The Nile River Basin

Geography

The Nile is a major river in northeast Africa which flows into the Mediterranean Sea. The Nile Basin is all land that drains into the Nile, and it covers 2,927,843 km2 – about 10% of the African continent (see adjacent map). Eleven countries are wholly or partially within the basin: Burundi, Democratic Republic of the Congo, Egypt, Eritrea, Ethiopia, Kenya, Rwanda, South Sudan, Sudan, Tanzania, and Uganda. The basin covers a wide variety of climates, ecosystems, and topographies – ranging from arid, sandy deserts in the north, to flat, swampy wetlands in the middle, to rainy, forested mountains in the south.

The basin can be divided into seven regions; five of these regions encompass the longest course of the Nile.

Hydrology

Although the Nile is the longest river in the world, it does not have the largest discharge. Its flow – about 87 km3 per year – is small compared to other major rivers. The Nile's discharge is only about 1% of the Amazon, 6% of the Congo, and 12% of the Yangtze.

The annual contributions to the main Nile from the three primary tributaries are: 54% from the Blue Nile, 32% from the White Nile (including contributions from the Bahr el Ghazal and Sobat tributaries), and 14% from the Atbarah.

The highlands of the White Nile and Blue Nile both experience seasonal rain, but the White Nile's flow into the main Nile is much more constant than the Blue Nile. This is due to the many lakes and wetlands on the White Nile, which moderate the cyclic effects of seasonal rainfall.

Geological history

The Nile – like most rivers – evolved from ancestral river systems that followed significantly different courses. Over millions of years, tectonic and environmental forces shifted, separated, and merged rivers. The river assumed its current form relatively recently, about 15,000 years ago. One hypothesis describes the Nile's geological history as a sequence of five evolutionary periods.

Around 20 million years ago (mya), the west flank of the East African Rift System began to uplift, separating the African Great Lakes region from the Congo basin. About 6 mya, the ancestral Nile in Egypt was a relatively short river, originating near the modern Wadi Howar and flowing north into the Mediterranean Sea.

Ecology

The total number of fish species found in the Nile Basin is estimated at over 800. In the river alone, 128 species are found, belonging to 27 families. The majority of species belong to four families: Cichlidae (cichlids), Cyprinidae (carp), Mormyridae (elephantfish), and Mochokidae (catfish). A number of Cichlid species are found in the African Great Lakes.

In addition to fish, animals that sometimes reside in the Nile's waters include hippos, crocodiles, and African buffalo. Other water dwellers include molluscs, crabs, and shrimp.