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The term "radar rivers" refers to almost fully aggraded Tertiary basins and valleys that lie beneath the sand sheet in southern Egypt and northern Sudan. These features were first recognized when radar images produced by the shuttle imaging radar (SIR-A) experiment aboard the November, 1981, flight of the space shuttle Columbia were examined. Field investigations have shown that at least the upper 40 m of these ancient valleys (which are hundreds of meters deep) are now filled with alluvium, mainly sand and fine gravels, that was deposited by running water as recently as the late Pleistocene. Concurrent with and subsequent to the last stages of aggradation, these sediments became cemented by calcium carbonate deposited under fluctuating groundwater conditions. The widespread alluvium and ubiquitous carbonate deposits imply climatic conditions with annual rainfall approaching 400-600 ram. The last stages of aggradation are archaeologically dated by the Acheulian remains incorporated within the alluvium. Uranium-series age determinations on 25 carbonate samples have yielded these modal dates for episodes of carbonate formation: <300,000 years B.P., 212,000 B.P., 141,000 B.P., and 45,000 B.P. The 212,000 and 141,000 B.P. dates are minimal age-estimates for two phases of the Acheulian cultural remains buried in the calichified alluvium. The findings reported here suggest that the southern Eastern Sahara had major flowing rivers during the Middle and Late Pleistocene and supported human populations over an extensive area during these periods. [ABSTRACT FROM AUTHOR]