1. Nile Valley and Delta
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 southern 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 southern and central Sudan, the White Nile passes through a wide, flat plain covered with swamp vegetation and slows almost to the point of 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 kilometres north of Khartoum, water closer to the eastern bank of the river, coming from the Blue Nile, is visibly muddy, while that closer to the western bank, and coming from the White Nile, is clearer.
The much shorter Atbara 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% 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% 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 kilometres 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 km to the border and for an additional 158 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 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 km north of Isna) the 300-m limestone cliffs force the Nile to change course towards the southwest for about 60 km before it turns northwest for about 160 km to Asyut. Northward from Asyut, the escarpments on both sides diminish, and the valley widens to a maximum of 22 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 delta some 250 km wide at its seaward extremity and extending about 160 km from north to south. The Nile Delta covers approximately 22,000 km2 (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 seen as Dumyat; 240 km long), and the west branch, Rosetta (235 km long). Both outlets are named after the ports 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 lakes, the most notable among which 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 m3 and as high as 4.25 billion m3 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
Nile and 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 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 Atbara – totals some 1,600 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 southern 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 southern and central Sudan, the White Nile passes through a wide, flat plain covered with swamp vegetation and slows almost to the point of 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 kilometres north of Khartoum, water closer to the eastern bank of the river, coming from the Blue Nile, is visibly muddy, while that closer to the western bank, and coming from the White Nile, is clearer.
The much shorter Atbara 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% 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% 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 kilometres 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 km to the border and for an additional 158 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 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 km north of Isna) the 300-m limestone cliffs force the Nile to change course towards the southwest for about 60 km before it turns northwest for about 160 km to Asyut. Northward from Asyut, the escarpments on both sides diminish, and the valley widens to a maximum of 22 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 delta some 250 km wide at its seaward extremity and extending about 160 km from north to south. The Nile Delta covers approximately 22,000 km2 (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 seen as Dumyat; 240 km long), and the west branch, Rosetta (235 km long). Both outlets are named after the ports 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 lakes, the most notable among which 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 m3 and as high as 4.25 billion m3 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
