NILE RIVER

Arabic Bahr An-nil, or Nahr An-nil, river, the longest of Africa and of the world, rising in highlands south of the equator and flowing northward through northeastern Africa to drain into the Mediterranean Sea. The Nile River basin covers about one-tenth of the area of the African continent and falls naturally into seven major regions: the Lake Plateau of East Africa, the Al-Jabal River (Mountain Nile), the White Nile (Al-Bahr al-Abyad), the Blue Nile (Al-Bahr al-Azraq), the Atbara ('Atbarah) River, the Nile north of Khartoum, and the Nile delta. The Nile's remotest headstream rises as the Kagera River in the eastern African lakes region of Burundi and flows northward, passing through or forming part of the boundaries of Tanzania, Rwanda, and Uganda before entering Lake Victoria from the west. The Nile proper rises from that lake as the Victoria Nile, passes through Lakes Kyoga and Albert, and leaves the latter as the Albert Nile. Entering southern Sudan at Nimule, this river's main course is then called the Al-Jabal, until its confluence with the Al-Ghazal and Sobat rivers near Malakal. Before reaching Malakal the Al-Jabal and Al-Ghazal rivers pass through As-Sudd, a swampy lowland region crowded with tall papyrus and other aquatic plants. After the river's confluence with the Sobat River, the main stream becomes the White Nile until its confluence with the Blue Nile, which rises in the central Ethiopian Plateau and enters the White Nile on its eastern bank near Khartoum. Downriver and northeast of Khartoum the Nile proper receives its last major tributary, the Atbara River, again on its eastern bank. Below its confluence with the Atbara the Nile describes a broad S-curve toward the northwest and passes over three cataracts before entering Lake Nasser (Lake Nubia in The Sudan) near the Egypt-Sudan border. After the Aswan High Dam has impounded the lake, the river flows northward through Egypt to the head of the Nile delta near Cairo, where it divides into several distributaries before discharging into the eastern end of the Mediterranean Sea. The principal feature of the Nile River's hydrological regime below the Aswan High Dam is the annual flood. In the northern Sudan, the river usually begins to rise in May, reaches its maximum level in August, and decreases thereafter, having a low level from January to May. Although the flood is a regular phenomenon, it varies in both its volume and in its date of onset. The flood is caused by the Blue Nile and Atbara rivers, whose waters come from heavy seasonal rainfall on the Ethiopian Plateau. The Ethiopian Plateau contributes more than 80 percent of the Nile's total water supply, while the remainder comes from the Lake Plateau of East Africa. The greatest part of the Nile's flow is contributed by the Blue Nile and the least by the Atbara, but, during the season when the river's water level is low, the White Nile becomes the most important stream. There is hardly an area within the Nile basin that experiences a true equatorial type of climate. The greater part of the region falls under the influence of the trade winds, which account for the prevailing aridity of most of the basin. The main stream of the Nile, from Khartoum northward to Aswan, flows between deserts and has a narrow strip of vegetation on either side. Where the soil permits, the river's banks and the neighbouring flatlands are cultivated by use of irrigation, thus supporting a modest population. From Aswan northward to Cairo the river is bordered by a floodplain of rich alluvial soil that gradually increases to a width of about 12 miles (19 km), all of which is cultivated by use of irrigation. Beyond this on both sides is the desert. The desert region outside the Nile valley extends from the Mediterranean southward to Atbarah, in the north-central Sudan. Much of this area is almost rainless and possesses no vegetation except in occasional oases. South of the Atbarah the country becomes dotted with small thorny shrubs, mostly acacias. These grow increasingly thick toward the south and merge into savanna country that supports grasses and small thorny trees. South of this is found true savanna country, consisting of open grassy plains and a few trees, and becoming swampy during the rainy season. Farther south, along the Blue Nile, are to be found savanna forests and areas of true rain forest. Many varieties of fish are found in the Nile River system. Notable species include the perchlike Tilapia, which is an important food source for humans; the giant Nile perch (Lates niloticus), which may reach weights of 300 pounds (140 kg); and several types of catfish. Reptiles include the crocodile, soft-shelled turtles, monitor lizards, and a wide variety of snakes, including two types of cobras. Nilot is the general term used to describe the people of the upper Nile basin. Much of southern Sudan is populated by Nilotic and Bantu-speaking peoples, central Sudan by pastoral Semitic people, and northern Sudan by Muslim Arabs and Nubians, as well as Hamitic people. Egyptians are primarily of Hamitic descent intermixed with European elements to the north and Nubian elements to the south. The first use of the Nile for irrigation in Egypt began when seeds were sown in the mud left after the river's annual floodwaters had subsided. Henceforth, the Nile supported a continuous human settlement for at least 5,000 years. Perennial irrigation was made possible by the completion of several barrages and waterworks on the river before the end of the 19th century, and, by the beginning of the 20th century, the canal system had been remodeled and the first dam at Aswan had been completed (in 1902). The Aswan High Dam (q.v.), constructed between 1959 and 1970, is built at a place where the river is 1,800 feet (550 m) wide and has steep banks of granite. The High Dam completed human control over the Nile's annual flooding and in the process created Lake Nasser, a reservoir that stretches more than 300 miles (500 km) upstream of the dam and extends into the northern Sudan. The High Dam provides protection for both population and crops against unusually high floods, and it also generates enormous amounts of hydroelectric power. The Nile River also serves as a vital waterway for the transportation of people and goods. Length 4,132 miles (6,650 km); drainage basin 1,293,000 square miles (3,349,000 square km); mean annual discharge 109,475 cubic feet per second (3,100 cubic m/sec). The Nile River basin and its drainage network. Arabic Bahr an-Nil, or Nahr an-Nil, river, the father of African rivers and the longest river in the world. It rises south of the equator and flows northward through northeastern Africa to drain into the Mediterranean Sea. It has a length of about 4,132 miles (6,650 kilometres) and drains an area estimated at 1,293,000 square miles (3,349,000 square kilometres). Its basin includes parts of Tanzania, Burundi, Rwanda, Congo (Kinshasa), Kenya, Uganda, and Ethiopia, most of The Sudan, and the cultivated part of Egypt. Its most distant source is the Kagera River in Burundi. The Nile is formed by three principal streams, the Blue Nile (Arabic: Al-Bahr al-Azraq; Amharic: Abay) and the Atbara (Arabic: Nahr 'Atbarah), which flow from the highlands of Ethiopia, and the White Nile (Arabic: Al-Bahr al-Abyad), the headstreams of which flow into Lakes Victoria and Albert. The name Nile is derived from the Greek Neilos (Latin: Nilus), which probably originated from the Semitic root nahal, meaning a valley or river valley, and hence, by an extension of the meaning, a river. The fact that the Nileunlike other great rivers known to themflowed from the south northward and was in flood at the warmest time of the year was an unsolved mystery to the ancient Egyptians and Greeks. The ancient Egyptians called the river Ar or Aur (Coptic: Iaro), or Black, in allusion to the colour of the sediments carried by the river when it is in flood. Nile mud is black enough to have given the land itself its oldest name, Kem or Kemi, which also means Black and signifies darkness. In The Odyssey, the epic poem written by the Greek poet Homer (7th century BC), Aigyptos is the name of the Nile (masculine) as well as the country of Egypt (feminine) through which it flows. The Nile in Egypt and the northern Sudan is now called An-Nil, Al-Bahr, and Bahr an-Nil or Nahr an-Nil. The Nile River basin, which covers about one-tenth of the area of the continent, served as the stage for the evolution and decay of advanced civilizations in the ancient world. On the banks of the river dwelled people who were among the first to cultivate the arts of agriculture and to use the plow. The basin is bordered on the north by the Mediterranean; on the east by the Red Sea Hills and the Ethiopian Plateau; on the south by the East African Highlands, which include Lake Victoria, a Nile source; and on the west by the less well-defined watershed between the Nile, Chad, and Congo basins, extending northwest to include the Marrah Mountains of The Sudan, the Al-Jilf al-Kabir Plateau of Egypt, and the Libyan Desert (part of the Sahara). The availability of water from the Nile throughout the year, combined with the area's high temperatures, makes possible intensive cultivation along its banks. Even in some of the regions in which the average rainfall is sufficient for cultivation, such as in The Sudan, marked annual variations in precipitation often make cultivation without irrigation risky. The Nile River is also a vital waterway for transport, especially at times when motor transport is not feasiblee.g., during the flood season. Improvements in air, rail, and highway facilities in the 20th century, however, have greatly reduced dependency on the waterway. Additional reading General works on the Nile River include H.E. Hurst, The Nile, rev. ed. (1957); and H.E. Hurst et al., The Nile Basin (1931 ). Studies of the river's geology and biology can be found in Julian Rzska (ed.), The Nile: Biology of an Ancient River (1976); Rushdi Said, The Geological Evolution of the River Nile (1981); and Martin A.J. Williams and Hugues Faure (eds.), The Sahara and the Nile (1980), on landforms and human settlement in the region. Hydrology is discussed in A. Azim Abul-Atta, Egypt and the Nile After the Construction of the High Aswan Dam (1978); Yusuf A. Shibl, The Aswan High Dam (1971); and John Waterbury, Hydropolitics of the Nile Valley (1979). Early accounts of the attempts to find the source of the Nile are described by John Hanning Speke, Journal of the Discovery of the Source of the Nile (1863, reissued 1971); and Richard F. Burton, The Nile Basin (1864, reprinted 1967). Alan Moorehead, The White Nile, rev. ed. (1971, reprinted 1983), and The Blue Nile, rev. ed. (1972, reprinted 1983), together offer a study of exploration in the 19th century. Other works on the river's history include Karl W. Butzer, Early Hydraulic Civilization in Egypt: A Study in Cultural Ecology (1976); and Tom Little, High Dam at Aswan: The Subjugation of the Nile (1965). Charles Gordon Smith Climate and hydrology Almost no area within the Nile basin experiences a true equatorial or a true Mediterranean type of climate. While the Nile basin in The Sudan and Egypt is rainless during the northern winter, its southern parts and the highlands of Ethiopia experience heavy rainmore than 60 inches (1,520 millimetres)during the northern summer. Most of the region falls under the influence of the northeast trade winds between October and May, which causes the prevailing aridity of most of the basin. Tropical climates with well-distributed rainfall are found in parts of the East African lakes region and southwestern Ethiopia. In the lake region there is little variation throughout the year in the mean temperature, which ranges from 60 F to 80 F (16 C to 27 C) depending on locality and altitude. Relative humidity, which varies similarly, is about 80 percent on the average. Similar climatic conditions prevail over the extreme southern parts of The Sudan, which receive as much as 50 inches of rain spread over a nine-month period (March to November), with the maximum occurring in August. The humidity reaches its highest at the peak of the rainy season and reaches its low level between January and March. Maximum temperatures are recorded during the dry season (December to February), with the minimums occurring in July and August. To the north, the rainy season gets shorter, and the amount of rainfall decreases. The rainy season, which occurs in the south from April to October, is confined to July and August in the northern part of the central Sudan, where three seasons may be distinguished. The first of these is the pleasant, cool, dry winter, which occurs from December to February; this is followed by hot and very dry weather from March to June; and this is followed, in turn, by a hot rainy period from July to October. The minimum temperature occurs in January and the maximum in May or June, when it rises to a daily average of 105 F (41 C) in Khartoum. Only about 10 inches of rainfall occurs annually in the Al-Jazirah area (between the White and Blue Nile rivers), as compared with more than 21 inches at Dakar, Senegal, which is at the same latitude. North of Khartoum less than five inches of rain falls annually, an amount insufficient for permanent settlement. In June and July the central parts of The Sudan are frequently visited by squalls during which strong winds carry large quantities of sand and dust. These storms, which are of three to four hours duration, are called haboobs. A desert-type climate exists over most of the remainder of the area north to the Mediterranean. The principal characteristics of the northern Sudan and the desert of Egypt are aridity, a dry atmosphere, and a considerable seasonal, as well as diurnal, temperature range in Upper Egypt. Temperatures often surpass 100 F (38 C); in Aswan, for example, the average daily maximum in June is 117 F (47 C). While no low temperatures are recorded anywhere in The Sudan or Egypt, winter temperatures decrease to the north. Thus, only Egypt has what could be called a winter season, which occurs from November to March, when the daily maximum temperature in Cairo is 68 to 75 F (20 to 24 C) and the night minimum is about 50 F (10 C). The rainfall in Egypt is of Mediterranean origin and falls mostly in the winter, with the amount decreasing toward the south. From eight inches on the coast, it falls gradually to a little over an inch in Cairo and to less than an inch in Upper Egypt. During the spring, from March to June, depressions from the Sahara or along the coast travel east, causing dry southerly winds, which sometimes results in a condition called khamsin. These are sandstorms or dust storms during which the atmosphere becomes hazy; on occasion they may persist for three or four days, at the end of which the phenomenon of a blue sun may be observed. The periodic rise of the Nile remained an unsolved mystery until the discovery of the role of the tropical regions in its regime. In effect, there was little detailed knowledge about the hydrology of the Nile before the 20th century, except for early records of the river level that the ancient Egyptians made with the aid of nilometers (gauges formed by graduated scales cut in natural rocks or in stone walls), some of which still remain. Today, however, no other river of comparable size has a regime that is so well known. The discharge of the main stream, as well as the tributaries, is regularly measured. The Nile swells in the summer, the floods rising as a result of the heavy tropical rains in Ethiopia. In the southern Sudan the flood begins in April, but the effect is not felt at Aswan, Egypt, until July. The water then starts to rise and continues to do so throughout August and September, with the maximum occurring in mid-September. At Cairo the maximum is delayed until October. The level of the river then falls rapidly through November and December. From March to May the level of the river is at its lowest. Although the flood is a fairly regular phenomenon, it occasionally varies in volume and date. Before it was possible to regulate the river, years of high or low floodparticularly a sequence of such yearsresulted in crop failure, famine, and disease. Following the river from its sources, an estimate can be made of the contribution of the various lakes and tributaries in the Nile flood. Lake Victoria forms the first great natural reservoir of the Nile system. The heavy rainfall over the lake is nearly balanced by surface evaporation, and the outflow from the lakesome 812 billion cubic feet (23 billion cubic metres)comes mostly from the rivers draining into it, particularly the Kagera. This water then flows via the Victoria Nile into Lake Kyoga, where there is little net loss of water, and then into Lake Albert. Water lost by evaporation is more than balanced by the rainfall over the lake and the inflow from other smaller streams, notably the Semliki. Thus the annual outflow from Lake Albert to the Al-Jabal River is about 918 billion cubic feet. In addition to the water it receives from the great lakes, the torrential tributaries of the Al-Jabal supply it with nearly 20 percent of its water. The discharge of the Al-Jabal varies little throughout the year because of the regulatory effect of the large swamps and lagoons of the As-Sudd region. About half of its water is lost in this stage by seepage and evaporation, but the flow of the Sobat River into the main stream just upstream of Malakal nearly makes up for the loss. The White Nile provides a regular supply of water throughout the year. During April and May, when the main stream is at its lowest level, more than 80 percent of its water comes from the White Nile. The White Nile obtains its water in roughly equal amounts from two main sources. The first source is the rainfall on the East African Plateau of the previous summer. The second source is the drainage of southwestern Ethiopia through the Sobat (contributed mainly by its two headstreams, the Baro and the Pibor) that enters the main stream below As-Sudd. The annual flood of the Sobat, a consequence of the Ethiopian summer rains, is to a great extent responsible for the variations in the level of the White Nile. The rains that swell its upper valley begin in April and cause widespread inundation over the 200 miles of plains through which the river passes, thus delaying the arrival of the rainwater in its lower reaches until NovemberDecember. Relatively small amounts of the mud carried by the Sobat's flood reach the White Nile. The Blue Nile, the most important of the three great Ethiopian affluents, plays an overwhelming part in bringing the Nile flood to Egypt. It receives two tributaries in The Sudanthe Ar-Rahad and the Ad-Dindarboth of which also originate in Ethiopia. The regime of the Blue Nile is distinguished from that of the White Nile by the more rapid passage of its floodwater into the main stream. The river level begins to rise in June, reaching a maximum level at Khartoum in about the first week in September. The Atbara River draws its floodwater from the rains on the northern part of the Ethiopian Plateau, as does the Blue Nile. While the floods of the two streams occur at the same time, the Blue Nile is a perennial stream, while the Atbara, as mentioned, shrinks to a series of pools in the dry season. The swelling of the Blue Nile causes the first floodwaters to reach the central Sudan in May. The maximum is reached in August, after which the level falls again. The rise at Khartoum averages more than 20 feet. When the Blue Nile is in flood it holds back the White Nile water, turning it into an extensive lake and delaying its flow. The Jabal al-Awliya' Dam south of Khartoum increases this ponding effect. The peak of the flood does not enter Lake Nasser until late July or August, when the average daily inflow from the Nile rises to some 25.1 billion cubic feet. Out of this amount the Blue Nile accounts for almost 70 percent, the Atbara more than 20 percent, and the White Nile 10 percent. In early May the inflow drops to its minimum; the total discharge of 1.6 billion cubic feet per day comes mainly from the White Nile and the remainder from the Blue Nile. On the average, about 85 percent of the water in Lake Nasser comes from the Ethiopian Plateau, and the rest is contributed by the East African Lake Plateau system. Lake Nasser has an enormous storage capacitymore than 40 cubic miles (about 168 cubic kilometres)although the content of the reservoir varies with the extent of the annual flood upstream. Because it is situated in a very hot and dry region, however, Lake Nasser can lose up to 10 percent of its volume to evaporation annually when it is full, decreasing to about one-third that amount when it is at minimum capacity. Plant and animal life In the areas where no irrigation is practiced, different zones of plant life may be roughly divided according to the amount of rainfall. Tropical rain forest is found along the NileCongo divide, in parts of the Lake Plateau, and in southwestern Ethiopia. Heat and copious rainfall produce thick forests with a great variety of tropical trees and plants, including ebony, banana, rubber, bamboo, and coffee shrub. Mixed woodland and grassland (savanna), characterized by a sparse growth of thinly foliaged trees of medium height and a ground covering of grass and perennial herbs, occurs in large parts of the Lake Plateau, in parts of the Ethiopian Plateau, in the area that fringes the Blue Nile near Ar-Rusayris, and in the southern Al-Ghazal River region. On the Sudanese plains, a mixture of thin bush, thorny trees, and open grassland prevails. This area is swampy during the rainy season, particularly in the Sudd region of the south-central Sudan, which has an area of nearly 100,000 square miles. The vegetation there includes papyrus, tall bamboolike grasses, reed mace ambatch, or turor, water lettuce, a species of convolvulus, and the South American water hyacinth. North of latitude 10 N there occurs a belt of thorny savanna or orchard shrub country characterized by small scattered tree stands, thornbush, andafter raingrass and herbs. North of this, however, rainfall decreases and the vegetation thins out, so that the countryside is dotted with small thorny shrubs, mostly acacias. From Khartoum northward there is true desert, with scanty and irregular rainfall and no permanent vegetation at all except for a few stunted shrubs. Grasses and small herbs may be scattered along drainage lines after rainfall, but these die away in a few weeks. In Egypt the vegetation near the Nile is almost entirely the result of irrigation and cultivation. Many varieties of fish are found in the Nile system. Notable among those found in the lower Nile system are the Nile perch (which may attain a weight of more than 175 pounds), the bolti (a species of Tilapia), the barbel, several species of catfish, the elephant-snout fish, and the tiger fish, or water leopard. Most of these species and the sardinelike Haplochromis, the lungfish, and mudfish are found as far upstream as Lake Victoria. The common eel penetrates as far south as Khartoum, and the spiny eel is found in Lake Victoria. The Nile crocodile, found in most parts of the river, has not yet penetrated the lakes of the upper Nile basin. Other reptiles found in the Nile basin include the soft-shelled turtle, three species of monitor lizard, and some 30 species of snakes, of which more than half are venomous. The hippopotamus, once common throughout the Nile system, is now found only in the As-Sudd region and to the south. Many schools of fish that fed in the waters of the Nile in Egypt during the flood season have been reduced or have disappeared since the construction of the Aswan High Dam. Most of the species of the Nile fish were migrants, and the dam has prevented many from migrating to Lake Nasser. The diminution in the number of anchovies in the eastern Mediterranean has also been attributed to the serious reduction in the outflow of waterborne nutrients due to the dam. Lake Nasser, however, has been developed into a commercial fishery, where the Nile perch and other species thrive. Study and exploration The ancient Egyptians were probably familiar with the Nile as far as Khartoum, The Sudan, and with the Blue Nile as far as its source in Lake Tana, Ethiopia, but they showed little or no interest in exploring the White Nile. The source of the Nile was unknown to them. The Greek historian Herodotus, who visited Egypt in 457 BC, traveled up the Nile as far as the first cataract (Aswan). About the second century BC the Greek scientific writer Eratosthenes sketched a nearly correct route of the Nile to Khartoum, showing the two Ethiopian affluents, and suggested lakes as the source of the river. In 25 BC the Greek geographer Strabo and a Roman governor of Egypt, Aelius Gallus, also explored the Nile as far as the first cataract. A Roman expedition to find the source of the Nile that took place in AD 66, during the reign of the emperor Nero, was impeded by the As-Sudd, and the attempt was therefore abandoned. Ptolemy, the Greek astronomer and geographer who lived in Alexandria, wrote in AD 150 that the White Nile originated in the high snow-covered Mountains of the Moon (since identified with the Ruwenzori Range). From the 17th century onward several attempts were made to explore the Nile. In 1618 Pedro Pez, a Spanish Jesuit priest, located the source of the Blue Nile. In 1770 the Scottish explorer James Bruce visited Lake Tana as well as the source of the Blue Nile. Magdi M. El-Kammash Charles Gordon Smith Modern exploration of the Nile basin began with the conquest of the northern and central Sudan by the Ottoman viceroy of Egypt, Muhammad 'Ali, and his sons from 1821 onward. As a result of this the Blue Nile was known as far as its exit from the Ethiopian foothills, and the White Nile as far as the mouth of the Sobat River. Three expeditions under a Turkish officer, Selim Bimbashi, were made between 1839 and 1842, and two got to the point about 20 miles (32 km) beyond the present port of Juba, where the country rises and rapids make navigation very difficult. After these expeditions, traders and missionaries penetrated the country and established stations in the southern Sudan. From an Austrian missionary, Ignaz Knoblecher, in 1850 came reports of lakes farther south. In the 1840s the missionaries Johann Ludwig Krapf, Johannes Rebmann, and Jacob Erhardt, traveling in East Africa, saw the snow-topped mountains Kilimanjaro and Kenya and heard from traders of a great inland sea that might be a lake or lakes. These reports led to fresh interest in the Nile source and to an expedition by the English explorers Sir Richard Burton and John Hanning Speke, who followed a trade route of the Arabs from the east coast and reached Lake Tanganyika. On the return journey Speke went north and reached the southern end of Lake Victoria, which he thought might be the origin of the Nile. This was followed in 1860 by another expedition by Speke and James A. Grant under the auspices of the Royal Geographical Society. They followed the previous route to Tabora and then turned toward Karagwe, the country west of Lake Victoria. There they saw the Virunga Mountains 100 miles to the west (they thought that these might be the Mountains of the Moon) and discovered the Kagera River. Continuing around the lake, Speke finally reached the Ripon Falls (1862), at which point he wrote, I saw that old Father Nile without any doubt rises in Victoria Nyanza. Speke then made his way northward with Grant, for part of the way traveling along the Nile, until the two reached Gondokoro, which lies nearly opposite the present Juba. They heard rumours on the way of another large lake to the west but were unable to visit it and passed the information on to Sir Samuel White Baker, who met them at Gondokoro, having come up from Cairo. Baker then continued his journey south and discovered Lake Albert. Neither Speke nor Baker had followed the Nile completely from the Ripon Falls to Gondokoro; and Baker, who saw the northern half of Lake Albert, was told that it extended a very long way to the south. The question of the source of the Nile was finally settled when, between 1874 and 1877, General Charles George Gordon and his officers followed the river and mapped part of it. In particular Lake Albert was mapped, and Charles Chaill-Long, an American, discovered Lake Kyoga. In 1875 Henry Morton Stanley traveled up from the east coast and circumnavigated Lake Victoria. His attempt to get to Lake Albert was not successful, but he marched to Lake Tanganyika and traveled down the Congo River to the sea. In another memorable journey in 1889, taken in order to relieve the German traveler Mehmed Emin Pasa (Pasha), Stanley traveled up the Congo and across to Lake Albert, where he met Emin and persuaded him to evacuate his Equatorial Province, which had been invaded by the Mahdist forces. They returned to the east coast by way of the Semliki valley and Lake Edward, and Stanley saw the snowy peaks of the Ruwenzori Range for the first time. Exploration and mapping has continued over the years: it was not until the 1960s, for example, that a detailed study of the upper gorges of the Blue Nile was completed. Harold Edwin Hurst Charles Gordon Smith The economy Irrigation A stand of sugarcane on the west bank of the Nile River, near Dandarah, Egypt. As an aid to cultivation, irrigation almost certainly originated in Egypt. A particular phenomenon that makes irrigation from the Nile feasible is the slope of the land from south to northwhich amounts to about five inches to the mileas well as the slightly greater slope downward from the riverbanks to the desert on either side. The first use of the Nile for irrigation in Egypt began when seeds were sown in the mud left after the annual floodwater had subsided. With the passing of time, these practices were refined until a traditional method emerged, known as basin irrigation. Under this system, the fields on the flat floodplain were divided by earth banks into a series of large basins of varying size but some as large as 50,000 acres (20,000 hectares). During the annual Nile flood, the basins were flooded and the water allowed to remain on the fields for up to six weeks. The water was then permitted to drain away as the river level fell, and a thin deposit of rich Nile silt was left on the land each year. Autumn and winter crops were then sown in the waterlogged soil. Under this system only one crop per year could be grown on the land, and the farmer was always at the mercy of annual fluctuations in the size of the flood. Along the riverbanks and on land above flood level, some perennial irrigation was always possible where water could be lifted directly from the Nile or from irrigation channels by such traditional means as the shadoof (a counterbalanced lever device that uses a long pole); the sakieh, or Persian waterwheel; or the Archimedean screw. Modern mechanical pumps have begun to replace such human- or animal-operated devices. Because of the limitations of the basin method of irrigation, perennial irrigationin which the water is controlled so that it can be made to run into the land at regular intervals throughout the yearhas largely replaced it. Perennial irrigation was made possible by the completion of several barrages and waterworks before the end of the 19th century. By the beginning of the 20th century, the canal system had been remodeled and the first dam at Aswan had been completed (see below, Dams and reservoirs). Since the completion of the Aswan High Dam, virtually all formerly basin-irrigated land in Upper Egypt has been brought under perennial irrigation. While the people of The Sudan make use of the waters of the Nile for irrigation, reliance on the river is not absolute, as a fair amount of rainfall occurs in the southern parts. Basin irrigation from the Nile floods is used to a small extent, but it is less satisfactory in these areas because the surface is more uneven, with less deposition of silt; the area inundated also varies from year to year. Since about 1950, these traditional methods of irrigation have been largely displaced by diesel-engined pumps, which are used on large tracts on the banks of either the main Nile, or above Khartoum, the White Nile. Perennial irrigation in The Sudan began with the completion of the combined dam and barrage near Sannar on the Blue Nile in 1925. This made possible the irrigation of the area of the clay plain called Al-Jazirah between the two Niles south of Khartoum. The success of this attempt encouraged the construction of more dams and barrages for large-scale irrigation schemes. Dams and reservoirs In 1843 it was decided to build a series of diversion dams (barrages or weirs) across the Nile at the head of the delta about 12 miles downstream from Cairo, so as to raise the level of water upstream to supply the irrigation canals and to regulate navigation. This delta barrage scheme was not fully completed until 1861, after which it was extended and improved; it may be regarded as marking the beginning of modern irrigation in the Nile valley. The Zifta Barrage, nearly halfway along the Damietta branch of the deltaic Nile, was added to this system in 1901. In 1902 the Asyut Barrage, more than 200 miles upstream from Cairo, was completed. This was followed in 1909 by the barrage at Isna (Esna), about 160 miles above Asyut, and in 1930 by the barrage at Naj' Hammadi, 150 miles above Asyut. The first dam at Aswan was constructed between 1899 and 1902; it has a series of four locks to allow navigation. The dam has twice been enlargedfirst between 1908 and 1911 and again between 1929 and 1934thus raising the water level and increasing the dam's capacity. It is also equipped with a hydroelectric plant with an installed power of more than 345 megawatts. The Aswan High Dam is located about 600 miles upstream from Cairo and 4 miles upstream from the first Aswan Dam. It is built at a place where the river is 1,800 feet wide and has steep banks of granite. The dam is designed to control the Nile water for the expansion of cultivation and for the generation of hydroelectric power and to provide protection downstream for both crops and population against unusually high floods. The work began in 1959 and was completed in 1970. The High Dam is 12,562 feet long at crest level and 3,280 feet wide at the base, with a height of 364 feet above the riverbed. It has a hydroelectric plant with an installed capacity of 2,100 megawatts. Lake Nasser stretches some 310 miles upstream from the dam site, extending 125 miles into The Sudan. The principal objective behind the construction of the High Dam is to store sufficient water in the reservoir in order to protect Egypt from the dangers of a series of years when the Nile flood is above or below the long-term average and thus to guarantee a steady flow of water from the Nile for both Egypt and The Sudan. An agreement concluded in 1959 between the two countries sets a maximum amount that can be drawn per year and apportions it in a ratio of three to one, with Egypt receiving the larger share. The quantities of water maintained and apportioned are based on the estimated worst possible sequence of flood and drought events over a period of 100 years; and generally, one-fourth of the total capacity of Lake Nasser is reserved as relief storage for the highest anticipated flood during such a period (called century storage). The High Dam was a source of considerable controversy during its construction, and since it began operation it has continued to have its critics. Opponents have charged that silt-free water flowing below the dam has caused erosion of the downstream barrages and bridge foundations; that the loss of silt downstream has caused coastal erosion in the delta; that the overall reduction in the flow of the Nile resulting from the construction of the dam has caused the inundation of the lower reaches of the river by saltwater from the Mediterranean Sea, with resulting deposition of salt in the delta soils; and that the creation of Lake Nasser has caused the water table along the river to rise, resulting in waterlogging and an increase in soil salinity in some areas. Already the fish population offshore of the delta has been reduced dramatically by the loss of the nutrient-laden silt. Proponents of the dam have maintained that these harmful effects are worth the security of dependable water and power supplies; and, indeed, Egypt would have suffered a severe water shortage in 198488 without the dam. In The Sudan the Sannar Dam on the Blue Nile provides water for the Al-Jazirah plain at the time of year when the water level of the Blue Nile is low. It also produces hydroelectric power. Another dam, at Jabal al-Awliya' on the White Nile, was completed in 1937; it was built to increase the water available to Egypt during the period of low water (January to June) and was not intended to provide irrigation water for The Sudan. Other damsincluding one on the Atbara at Khashm al-Qirbah (completed in 1964) and the Ar-Rusayris Dam on the Blue Nile (1966)have enabled The Sudan to take maximum advantage of its allocation of waters from Lake Nasser. In Uganda, Lake Victoria was made into a reservoir by the completion in 1954 of the Owen Falls Dam; the dam is situated on the Victoria Nile just below the point where the lake waters flow into the river. This permits the storage of surplus water in high-flood years to meet the deficit in years when the waters are low. The fall from the lake is harnessed by a hydroelectric plant that provides power for industries in Uganda and Kenya. The people The Nile flows through regions inhabited by a wide variety of peoples, from the Bantu-speaking populations of the Lake Victoria area to the Arabs of the Sahara and the Nile delta. The wide ethnic and linguistic diversity is mirrored in the numerous ecological relationships between these peoples and the river. In the southern Sudan are Nilotic-speaking peoples including the Shilluk, Dinka, and Nuer. The Shilluk are sedentary agriculturists whose land is watered by the Nile. The Dinka and Nuer are pastoralists whose movements are dictated by the Nile's seasonal flow. They migrate with their herds from the river's shores during the dry season, to high ground during the wet season, and back to the river when the dry season returns. Perhaps nowhere is the relationship between people and the river so intense as in the Nile floodplain. The average population density in the cultivated parts of the floodplain south of the delta is more than 3,320 per square mile (1,280 per square kilometre). This great population, composed mostly of peasant farmers (fellahin), can survive only by making the most careful use of the available land and water. Before the completion of the Aswan High Dam, the large quantities of silt washed down from the rich highlands of Ethiopia were deposited by the floodwaters in Egypt, where the fertility of the riverine lands was maintained over the centuries, despite intensive cultivation. Thus, a vital feature in the life of the Egyptian people was the river's behaviour, since a good harvest followed a good flood, and a poor flood often meant a later food shortage.