Researchers believe that the Nile originated 30 million years ago in the mid-Tertiary Period. Its headstream was probably the Atbara River. The river basin continued to evolve and now has seven major regions:
At Nimule, the Nile enters The Sudan and becomes the Al-Jabal River (Mountain Nile). This portion of the Nile is unnavigable due to the series of rapids that are formed while the river descends through narrow gorges. Once it reaches Juba it flows over a large and level clay plain. The mainstream flows through its center and during the rainy season flooding will occur. This allows aquatic vegetation to grow in abundance. Eventually, as the water flows through this area, large amounts of vegetation are dislodged and carried downstream where they fill the channels and prevent navigation. The basin in this area also receives drainage from other rivers and streams. Some of the contributors lose tremendous amounts of water from evaporation and only a small amount of water is added to the Nile.
The White Nile begins near Malakal, where the Sobat River (Baro in Ethiopia) enters the Nile's mainstream. It continues for about 500 miles with very little water being added from other sources along the way. This section of the Nile is wide and the water flows slowly along its course. Narrow swamps occur frequently along the edges. These conditions contribute to the massive amounts of water lost through evaporation and seepage.
The water of the Blue Nile drains from the Ethiopian Plateau, where it descends from 6,000 feet above sea level. It continues in a north-northwesterly direction through The Sudan till it reaches the White Nile at Khartoum. The White Nile has a relatively constant flow, the Blue Nile in contrast is affected by the tremendous runoff resulting from the late July to October rains on the Ethiopian Plateau. The annual floods in Egypt are a direct result of this increase in the river's water volume.
The Atbara River flows into the Nile 200 miles north of Khartoum. The Atbara is fed by the Angereb and Tekezo. These tributaries bring tremendous amounts of water from the Ethiopian highlands between July and October and contribute to the flooding downstream.
The United Nile is north of Khartoum and has two distinct parts. One part is 830 miles long and flows through a desert region. Irrigation takes place along this stretch. The second part involves Lake Nasser (2,600 sq. miles, the world's second largest man-made lake). Here the water is held back by Egypt's Aswan Dam. It is about 500 miles from the dam to Cairo. As the river runs its course through this area it averages 10 to 14 miles in width, with scarps that rise to heights of 1,500 feet above the river's level. About 200 miles from Cairo the river flows and hugs the eastern edge of the valley's floor. This has resulted in more cultivated land along the left bank of the Nile.
Another stretch of the Nile flows northward below Khartoum. A series of cataracts occur leading towards Lake Nasser. For the 800 miles that the river flows through this portion it changes from gentle sections to a series of rapids. It is the crystalline rocks that have caused five of the famous cataracts in the river making portions of it unnavigable.
Finally, the Nile delta was formed. It is composed of the silt carried by the river from the Ethiopian Plateau. This silt is 50 to 75 feet deep and has created Africa's most fertile soil. A 100 mile plain was formed and extends north and south along the Nile delta. As the land reaches the Mediterranean Sea in the north, lakes and salt marshes occur.
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