Wadi Kufra, Libya The ability of a sophisticated radar instrument to image large regions of the world from space, using different frequencies that can penetrate dry sand cover, produced the discovery in this image: a previously unknown branch of an ancient river, buried under thousands of years of windblown sand in a region of the Sahara Desert in North Africa. This area is near the Kufra Oasis in southeast Libya, centered at 23.3 degrees north latitude, 22.9 degrees east longitude. The image was acquired by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture (SIR-C/X-SAR) imaging radar when it flew aboard the space shuttle Endeavour on its 60th orbit on October 4, 1994. This SIR-C image reveals a system of old, now inactive stream valleys, called "paleodrainage systems," which, during periods of wetter climate, carried running water northward across the Sahara. The region is now hyper-arid, receiving only a few millimeters of rainfall per year, and the valleys are now dry "wadis," or channels, mostly buried by windblown sand. Prior to the SIR-C mission, the west branch of this paleodrainage system, known as the Wadi Kufra (the dark channel along the left side of the image), was recognized and much of its course outlined. The broader east branch of Wadi Kufra, running from the upper center to the right edge of the image, was, however, unknown until the SIR-C imaging radar instrument was able to delineate its dimensions: at least 5 kilometers wide and nearly 100 kilometers long (3 miles by 62 miles). The two branches of Wadi Kufra converge at the Kufra Oasis, the cluster of circular fields at the top of the image. The farms at Kufra depend on irrigation water from the Nubian Aquifer System. The paleodrainage pattern unveiled by SIR-C suggests that the location of productive wells at the confluence of the old river valleys is no accident. Quite likely, the water supply for the Kufra Oasis has been recharged by episodic runoff and by groundwater that moved northward in the alluvial fill of the old stream channels. Rainfall was more abundant in this region during parts of the late Quaternary when Stone Age (Paleolithic and later Neolithic) people left their implements along the riverbanks. The SIR-C image, which clearly shows river channels cut into the surrounding bedrock, provides a "road map" for geoarchaeologists to locate artifacts and to better interpret the history of early people and climatic conditions in this region. The area shown is approximately 120 kilometers by 78 kilometers (74 miles by 48 miles). North is toward the upper left. The colors in this image were obtained using the following radar channels: red represents the L-band (horizontally transmitted and received); green represents the average of the C- and L-band (horizontally transmitted and received); blue represents the C-band (horizontally transmitted and received).