Editing Geography of Egypt (section)

===Nile Valley and Delta===
[[File:Nile composite NASA.jpg|thumb|right|200px|Nile delta, and the entire course of the Nile]]
{{Main|Nile|Nile Delta}}
The Nile Valley and Delta, the most extensive [[oasis]] on earth, was created by the world's longest river and its seemingly inexhaustible sources. Without the [[topographic]] [[Stream channel|channel]] that permits the Nile to flow across the [[Sahara]], Egypt would be entirely desert. The length within Egypt of the River Nile in its northwards course from three central African sources – the [[White Nile]], the [[Blue Nile]], and the [[Atbarah River|Atbara]] – totals some 1,600&nbsp; km.

The White Nile, which begins at [[Lake Victoria]] in [[Uganda]], supplies about 28% of the Nile's Egyptian waters. In its course from Lake Victoria to [[Juba]] in [[South Sudan]], the White Nile's channel drops more than 600 m. In its 1,600-km course from Juba to [[Khartoum]], [[Sudan]]'s capital, the river descends just 75 m. In South Sudan, the White Nile passes through the [[Sudd]], a wide, flat plain covered with [[swamp]] [[vegetation]] and slows almost to the point of [[Water stagnation|stagnation]].

The Blue Nile, which originates at [[Lake Tana]] in [[Ethiopia]], provides on average some 58% of the Nile's Egyptian waters. This river has a steeper gradient and therefore flows more swiftly than the White Nile, which it joins at Khartoum. Unlike the White Nile, the Blue Nile carries a considerable amount of [[sediment]]. For several kilometers north of Khartoum, water closer to the eastern bank of the river, coming from the Blue Nile, is visibly [[mud]]dy, while that closer to the western bank, and coming from the White Nile, is clearer.

The much shorter Atbarah River, which also originates in Ethiopia, joins the main Nile north of Khartoum between the fifth and sixth [[Cataracts of the Nile|cataracts]] (areas of steep rapids) and provides about 14% of the Nile's waters in Egypt. During the low-water season, which runs from January to June, the Atbarah shrinks to a number of pools. But, in late summer, when torrential rains fall on the [[Ethiopian Highlands]], the Atbarah provides 22% of the Nile's flow.

The Blue Nile has a similar pattern. It contributes 17% of the Nile's waters in the low-water season and 68% during the high-water season. In contrast, the White Nile provides only 10% of the Nile's waters during the high-water season but contributes more than 80% during the low-water period. Thus, before the [[Aswan High Dam]] was completed in 1971, the White Nile watered the Egyptian stretch of the river throughout the year, whereas the Blue Nile, carrying seasonal rain from Ethiopia, caused the Nile to overflow its banks and deposit a layer of fertile mud over adjacent fields. The great flood of the main Nile usually occurred in Egypt during August, September, and October, but it sometimes began as early as June at [[Aswan]] and often did not completely wane until January.

The Nile enters Egypt a few kilometers north of [[Wadi Halfa]], a Sudanese town that was completely rebuilt on high ground when its original site was submerged in the [[reservoir]] created by the Aswan High Dam. As a result of the dam's construction, the Nile actually begins its flow into Egypt as [[Lake Nasser]], which extends southwards from the dam for 320&nbsp; km to the border and for an additional 158&nbsp; km within Sudan. Lake Nasser's waters fill the area through [[Lower Nubia]] (Upper Egypt and northern Sudan) within the narrow [[canyon]] between the [[cliffs]] of [[sandstone]] and [[granite]] created by the flow of the river over many centuries.

Below Aswan, the cultivated [[floodplain]] strip widens to as much as twenty km. North of Isna (160&nbsp; km north of Aswan), the plateau on both sides of the valley rises to as much as 550 m above sea level; at Qina (some 90&nbsp; km north of Isna) the 300-m limestone cliffs force the Nile to change course towards the southwest for about 60&nbsp; km before it turns northwest for about 160&nbsp; km to Asyut. Northward from Asyut, the [[escarpment]]s on both sides diminish, and the valley widens to a maximum of 22&nbsp; km.

At Cairo, the Nile spreads out over what was once a broad [[estuary]], subsequently filled by silt deposits to form what is now a fertile, fan-shaped [[River delta|delta]] some 250&nbsp; km wide at its seaward extremity and extending about 160&nbsp;km from north to south. The Nile Delta covers approximately 22,000&nbsp;km<sup>2</sup> (roughly equivalent in area to that of [[Massachusetts]]). According to historical accounts from the first century AD, seven branches of the Nile once ran through the delta. According to later accounts, the Nile had, by around the twelfth century, just six branches.

Since then, nature and man have closed all but two main outlets: the east branch, [[Damietta]] (also known as Dumyat; 240&nbsp; km long), and the west branch, [[Rosetta]] (235&nbsp; km long). Both outlets are named after the [[port]]s located at their respective mouths. A network of [[drainage]] and [[irrigation]] [[canals]] supplements these remaining outlets. In the north, near the coast, the Nile delta embraces a series of [[salt marshes]] and [[lake]]s, the most notable among which are [[Idku]], [[Al Burullus]], and [[Manzilah]].

The fertility and productivity of the land adjacent to the Nile depend largely on the [[silt]] deposited by floodwaters. [[Archaeological]] research indicates that people once lived at a much higher elevation along the river than they do today, probably because the river was higher or the floods more severe. The timing and amount of annual flow were always unpredictable. Measurements of annual flows as low as 1.2 billion m<sup>3</sup> and as high as 4.25 billion m<sup>3</sup> have been recorded. For centuries Egyptians attempted to predict and take advantage of these flows and thereby moderate the severity of floods.

The construction of [[dams]] on the Nile, particularly the Aswan High Dam, transformed the mighty river into a large and predictable [[irrigation]] [[ditch]]. Lake Nasser, the world's largest artificial lake, has enabled planned use of the Nile regardless of the amount of rainfall in [[Central Africa]] and [[East Africa]]. The dams have also affected the Nile Valley's [[fertility]], which was dependent for centuries not only on the water brought to the [[arable land]] but also on the materials left by the water.

Researchers have estimated that beneficial silt deposits in the valley began about 10,000 years ago. The average annual deposit of arable soil through the course of the river valley amounted to some nine metres. Analysis of the flow revealed that 10.7 million tons of solid matter passed Cairo each year.

Today the Aswan High Dam obstructs most of this sediment, now retained in Lake Nasser. The reduction in annual silt deposits has contributed to rising water tables and increasing [[soil salinity]] in the Delta, the erosion of the river's banks in [[Upper Egypt]], and the erosion of the [[alluvial fan]] along the shore of the Mediterranean Sea.