The Nile River flows through eleven countries (Burundi, DR Congo, Egypt, Ethiopia, Eritrea, Kenya, Rwanda, South Sudan, Sudan, Tanzania, and Uganda). The Nile basin comprises two broad sub-systems, these are the Eastern Nile sub-system and the Equatorial Nile sub-system. The basin was delineated into ten sub-basins (Main Nile, Atbara, Blue Nile, White Nile, Baro- Akobo-Sobat, Bahr El Jebel, Bahr El Ghazal, Lake Albert, Victoria Nile, Lake Victoria). These sub-basins featured five broad physiographic regions with diverse topography, drainage patterns and geomorphology.
These physiographic regions include (1) highlands – plateaus and mountains; (2) open water surfaces (lakes – both natural and man-made); (3) wetlands and swamps; (4) flat lands; and (5) deserts. Each physiographic region has a more or less unique combination of surface, slope, soils, topography and vegetation. The first two physiographic regions mainly in the upper subbasin, and the later three regions covers mostly the mid and lower sub- basins. The Nile River is the longest river in the world at 6,695 km, flowing northward through the tropics and the highlands of eastern Africa and drains into the Mediterranean Sea. The basin covers about one-tenth of the area of the continent, drains a total land area of 3,200,000 km2. Beside its length, the Nile River basin contains other unique features among the world large river basins, e.g. the Sudd wetland, Lake Victoria; 17 wetlands sites registered by Ramsar and diverse species of flora and fauna.
Moving through the basin from south to north, there is a gradual change in elevation and slope (ranges 0 to 33 degrees) and climatic conditions, producing a striking latitudinal gradation in vegetation and fauna. The Nile River basin supports a range of wetland ecosystems and protected areas distributed across the entire length of the basin (national parks, wilderness areas, community conserved areas, nature reserves, and privately owned reserves). This gradation in ecoregions is accompanied by a marked decrease in the diversity of plant and animal species in northward direction. The hydrological cycle of the Nile basin supports and maintains high productivity of biodiversity within the lakes and in the wetlands and swamps – particularly of fish, plant communities and wildlife. In the basin a large population depends on the biodiversity and flood plains for their livelihoods.
The topography of the Nile basin includes mountain ranges of the upper Kagera, White Nile, Blue Nile and Tekeze-Atbara rivers. The upper parts of the basin have a ridged topography and steep slopes. Most rivers in the Eastern Nile exhibit much steeper slope in their upper reaches compared to the rivers that originate in the Equatorial Lakes region. These steeper slopes, beside high contribution of flow, also contribute to erosion, land degradation of watersheds and downstream sediment transport.
Changes in land cover are determined by a complex set of interactions between environmental and socioeconomic factors. Land cover change in sub-basins indicates the decline of forest areas and increase of cultivated land in almost all the sub-basins, indicative of increasing human activity in the basin. The Nile basin has 17 soil groups, the dominant soil group in the basin is vertisols (18.5% of basin area), followed by yermosols (16.7%). Bare areas are dominant in low lying areas, mainly desert area of the Main Nile but there are also significant bare areas in steep slopes. Soil moisture is highest in the three upper southern subbasins (the lakes area) and lowest in the Main Nile and Tekeze- Atbara sub-basins. Agriculture is found in all categories of elevations but mainly in low lying areas (less than 502 m) and medium elevation areas (890 – 1,454 m) and also practiced in some steep slope areas. Forest is dominant in the elevation range between 500 m and 2,159 m and shrub-land is dominant in the elevation range between -47 and 1,454 m and in steep slope areas (30 – 33 degrees).