The rest of the tributaries are wadis, dry except after rains.
From the inner slopes of the plateau numerous wadis take a direction towards the Sahara.
It is probable that certain rudely chipped flints, so-called eoliths, in the alluvial gravels (formed generally at the mouth of wadis opening on to the Nile) at Thebes and elsewhere, are the work of primitive man; but it has been shown that such are produced also by natural forces in the rush of torrents.
Though there are no perennial rivers, there are watercourses (khors or wadis) in the rainy season; the chief being the Khor Abu Habl, which traverses the southcentral region.
Both in the steppe and in the desert, small ephemeral species occur on the bare ground away from the large plants and especially in the wadis.
