Rivers in the Sand

Archaeological remains indicate the streams drew groups of early humans until about 4,000 years ago, when climate changes made the area unlivable, and USGS researchers hold, ensuing rainy periods temporarily reactivated some of the rivers and streams in the Eastern Sahara.

Abstract

The Eastern Sahara is one of the hottest places on Earth, its par. ched sands moistened by a rair shower every few decades. It may come as a surprise, then, that stretches of this African desert have inspired a scientific debate over water. More specifically, the argument concerns ancient water. From around 2 million to 4,000 years ago, one group ol investigators contends, a braided network of channels set into large valleys in the Eastern Sahara filled with flowing water during extended spells of humidity and substantial rainfall. The valleys attracted early humans at least 200,000 years ago, they say. Another team maintains no such rivei system ever existed. In their view, fierce desert winds hollowed out depressions in the earth that became temporary ponds or lakes after rains. Human activity al these oases was minimal at best, they conclude. One thing is sure: The debate owes its existence to the U.S. space program. The first inkling that the Eastern Sahara once possessed some type ol water drainage system came in 1982, when scientists at the U.S. Geological Survey (USGS) in Flagstaff, Ariz., examined images of Earth taken during a flight of the space shuttle Columbia. The penetrated large areas of the Sahara where the sand sheet is no more than several feet thick, revealing a web ol valleys and smaller channels winding beneath the desert sands (SN: 4/21/84, p.244). In 1984, guided by specially processed maps from Landsat, and other sources, USGS researchers located some of the radar-exposed channels with the aid of a satellite navigation device modified for land use. Excavations on the shores of two sand-covered valleys, directed by archaeologist William P McHugh (who died in May), uncovered hand axes and other stone artifacts dating to approximately 200,000 years ago. The archaeological evidence fits with geological data, described by USGS scientist John F McCauley and his colleagues in the July 1986 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, indicating the radar are missing links of a previously unrecognized trans-African river system. At least 30 million years ago, the researchers maintain, volcanic eruptions and shifts in the Earth's crust carved out the system's major streams, which flowed southwest from headwaters in Egypt and the Sudan, across northern Africa and into the Atlantic Ocean at the Gulf of Guinea. The Amazonian proportions of the river system gradually receded. Geological disturbances cut off stream flow at several points in the valleys around 15 million years ago, McCauley's group says. A northward-flowing river arose about 6 million years ago and amputated stilloperational sections of the river complex from its headwaters. The onset of extreme heat and virtually no rain around 2 million years ago dealt the final blow. Nevertheless, the USGS researchers hold, ensuing rainy periods temporarily reactivated some of the rivers and streams in the Eastern Sahara. Archaeological remains indicate the streams drew groups of early humans until about 4,000 years ago, when climate changes made the area unlivable.