The Nile Valley and Delta, the most extensive oasis on earth, was created by the world's second-longest river and its seemingly inexhaustible sources. Without the topographic channel that permits the Nile to flow across the Sahara, Egypt would be entirely desert; the Nile River traverses about 1,600 kilometers through Egypt and flows northward from the Egyptian-Sudanese border to the Mediterranean Sea. The Nile is a combination of three long rivers whose sources are in central Africa: the White Nile, the Blue Nile, and the Atbarah.

The White Nile, which begins at Lake Victoria in Uganda, supplies about 28 percent of the Nile's waters in Egypt. In its course from Lake Victoria to Juba in southern Sudan, the elevation of the White Nile's channel drops more than 600 meters. In its 1,600-kilometer course from Juba to Khartoum, Sudan's capital, the river descends only 75 meters. In southern and central Sudan, the White Nile passes through a wide, flat plain covered with swamp vegetation and slows almost to stagnation.

The Blue Nile, which originates at Lake Tana in Ethiopia, provides an average of 58 percent of the Nile's waters in Egypt. It has a steeper gradient and 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 is visibly muddy and comes from the Blue Nile, while the water closer to the western bank is clearer and comes from the White Nile.

The much shorter Atbarah River, which also originates in Ethiopia, joins the main Nile north of Khartoum between the fifth and sixth cataracts (areas of steep rapids) and provides about 14 percent 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 plateau, the Atbarah provides 22 percent of the Nile's flow.

The Blue Nile has a similar pattern. It contributes 17 percent of the Nile's waters in the low-water season and 68 percent during the high-water season. In contrast, the White Nile provides only 10 percent of the Nile's waters during the highwater season but contributes more than 80 percent during the lowwater 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 south from the dam 320 kilometers to the border and an additional 158 kilometers into Sudan. Lake Nasser's waters fill the area through Lower Nubia (Upper Egypt and northern Sudan) within the narrow gorge 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 kilometers. North of Isna (160 kilometers north of Aswan), the plateau on both sides of the valley rises as high as 550 meters above sea level; at Qina (about 90 kilometers north of Isna) the 300-meter limestone cliffs force the Nile to change course to the southwest for about 60 kilometers before turning northwest for about 160 kilometers to Asyut. Northward from Asyut, the escarpments on both sides diminish, and the valley widens to a maximum of twenty-two kilometers. The Nile reaches the Delta at Cairo.

At Cairo the Nile spreads out over what was once a broad estuary that has been filled by silt deposits to form a fertile, fan-shaped delta about 250 kilometers wide at the seaward base and about 160 kilometers from north to south. The Nile Delta extends over approximately 22,000 square kilometers (roughly equivalent in area to Massachusetts). According to historical accounts from the first century A.D., seven branches of the Nile once ran through the Delta. According to later accounts, the Nile had only six branches by around the twelfth century. Since then, nature and man have closed all but two main outlets: the east branch, Damietta (also seen as Dumyat; 240 kilometers long), and the west branch, Rosetta (235 kilometers long). Both outlets are named after the ports located at their mouths. A network of drainage and irrigation canals supplements these remaining outlets. In the north near the coast, the Delta embraces a series of salt marshes and lakes; most notable among them are Idku, Al Burullus, and Manzilah.

The fertility and productivity of the land adjacent to the Nile depends 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 the amount of annual flow were always unpredictable. Measurements of annual flows as low as 1.2 billion cubic meters and as high as 4.25 billion cubic meters have been recorded. For centuries Egyptians attempted to predict and take advantage of the flows and 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 was about nine meters. 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, which is 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.