Few indicators offer a more mixed picture of Africa than that of waterways. Represented according to their permanent or seasonal nature, African waterways seem to cut the continent into a succession of zones of unequal thickness. North of the Equator, the Atlas forms a narrow humid strip facing the Saharan expanses, which cover almost a third of the continent and whose numerous wadis, dried out since the late Neolithic, flow only episodically. South of the Sahara, permanent watercourses dominate, except in the Horn and the Kalahari. Only a handful of rivers born in the Great Lakes Region (Nile), Ethiopian plateaus (Juba, Sheebele) and the Drakensberg (Orange) manage to cross the arid dams of the Sahara, Somalia and Namibia. Limited to surface water, the map cannot show the existence, sometimes considerable as in the Sahara, of underground reserves that were formed millennia ago and give witness to more favourable climatic episodes.