Interactions between fluvial and eolian geomorphic systems and processes: Examples from the Sahara and Australia

The remains of once integrated drainage systems are still discernible in many of the hot deserts of the world. Some of these date back to the Mesozoic, as in arid Western Australia, where they now form chains of salt lakes. Others were active in Miocene times, such as the Sahabi Rivers that once flowed from Chad to the Mediterranean. A few remained active during Quaternary interglacials, as in the Sahara. All contributed sediments that were reworked by wind during drier phases to form desert dunes. The desert dunes of Australia and of the 9500 km long chain of deserts extending from the Sahara through Arabia to the Thar desert of India originated from sediments eroded from the uplands and deposited in closed interior basins formed during the course of Late Mesozoic and Cenozoic faulting, rifting and epeirogenic movements. It was the unconsolidated Neogene sediments laid down in large subsiding sedimentary basins such as the Kufra-Sirte basin in Libya and the Chad basin which provided the source material for the Late Pliocene and Quaternary desert dunes of the Sahara. One effect of the progressive build up of high latitude ice sheets during the Neogene was to steepen the temperature and pressure gradients between the equator and the poles, resulting in increased Trade Wind velocities. Stronger winds were better able to mobilize the alluvial sands of the increasingly dry Sahara and adjacent deserts and to fashion them into desert dunes.A more subtle form of interaction between eolian and fluvial processes concerns the interplay between desert dust (loess) accumulation on hill slopes in arid regions and the onset of fine-grained valley-fill accumulation by sluggish, low energy streams in mountain valleys. Some of these Late Pleistocene wetlands served as refuge areas for plants, reptiles and invertebrates at a time when the surrounding desert plains were very cold and dry, as in the Flinders Ranges of South Australia. In the arid Flinders Ranges such valley-fills were built up between 35 kyr and 15 kyr ago, after which the return of the summer monsoon led to stream channel incision and the formation of a network of gullies, often erroneously attributed to human activities. Understanding the causes of such accelerated erosion can prevent unnecessary and expensive attempts at restoration, since the latter are based on the incorrect assumption that overgrazing and present-day land use are responsible.