THE OKAVANGO DELTA

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 ...