Drainage

Six major drainage networks exist in Africa. With the exception of the Chad Basin, all have outlets to the sea and all are cut by steep cataracts or rapids that impede navigation.

The Nile River, with a length of 6671 km (4145 mi), drains northeastern Africa and is the longest river in the world. Formed from the Blue Nile, which originates at Lake T'ana in Ethiopia, and the White Nile, which originates at Lake Victoria in eastern Africa, the Nile flows west and north before emptying into the Mediterranean Sea. The Congo River, some 4670 km (about 2900 mi) long, drains much of central Africa. It originates in Zambia and flows north, west, and south to empty into the Atlantic Ocean.

The third longest African river, the Niger River in western Africa, is about 4180 km (about 2600 mi) long; its upper portions are navigable only during rainy seasons. The Niger rises in the highlands of the Fouta Djallon and flows north and east before turning south to empty into the Gulf of Guinea. The Zambezi River, about 3540 km (about 2200 mi) long, originates in Zambia in southeastern Africa and flows south and east to empty into the Indian Ocean.

The Zambezi is cut by various rapids, the most spectacular of which is Victoria Falls. Draining southern Africa is the Orange River, which, with its tributary, the Vaal River, has a length of about 2100 km (about 1300 mi). It rises in the Drakensberg Mountains and flows west to the Atlantic.

Lake Chad, a shallow freshwater lake with an average depth of only about 1.2 m (about 4 ft), drains nearby rivers and constitutes one of the largest inland drainage areas on the continent. The deep rift valleys of the Eastern Highlands hold a great series of lakes. This equatorial lake system includes Lakes Turkana, Albert, Tanganyika, and Nyasa (Malawi). Lake Victoria, the largest lake in Africa and the third largest in the world, is, however, not part of this system; it occupies a shallow depression in the Eastern Highlands. Achieving effective control of the water supply is a major problem in Africa.

Vast areas suffer low rainfall; still larger areas receive only irregular rainfall and must store water as insurance against delayed or deficient rains. Other areas have an overabundance of water; great swamps exist, and large areas suffer from periodic flooding. In recent years, numerous dams and reservoirs have been constructed to channel water for irrigation and for hydroelectric power.

The continent's numerous rivers and the abrupt descents of the waterways have led to estimates that Africa has approximately 40 percent of the total world hydroelectric potential.