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 head- waters 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 dis- charge 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 km 3 from a catchment area of 275,000 km 2 . 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 vol- ume of the Nile delta and submerged Nile cone in the eastern Mediterranean, which amounts to 150,000 ± 50,000 km 3 . The concordance between these two independ- ent 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