The Ghost of Water
Most of the rugged, sandy terrain of the Sahara drains into expansive internal basins or, in the north, into the ghostly tributaries of the Nile River. A few rivers that arise beyond the limits of the desert drain into it, contributing to its fitful supply of streams and its ancient supply of groundwater.
The greatest inflow of water comes from the high, tropical areas to the south, most of which feed their waters into the Nile to flow north along the eastern edge of the Sahara and into the Mediterranean. Several empty into Lake Chad in the southern part of the desert and others empty into the internal basins. Very few flow year round. Mostly, the drainage system of the Sahara is dry and fitful, save on the margins of the great, interior desert space. The desert is embroidered with mostly dry washes called wadis, the fitfully active remnants of the system of rivers and streams that sustained the hunters and gatherers who cut grass with stone tools 6,000 years ago. Now, those wadis generally carry rain after the stray storm that struggles past the barrier of rising, heated desert air. Sometimes, those dry watercourses carry substantial flash floods, since rooted plants do not hold back the runoff.
The Sahara's lack of topsoil as a result of the breakdown of the nutrients from the remains of plants in the upper layers of soil renders most of the desert barren. Few areas can yield decent crops even when wells provide water, mostly for the lack of nitrogen and organic remains that sustain plants. In some areas, bacteria ekes out a living on the few organic remains. That bacteria in the soil can produce the free nitrogen plants need to survive. But those barely fertile areas lie mostly along the wadis or atop underground reservoirs of water. The low-lying areas that might collect organic debris that could make for good topsoil often also have high concentrations of salts and minerals from the evaporation of water from the surface. As a result, even where soils might build up, salts sharply limit plant growth. Free carbonates in the upper levels of soil demonstrate that in its current desert state, very little water soaks into the soil to reach the hidden water table. Most of the water that does make it to the upper few inches of soil quickly evaporates back into the atmosphere, which is the defining hallmark of a desert.