Cenozoic Evolution of the Nile Basin

Abstract

The Cenozoic evolution of the Nile basin reflects a complex interaction between tectonic, volcanic and climatic events. The Ethiopian and Ugandan headwaters of the Nile attain elevations in excess of 2 km, while the watersheds rise to over 5 km. The Ethiopian tributaries of the Nile (notably the Blue Nile/Abbai and the Atbara/Tekezze) provide the bulk of the flood discharge and sediment load, but the Ugandan tributaries, via the White Nile, provide the reliable dry season discharge responsible for maintaining perennial flow in the main Nile. The hydrologi-cal differences between the Blue and White Nile rivers reflect their very different geomorphic histories, reflecting the Cenozoic tectonic and volcanic evolution of their respective headwaters. The volume of rock eroded from the Ethiopian headwaters of the Blue Nile/Abbai and the Atbara/Tekezze river basins since the extrusion of the Oligocene Trap Series basalts 30 Ma ago amounts to 102,000 ± 50 km3 from a catchment area of 275,000 km2. The volume of rock eroded from the Ethiopian headwaters region of the Blue Nile and Atbara rivers is comparable to that estimated for the total volume of the Nile delta and submerged Nile cone in the eastern Mediterranean, which amounts to 150,000 ± 50,000 km3. The concordance between these two independent estimates is consistent with an Ethiopian source for the bulk of the Nile cone sediment since the Pliocene, when there seems to have been an increase in erosion in the Blue Nile headwaters linked to an increase in the rate of tectonic uplift. The origin of the White Nile as a large, permanent river is intimately connected with the development of the western branch of the East African rift system. Prior to the onset of rifting much of the drainage from central Africa was directed westward, into the Congo basin. Rifting led to the capture of a number of major tributaries of the Congo River and the redirection of river systems, producing south-to-north directed axial drainage within the rift and radial drainage into the Victoria basin. Rifting and associated rift-flank uplift was also responsible for the development of the large lake basins (Victoria, Albert, Edward, George and Kyoga), which characterise the upper White Nile catchment. Exactly when drainage from the present headwaters became linked to the White Nile is still unclear, but it seems likely that Lake Victoria, which today provides around 90% of the runoff to the White Nile, may be no more than about 0.5 million years old. While rift development and ongoing rift-related tectonic activity have controlled the long-term evolution of the White Nile headwaters, it is clear that much of the shorter term variations in discharge are probably related to climate change. There is evidence of a progressive tendency to drier conditions over the past 1.5 Ma, which may have been related to the development of topographic barriers that limited the penetration of humid air masses into the continental interior. Superimposed on this long-term trend are astronomically driven changes and brief, abrupt climatic excursions that were probably driven by events outside tropical Africa.