Thursday, March 25, 2010

The Nile

The Nile is a major north-flowing river in Africa, generally regarded as the longest river in the world.
The Nile has two major tributaries, the White Nile and Blue Nile, the latter being the source of most of the Nile's water and fertile soil, but the former being the longer of the two. The White Nile rises in the Great Lakes region of central Africa, with the most distant source in southern Rwanda at 2°16′55.92″S 29°19′52.32″E, and flows north from there through Tanzania, Lake Victoria, Uganda and southern Sudan, while the Blue Nile starts at Lake Tana in Ethiopia at 12°2′8.8″N 37°15′53.11″E, flowing into Sudan from the southeast. The two rivers meet near the Sudanese capital of Khartoum.
The northern section of the river flows almost entirely through desert, from Sudan into Egypt, a country whose civilization has depended on the river since ancient times. Most of the population and cities of Egypt lie along those parts of the Nile valley north of Aswan, and nearly all the cultural and historical sites of Ancient Egypt are found along the banks of the river. The Nile ends in a large delta that empties into the Mediterranean Sea.

The source of the Nile is sometimes considered to be Lake Victoria, but the lake itself has feeder rivers of considerable size. The most distant stream—and thus the ultimate source of the Nile—emerges from Nyungwe Forest in Rwanda, via the Rukarara, Mwogo, Nyabarongo and Kagera rivers, before flowing into Lake Victoria in Tanzania near the town of Bukoba.
The Nile leaves Lake Victoria at Ripon Falls near Jinja, Uganda, as the Victoria Nile. It flows for approximately 500 kilometres (300 mi) farther, through Lake Kyoga, until it reaches Lake Albert. After leaving Lake Albert, the river is known as the Albert Nile. It then flows into Sudan, where it is known as the Bahr al Jabal . The Bahr al Ghazal, itself 716 kilometres (445 mi) long, joins the Bahr al Jabal at a small lagoon called Lake No, after which the Nile becomes known as the Bahr al Abyad, or the White Nile, from the whitish clay suspended in its waters. When the Nile flooded it left a rich silty deposit which fertilised the soil. The Nile no longer floods annually since the completion of the Aswan Dam in 1970. From Lake No, the river flows to Khartoum. An anabranch river, the Bahr el Zeraf, flows out of the Nile's Bahr al Jabal section and rejoins the White Nile.
The term "White Nile" is used in both a general sense, referring to the entire river above Khartoum, and in a limited sense, describing the section between Lake No and Khartoum.
The flow rate of the Albert Nile at Mongalla is almost constant throughout the year and averages 1,048 m3/s (37,000 cu ft/s). After Mongalla, the Nile is known as the Bahr El Jebel, which enters the enormous swamps of the Sudd region of Sudan. More than half of the Nile's water is lost in this swamp to evaporation and transpiration. The average flow rate in the Bahr El Jebel at the tails of the swamps is about 510 m3/s (18,000 cu ft/s). From here it soon meets with the Sobat River and forms the White Nile.
The Bahr al Ghazal and the Sobat River are the two most important tributaries of the White Nile in terms of drainage area and discharge. The Bahr al Ghazal's drainage basin is the largest of any of the Nile's sub-basins, measuring 520,000 square kilometres (200,000 sq mi) in size, but it contributes a relatively small amount of water, about 2 m3/s (71 cu ft/s) annually, due to tremendous volumes of water being lost in the Sudd wetlands. The Sobat River, which joins the Nile a short distance below Lake No, drains about half as much land, 225,000 km2 (86,900 sq mi), but contributes 412 cubic metres per second (14,500 cu ft/s) annually to the Nile.[4] When in flood the Sobat carries a large amount of sediment, adding greatly to the White Nile's color.
The average flow of the White Nile at Malakal, just below the Sobat River, is 924 m3/s (32,600 cu ft/s); the peak flow is approximately 1,218 m3/s (43,000 cu ft/s) in early March and minimum flow is about 609 m3/s (21,500 cu ft/s) in late August. This fluctuation is due the substantial variation in the flow of the Sobat, which has a minimum flow of about 99 m3/s (3,500 cu ft/s) in August and a peak flow of over 680 m3/s (24,000 cu ft/s) in early March. During the dry season (January to June) the White Nile contributes between 70% and 90% of the total discharge from the Nile.
The Blue Nile to Ethiopians springs from Lake Tana in the Ethiopian Highlands. The Blue Nile flows about 1,400 kilometres (870 mi) to Khartoum, where the Blue Nile and White Nile join to form the Nile. 90% of the water and 96% of the transported sediment carried by the Nile originates in Ethiopia, with 59% of the water from the Blue Nile (the rest being from the Tekezé, Atbarah, Sobat, and small tributaries). The erosion and transportation of silt only occurs during the Ethiopian rainy season in the summer, however, when rainfall is especially high on the Ethiopian Plateau; the rest of the year, the great rivers draining Ethiopia into the Nile (Sobat, Blue Nile, Tekezé, and Atbarah) have a weaker flow.
The Blue Nile contributes approximately 80-90% of the Nile River discharge. The flow of the Blue Nile varies considerably over its yearly cycle and is the main contribution to the large natural variation of the Nile flow. During the wet season the peak flow of the Blue Nile will often exceed 5,663 m3/s (200,000 cu ft/s) in late August. During the dry season the natural discharge of the Blue Nile can be as low as 113 m3/s (4,000 cu ft/s), although upstream dams regulate the flow of the river.
Before the placement of dams on the river the yearly discharge varied by a factor of 15 at Aswan. Peak flows of over 8,212 m3/s (290,000 cu ft/s) would occur during late August and early September and minimum flows of about 552 m3/s (19,500 cu ft/s) would occur during late April and early May.

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