The great inland deltas of Africa

Abstract

At least three large (> 30000 km²), low gradient alluvial fans, termed “Inland Deltas” by early travellers, are developed on the African continent. They have several features in common: (i) all occur in half-graben structures and are fault-bounded at their distal ends; (ii) they have low topographic gradients (0.2 to 0.05 m/km), and an extensive network of distributary channels on the fan surface which sustain vast wetland systems; (iii) all occur in semi-arid settings in which evapotranspiration greatly exceeds rainfall and inflow is derived from high rainfall sub-tropical areas; between 50 and 95% of inflow is lost to the atmosphere; (iv) the feeder rivers are low in suspended load and water chemistry is dominated by bicarbonate and silica; (v) they have extensive seasonal swamps, vegetated by grasses and sedges with numerous tree-covered islands, and variable areas of permanent swamp. Sedimentation is dominated by clastic accumulation in and near channels and by chemical sedimentation (calcrete and silcrete) in the seasonal swamps. Organic material is not preserved because of destruction by fire. The occurrence of these features is a result of the geomorphic influences of plate tectonics and global climatic structure.