a variety of habitats which vary in their duration of flooding, and hence in their species composition.