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Several rivers originating outside the Sahara contribute to both the surface water and groundwater regimes of the desert and receive the discharge of its drainage networks. Rivers rising in the tropical highlands to the south are particularly prominent: the main tributaries of the Nile join in the Sahara, and the river flows northward along the desert’s eastern margin to the Mediterranean; several rivers discharge into Lake Chad in the southern Sahara, and a significant quantity of water continues northeastward and contributes to the recharge of regional aquifers; and the Niger rises in the Fouta Djallon region of Guinea and flows through the southwestern Sahara before turning southward to the sea. Streams and wadis (ephemeral streams) flowing from the Atlas Mountains and coastal highlands of Libya, Tunisia, Algeria, and Morocco contribute additional water. Prominent among these are the Saoura and Drâa. Many of the smaller wadis discharge into the chotts of the northern Sahara. Within the desert itself, there are extensive networks of wadis: some are seasonally active remnants of systems formed during more humid periods in the past; some, however, have been shaped by the sudden discharge of historically documented storms, such as the flood that destroyed Tamanrasset, Algeria, in 1922. Particularly significant are the complex network of wadis, lakes, and pools associated with the Tibesti Mountains and those associated with the Tassili n’Ajjer region and the Ahaggar Mountains, such as Wadi Tamanrasset. The sand dunes of the Sahara store considerable quantities of rainwater, and seeps and springs issue from various escarpments in the desert.
Soils
The soils of the Sahara are low in organic matter, exhibit only slightly differentiated horizons (strata), and are often biologically inactive, although nitrogen-fixing bacteria are present in some areas. The soils in depressions are frequently saline. At the margins of the desert are soils containing greater concentrations of organic matter. Weatherable minerals are a prominent constituent of these soils, and chemically active expanding-lattice clays are common. Free carbonates are often present, indicating that little leaching has occurred. Compact and indurated layers, or crusts, are largely restricted to the northwestern section of the desert in association with calcareous bedrock. Fine materials, including deposits of diatomaceous earth, are limited to basins and depressions.