Boundaries

The lower Nile River provides a vital oasis for terrestrial and aquatic wildlife as it runs through the semi-arid Sahel and arid Saharan Desert of northern Sudan and Egypt. The boundaries of this ecoregion are defined by the lower Nile River from Khartoum, where the White and Blue Nile rivers converge, downstream to the Nile Delta [524]. Notable features of the ecoregion include four major cataracts over which the Nile flows before emptying into the Aswan High Dam’s massive reservoir, named Lake Nubia in Sudan and Lake Nasser in Egypt.

Freshwater habitats

In the late spring, rain clouds from the South Atlantic reach the Ethiopian highlands and drop their precipitation, providing the annual Nile flood that reaches Egypt between mid-May and early July. Thereafter, the Nile surges with increasing volume for an average of 110 days, reaching its maximum height and volume in September. At the beginning of the flood in June, a parcel of water in the lower Nile will take twelve days to flow the six hundred miles from the Aswan High Dam to Cairo, but the same journey requires only six days in full flood in September. By November, the flood returns to its lower discharge levels (Collins 2002).

The total mean average annual flow of the Nile is 88 billion cubic meters (bilm³) of which the waters from Ethiopia (Blue Nile, Sobat, and Atbara) contribute 76 bilm³ or 86% of the total Nile discharge (Kendie 2001; Collins 2002). The Blue Nile flood is so large that it effectively blocks the flow of the White Nile at Khartoum, contributing an average 59%, or 52 billion m³, to the Nile’s total flow. Further north, the discharge from the Atbara contributes another 13% or 11 bilm³ (Collins 2002). Below the Atbara, the Nile flows through the harsh Nubian and Egyptian deserts for 2,415 kilometers without receiving any more water before discharging into the Mediterranean Sea. 

Lake Nubia/Nasser sits downstream of the Nile’s great “S” bend that is located between the Atbara confluence and the third cataract. When the reservoir behind the Aswan High Dam (Sadd al-Aali) is full, it covers an area of 6,850 km² and holds 140 billion m³ of water. The reservoir can be divided into three sections. The southern portion is situated in Sudan and is mainly riverine in nature; the middle section extends from Sudan north to Tushka in Egypt and has riverine characteristics only during seasonal floods; and the northern section, which lies entirely in Egypt, and is fully lacustrine (Hughes & Hughes 1992).

The Aswan High Dam has significantly altered the Nile\'s hydrologic regime (Beadle 1981). Previously, annual floods laden with nutria

nt-rich sediments sustained swamps between Khartoum and lower Egypt, and papyrus extended to the Nile Delta (Dumont 1986). The dam has caused the floodplains to virtually disappear (Welcomme 1979). When not in flood, the shallow (7-8 m average depth) lower Nile today has little energy and meanders through the wide, older channel, creating many in-channel islands (Abdelbary 1996; El-Sherbini et al. 1996).

A flat, featureless plain covers a majority of the ecoregion, though in the southeast the landscape rises sharply into the Ethiopian highlands through which flows the Great Abbai (Blue Nile) from Lake Tana. Fed by powerful tributaries, the Abbai runs through a deep canyon before entering onto the plains of the Sudan. The Blue Nile gorge has its own unique habitat with a narrow fringe of forest and scrub lining the river in its upper reaches (Rzóska 1978). From Khartoum downstream, the Nile valley is a broad flat plain over 300 km wide at its narrowest point and almost devoid of vegetation throughout the desert of northern Sudan. However, drought-tolerant plant species, such as Polygonum spp. and Potamogeton spp., occur in local stands and create narrow fringes along the mainstem Nile. Stands of Phragmites proliferate where the Nile flows into Lake Nubia (Dumont 1986; Hughes & Hughes 1992).

Below Lake Nasser, the Nile flows across a desert plateau, dividing it into two regions: the western desert, Sahara el Gharqiya, and the eastern desert, Sahara el Sharqiya (Hughes & Hughes 1992). The western desert is an arid sea of blown sand, becoming rocky towards the mainstem Nile. The eastern desert is bound by a discontinuous range of mountains that separates the Nile valley from the Red Sea. These mountains are extensively dissected by wadis (dry riverbeds that sometimes flow during flood events). The valley of the Nile River below Aswan Dam varies between 20-30 km in width and is confined by steep sides (Collins 2002).

At the northern edge of the ecoregion, west of the Nile mainstem, Lake Qârûn lies at the bottom of the Fayum depression, which is 71 km long and 20 km wide. Once fed by the Nile, Lake Qârûn now receives most of its flow as runoff from surrounding irrigated lands. As a result, its waters are becoming increasingly saline (Collins 2002).

Description of endemic fishes

No fish are endemic to the lower Nile ecoregion. 

Justification for delineation

The headwaters of the Blue Nile and Atbara Rivers are also separated into the Ethiopian Highlands ecoregion [525] due to their swift-flowing, steep nature and different aquatic fauna. The valley of the Nile River was inundated by the Tethys Sea up through the Cretaceous period (approximately 65 million years ago) (Dumont 1986). Five geologic phases of the Egyptian Nile can be distinguished, each separated by a dry period of no flow: the Eonile, Palaeonile, Protonile, Prenile, and the present Neonile (Rzóska 1978; Dumont 1986). The present Nile valley developed at the end of the Miocene. The rise of the high volcanic plateaus in Ethiopia, probably during the Oligocene, is responsible for the origin and direction of the Blue Nile and the Atbara River (Rzóska 1978). Tectonic movement and climatic changes have changed the Nile’s course and flow many times. The Nile Basin has few endemic fish, due to the frequent cessation of flow that inhibited the evolution and persistence of aquatic species (Beadle 1981; Dumont 1986).

Level of taxonomic exploration

Good