ANCIENT MEGA RIVERS, INLAND DELTAS AND LAKE BASINS OF THE EASTERN SAHARA: A RADAR REMOTE SENSING INVESTIGATION

The Sahara of North Africa includes some of the driest areas on Earth. Yet, its landforms kept a record of past humid climates. During wet phases, the region was green and contained permanent rivers, inland deltas and lakes. When the wet periods ended at about 5.5 ka, the Sahara transformed into a hyperarid desert and its original surface was covered by sand. The vast sand cover has inhibited revealing these fluvial features. Radar waves from space, however, can penetrate the sand cover and reveal underlying topography. Here, radar images from Radarsat, as well as topographic data from SRTM, were used along with a GIS surface-flow model to map such sand-buried features. Data processing unveiled two distinct river systems, and two large lake basins.

The Kufrah River, which straddles the border between Egypt and Libya, flowed north with a gentle gradient (0.6 m/km) comparable to that of the adjacent modern Egyptian Nile. With a length of 950 km and an area of 236,000 km², the Kufrah is the largest river yet identified and mapped in the Eastern Sahara. Based on Geomorphological evidence, observed in space data, it is believed that the Kufrah at some point served as a spillway for overflow from Megalake Chad to the Mediterranean Sea. It is, thus, may have acted as a natural corridor used by human ancestors and animals to migrate northward across the Sahara. The second river of Gilf-Kebir emanates from the Gilf Kebir Plateau in Egypt, and trends northward with an enormous width of 16 km across. Both the Kufrah and Gilf-Kebir rivers terminate with vast inland deltas (47,500 km² and 23,000 km², respectively) at the southern margin of the Great Sand Sea. The trends of their distributary channels indicate that both rivers drained to a topographic depression that was periodically occupied by a massive lake, (the Sand Sea Lake). During dry climates, the lake dried up and roofed by sand deposits which today forming the Great Sand Sea. The enormity of the lake basin provides explanation as to why continuous extraction of groundwater in the oases of Siwa and Al-Jaghbub is possible. A similar lake basin, delimited by former shorelines, was found just across the border of Sudan. This lake (the Northern Darfur Megalake) has a massive size of 30,750 km². These former lakes and rivers could potentially hold vast reservoirs of groundwater, oil and natural gas at depth.