Africa is home to several great rivers. The Nile, the longest river in the world, winds its way from its headwaters in East Africa to empty into the eastern Mediterranean Sea. Africa’s second longest river, the Congo, begins in Zambia, but it flows westward through dense rainforests into the Atlantic Ocean. The Niger River drains a large part of West Africa, flowing into the Atlantic Ocean by way of a wide delta in southern Nigeria.
The Congo River (formerly the Zaire River) is a river in west-central Africa. With a length of 4,700 km (2,900 miles), it is the continent’s second longest river, after the Nile. It rises in the highlands of northeastern Zambia between Lakes Tanganyika and Nyasa (Malawi) as the Chambeshi River at an elevation of 1,760 metres (5,760 feet) above sea level and at a distance of about 700 km (430 miles) from the Indian Ocean. Its course then takes the form of a giant counterclockwise arc, flowing to the northwest, west, and southwest before draining into the Atlantic Ocean at Banana (Banane) in the Democratic Republic of the Congo. Its drainage basin, covering an area of 3,457,000 square km (1,335,000 square miles), takes in almost the entire territory of that country, as well as most of the Republic of the Congo, the Central African Republic, eastern Zambia, and northern Angola and parts of Cameroon and Tanzania.
With its many tributaries, the Congo forms the continent’s largest network of navigable waterways. Navigability, however, is limited by an insurmountable obstacle: a series of 32 cataracts over the river’s lower course, including the famous Inga Falls. These cataracts render the Congo unnavigable between the seaport of Matadi, at the head of the Congo estuary, and Malebo Pool, a lakelike expansion of the river. It was on opposite banks of Malebo Pool—which represents the point of departure of inland navigation—that the capitals of the former states of the French Congo and the Belgian Congo were founded: on the left bank Kinshasa (formerly Léopoldville), now the capital of the Democratic Republic of the Congo, and on the right bank Brazzaville, now the capital of the Republic of the Congo.
The Amazon and the Congo are the two great rivers of the world that flow out of equatorial zones where heavy rainfall occurs throughout all or almost all of the year. Upstream from Malebo Pool, the Congo basin receives an average of about 150 cm (60 inches) of rain a year, of which more than one-fourth is discharged into the Atlantic. The drainage basin of the Congo is only about half the size of that of the Amazon, however. And the Congo’s rate of flow—41,000 cubic metres (1.45 million cubic feet) per second at its mouth—is considerably less than the Amazon’s flow of more than 175,000 cubic metres (6.18 million cubic feet) per second.
While the Chambeshi River, as the remotest source, may form the Congo’s original main stream in terms of the river’s length, it is another tributary—the Lualaba, which rises near Musofi in southeastern Democratic Republic of the Congo—that carries the greatest quantity of water and thus may be considered as forming the Congo’s original main stream in terms of water volume.
When the river first became known to Europeans at the end of the 15th century, they called it the Zaire, a corruption of a word that is variously given as nzari, nzali, njali, nzaddi, and niadi and that simply means “river” in local African languages. It was only in the early years of the 18th century that the river was first called the “Rio Congo,” a name taken from the kingdom of Kongo that had been situated along the lower course of the river. During the period (1971–97) when the Democratic Republic of the Congo was called Zaire, the government also renamed the river the Zaire. Even during that time, however, the river continued to be known throughout the world as the Congo. To the literary-minded the river is evocative of the famous 1902 short story “Heart of Darkness” by Joseph Conrad. His book conjured up an atmosphere of foreboding, treachery, greed, and exploitation. Today, however, the Congo appears as the key to the economic development of the central African interior.
The expression “Congo basin,” strictly speaking, refers to the hydrographic basin. This is not only vast but is also covered with a dense and ramified network of tributaries, subtributaries, and small rivers—with the exception of the sandy plateaus in the southwest.
The Congo basin is the most clearly distinguished of the various geographic depressions situated between the Sahara to the north, the Atlantic Ocean to the south and west, and the region of the East African lakes to the east. In this basin, a fan-shaped web of tributaries flows downward along concentric slopes that range from 275 to 460 metres (900 to 1,500 feet) in elevation and that enclose a central depression. The basin itself stretches for more than 1,900 km (1,200 miles) from north to south (from the Congo–Lake Chad watershed to the interior plateaus of Angola) and also measures about 1,900 km (1,200 miles) from the Atlantic in the west to the Nile-Congo watershed in the east.
The central part of the Congo basin—often called the cuvette (literally “saucer” or “shallow bowl”)—is an immense depression containing Quaternary alluvial deposits that rest on thick sediments of continental origin, consisting principally of sands and sandstones. These underlying sediments form outcrops in valley floors at the eastern edge of the cuvette. The filling of the cuvette, however, began much earlier. Boreholes have revealed that since late Precambrian times (i.e., since at least 570 million years ago) considerable sediment has accumulated, derived from the erosion of formations situated around the periphery of the cuvette. The arrangement of surface relief, thick depositional strata, and substratum in amphitheatre-like fashion around the main Congo channel, which has been uniform across time, is evidence of a persistent tendency to subsidence in this part of the continent. This subsidence is accompanied by uplifting on the edges of the cuvette, principally on its eastern side—which has also been influenced by the formation of the Western Rift Valley.
From its sources to its mouth, the Congo River system has three contrasting sections—the upper Congo, middle Congo, and lower Congo. The upper reaches are characterized by three features—confluences, lakes, and waterfalls or rapids. To begin with, several streams of approximately equal size unite to form the river. In a little more than 100 km (60 miles), the upper Lualaba joins the Luvua and then the Lukuga. Each stream for part of its course undergoes at least a lacustrine type of expansion, even when it does not form a lake. Thus, Lake Upemba occurs on the upper Lualaba; Lakes Bangweulu and Mweru occur on the Chambeshi–Luapula–Luvua system; and Lake Tanganyika, which is fed by the Ruzizi (flowing from Lake Kivu) and by the Malagarasi, itself flows into the Lukuga. Rapids occur not only along the headstreams but also in several places along the course of the main stream. Navigation thus is possible only along sections of the upper Congo by vessels of low tonnage. Even so, these stretches are in danger of being overgrown by aquatic vegetation, particularly water hyacinths.
Kisangani (formerly Stanleyville)—located just downstream of the Boyoma Falls, a series of seven cataracts—marks the real beginning, upriver, of the navigable Congo. This central part of the river flows steadily for more than 1,600 km (1,000 miles) to within 35 km (22 miles) of Kinshasa. Its course at first is narrow but soon grows wider, after which many islands occur in midstream. This change in the character of the river corresponds to its entry into its alluvial plain. From that point onward, with the exception of a few rare narrow sections, the Congo divides into several arms, separated by strings of islands. It increases from a width of more than 5.5 km (3.5 miles) downstream from Isangi (where the Lomami enters the Congo) to a width of 8 to 11 km (5 to 7 miles) and on occasion—for example, at the mouth of the Mongala—to 13 km (8 miles). Beyond the natural levees (formed by silt deposits) occurring on either bank, some areas are subjected to extensive flooding that increases the river’s bounds still further. It is not always easy to distinguish such areas from the “rain swamps” in regions lying between rivers. The middle course of the Congo ends in a narrow section called the Chenal (“Channel”), or Couloir (“Corridor”). Between banks no more than 0.8 to 1.6 km (0.5 to 1 mile) wide, the riverbed deepens and the current becomes rapid, flowing through a valley that cuts down several hundreds of yards deep into the soft sandstone bedrock of the Batéké Plateau. Along this central reach the Congo receives its principal tributaries, primarily the Ubangi and the Sangha on the right bank and the Kwa on the left bank. An enormous increase in the average rate of flow results, rising from less than 250,000 cubic feet (7,000 cubic metres) a second at Kisangani to nearly its maximum flow at Kinshasa.
Upon leaving the Chenal, the Congo divides into two branches, forming Malebo Pool, a vast lacustrine area about 24 by 27 km (15 by 17 miles), which marks the end of the middle Congo. Immediately downstream occur the first waterfalls of the final section of the river’s course. Cataracts and rapids are grouped into two series, separated by a fairly calm central reach, in which the elevation drops from a little less than 275 metres (900 feet) to a few yards above sea level. The Congo’s estuary begins at Matadi, downstream from the rapids that close off the interior Congo; 134 km (83 miles) in length, it forms the border between Angola and the Democratic Republic of the Congo. At first the estuary is narrow—less than 0.8 to 2.4 km (0.5 to 1.5 miles) in width—with a central channel 20 to 24 metres (65 to 80 feet) deep, but it widens downstream of Boma. There the river, obstructed by islands, divides into several arms, and in some places the depth does not exceed 6 to 7.5 metres (20 to 25 feet), which makes dredging necessary to allow oceangoing vessels to reach Matadi. Beyond the estuary’s mouth, the course of the Congo continues offshore as a deep underwater canyon that extends for a distance of about 200 km (125 miles).
The Congo has a regular flow, which is fed by rains throughout the year. At Kinshasa the flow has for many years remained between the high level of 65,000 cubic metres (2.31 million cubic feet) per second, recorded during the flood of 1908, and the low level of 21,000 cubic metres (756,000 cubic feet) per second, recorded in 1905. During the unusual flood of 1962, however, by far the highest for a century, the flow probably exceeded 73,000 cubic metres (2.6 million cubic feet) per second.
At Brazzaville and Kinshasa, the river’s regime is characterized by a main maximum at the end of the year and a secondary maximum in May, as well as by a major low level during July and a secondary low level during March and April. In reality, the downstream regime of the Congo represents climatic influence extending over 20° of latitude on both sides of the Equator a distance of some 2,250 km (1,400 miles). Each tributary in its course modifies the level of the main stream. Thus, for example, the low level in July at Malebo Pool results from two factors: a drought that occurs for several months in the southern part of the basin at that time, as well as a delay before the floods of the Ubangi tributary flowing down from the north arrive, which does not happen before August. The Congo basin is so vast that no single meteorologic circumstance is capable of disturbing the slow movement of the waters’ rise and fall. The annual fluctuations may alter drastically, however, when floodwaters from different tributaries that normally coincide with each other arrive at different times.
Lake Tanganyika, apart from brief seiches caused by wind drift and sudden changes in atmospheric pressure, may experience considerable variations in its water level from year to year. In 1960, for example, its waters flooded parts of Kalemi, Democratic Republic of the Congo, and Bujumbura, Burundi. A series of particularly rainy years followed by a blocking of the outlet by floating vegetation may explain this phenomenon.
Typical climate in regions through which the Congo flows is that of Yangambi, a town situated on the river’s right bank slightly north of the Equator and a little downstream of Kisangani. Humidity is high throughout the year, and annual rainfall amounts to 170 cm (67 inches) and occurs fairly regularly. Even in the driest month the rainfall totals more than 7.6 cm (3 inches). Temperatures are also uniformly high throughout the year, and there is little diurnal variability. The average temperature at Yangambi is in the mid-70s F (mid-20s C).
From the pluviometric equator (an imaginary east-west line indicating the region of heaviest rainfall), which is situated slightly to the north of the geographic equator, the amount of rainfall decreases regularly in proportion to latitude. The northernmost points of the basin, situated in the Central African Republic, receive only from 20 to 40 cm (8 to 16 inches) less during the course of a year than points near the Equator. The dry season, however, lasts for four or five months, and there is only one annual rainfall maximum, which occurs in summer. In the far southern part of the basin, at a latitude of 12° S, the climate becomes definitely Sudanic in character, with marked dry and wet seasons of approximately equal length and with rainfall of about 125 cm (49 inches) a year.
The question of the source of the Congo confronted European explorers from the time that the Portuguese navigator Diogo Cão encountered the river’s mouth in 1482, which he believed to be a strait providing access to the realm of the mythical Prester John, a Christian priest-king. It is virtually certain that, well before the Welsh explorer Henry Morton Stanley arrived in 1877, some 17th-century Capuchin missionaries reached the shores of Malebo Pool. This exploit, however, was not followed up, even by the amply supplied expedition led by James Kingston Tuckey, which was sent out by the British Admiralty in 1816 but was decimated and had to retrace its footsteps even before it had surmounted the cataracts. Preposterous hypotheses about the river continued to be entertained, connecting, for example, the upper Niger to the Congo or maintaining that the Congo and the Nile both flowed from a single great lake in the heart of Africa.
Even after the European discovery of Lake Tanganyika by the British explorers Richard Burton and John Speke (1858), then of the Lualaba (1867) and of Lake Bangweulu (1868) by the Scottish explorer David Livingstone, uncertainty remained—uncertainty that Stanley was to dissipate in the course of his famous expedition in 1876 and 1877 that took him by water from the Lualaba to the Congo’s mouth over a period of nine months. In the interior of the Congo basin and above all on the right bank, the final blank spaces on the map could not be filled in until about 1890, when the exploration of the upper course of the Ubangi was completed.
The Niger River is the principal river of western Africa. With a length of 4,200 km (2,600 miles), it is the third longest river in Africa, after the Nile and the Congo. The Niger is believed to have been named by the Greeks. Along its course it is known by several names. These include the Joliba (Malinke: “great river”) in its upper course; the Mayo Balleo and the Isa Eghirren in its central reach; and the Kwarra, Kworra, or Quorra in its lower stretch.
The Niger rises in Guinea at 9°05´ N and 10°47´ W on the eastern side of the Fouta Djallon (Guinea) highlands, only 240 km (150 miles) inland from the Atlantic Ocean. Issuing as the Tembi from a deep ravine 850 metres (2,800 feet) above sea level, it flows due north over the first 160 km (100 miles). It then follows a northeasterly direction, during the course of which it receives its upper tributaries—the Mafou, the Niandan, the Milo, and the Sankarani on the right and the Tinkisso on the left—and enters Mali. Just below Bamako, Mali’s capital, the Sotuba Dam marks the end of the upper river. From there the Niger once dropped more than 300 metres (1,000 feet) in about 60 km (40 miles) into a valley formed by tectonic subsidence. The rapids in this stretch have been submerged by the waters backed up by the Markala Dam, however, located some 240 km (150 miles) downstream of the Sotuba Dam near Sansanding. In this stretch, at Koulikoro, the river takes a more east-northeasterly direction, and its bed becomes fairly free from impediments for about 1,600 km (1,000 miles).
At Mopti the Niger is joined by the Bani, its largest tributary on the right, after which it enters a region of lakes, creeks, and backwaters that is often called the “internal delta” of the Niger. These lakes are chiefly on the left bank and are connected to the river by channels that undergo seasonal changes in the direction of flow. At high water most of the lakes become part of a general inundation. Largest of the lakes in this region is Lake Faguibine, which is nearly 120 km (75 miles) long, 25 km (15 miles) wide, and more than 50 metres (160 feet) deep in places.
The labyrinth of lakes, creeks, and backwaters comes to an end at Kabara, the port of Timbuktu (Tombouctou). There, the river turns almost due east, passing its most northern point at latitude 17°05´ N. Some 400 km (250 miles) downstream from Timbuktu, a rocky ridge that obstructs the course of the river is pierced by a defile (narrow gorge) more than 1.6 k (1 mile) long, with an average width of about 240 metres (800 feet) and a depth of more than 30 metres (100 feet) in places. At low water the strong current there endangers navigation. A short way downstream the river turns to the southeast and widens considerably, flowing to Gao across a floodplain 5 to 10 km (3 to 6 miles) wide. This most northerly bend of the Niger flows through the southern fringe of the Sahara.
The middle course of the Niger River is navigable to small craft during high water as far downstream as Ansongo—some 1,770 km (1,100 miles) in all. Below Ansongo, 690 km (430 miles) downstream from Timbuktu, navigability is interrupted by a series of defiles and rapids. The river becomes navigable to small vessels again at Labbezanga—from which it flows into Niger—and continues to be navigable to the Atlantic Ocean. Navigation is seasonal, however, because of the fluctuations in the water level in the rainy and dry seasons.
Downstream from Jebba, in Nigeria, the Niger enters its lower course, flowing east-southeast through a broad and shallow valley 8 to 16 km (5 to 10 miles) wide. About 110 km (70 miles) from Jebba it is joined by the Kaduna River—an important tributary that contributes about one-fourth of the annual discharge of the river below the Niger-Kaduna confluence—and about 40 km (25 miles) above Lokoja the river turns to the south. At Lokoja the river receives the water of its greatest tributary, the Benue, thereby approximately doubling the volume of its annual discharge. At their confluence the Niger is about 1 km (0.75 mile) wide, and the Benue more than 1.6 km (1 mile). Together they form a lakelike stretch of water about 3.2 km (2 miles) wide that is dotted with islands and sandbanks. From Lokoja downstream to the town of Idah, the Niger flows in a restricted valley, enclosed by hills and in some places flanked by sandstone cliffs up to 45 metres (150 feet) high. Between Idah and Onitsha the banks are lower and the country flatter. At Onitsha, the largest town on the Niger’s banks in Nigeria and the third largest riverine town after Bamako and Niamey (Niger), the valley narrows as the river flows through what is probably a fault in the area’s sandstone. It emerges at Aboh, separating into many branches before reaching the Gulf of Guinea via Africa’s largest delta.
The Niger delta, which stretches for nearly 240 km (150 miles) from north to south and spreads along the coast for about 320 km (200 miles), extends over an area of 36,000 square km (14,000 square miles). Within the delta the river breaks up into an intricate network of channels called rivers. The Nun River is regarded as the direct continuation of the river, but some of the other important channels include (from west to east) the Forcados, the Brass, the Sambreiro, and the Bonny. The mouths of these channels are almost all obstructed by sandbars. The Forcados, for instance, which supplanted the Nun as the most traveled channel in the early 20th century, was in turn displaced by the Escravos River in 1964. The delta is being gradually extended seaward by the increments of silt brought down by the river, and mangrove swamps extend beyond its outer edge.
The Benue (meaning “Mother of Water” in the Batta language) rises at 1,350 metres (4,400 feet) above sea level on the Adamawa Plateau in northern Cameroon at about 7°40´ N and 13°15´ E. In its upper course, which extends north-northwest to its confluence with the Mayo Kébi, close to the town of Garoua, it is a mountain torrent, falling more than 600 metres (2,000 feet) over a distance of 180 km (110 miles). The river then turns westward into Nigeria and, for the greater part of its course, flows over a broad and fertile floodplain. At Yola, a town 180 metres (600 feet) above sea level and some 1,370 km (850 miles) inland, the width of the river in flood is from 910 to 1,370 metres (3,000 to 4,500 feet). Near Numan, some 50 km (30 miles) downstream from Yola, the Benue is joined on its north bank by its most important tributary, the Gongola. Other important tributaries include the Shemankar, the Faro, the Donga, and the Katsina Ala.
Together with its tributaries, the Niger drains a total area of some 1.9 million square km (730,000 square miles). The Niger drainage system is bounded in the south by such highlands as the Fouta Djallon, the Banfora Cliffs in Burkina Faso, the Plateau of Yorubaland, and the Cameroon highlands. This southern rampart forms a watershed separating the rivers of the Niger system from others that flow directly southward to the Atlantic Ocean. With the exception of such highlands as the Jos Plateau, the Iforas and Aïr massifs, and the Ahaggar Mountains to the north and east, the northern edge of the Niger basin is less clearly defined than the southern edge.
Within the Niger basin, climate shows great variability. Mean annual precipitation levels decrease northward from more than 410 cm (160 inches) in the delta area to less than 25 cm (10 inches) in Timbuktu. Both the upper and the lower stretches of the river, however, drain areas with more than 130 cm (50 inches) of precipitation per year. The middle Niger is an area where precipitation decreases and is also the sector where the greatest amount of evaporation takes place. It is estimated that in the lake region the Niger loses some two-thirds of the annual volume of discharge that flows past Mopti.
Because of climatic variations the annual river flood does not occur at the same time in different parts of the basin. In the upper Niger the high-water discharge occurs in June, and the low-water season is in December. In the middle Niger, a first high-water discharge—the white flood (so called because of the light sediment content of the water)—occurs soon after the rainy season between July and October. A second rise—the black flood (so called because of the greater sediment content)—begins in December with the arrival of floodwaters from upstream. May and June are the low-water months in the middle stretch. On the Benue there is only one high-water season. Because of the Benue’s more southerly location, this normally occurs from May to October—earlier than on the middle Niger. The lower Niger below its confluence with the Benue consequently has a high-water period that begins in May or June—about a month earlier than on the middle Niger—and a low-water period that is at least a month shorter, as the rains in the south start earlier. In January a slight rise occurs because of the arrival of floodwaters from the upper Niger. The difference between high and low water often measures as much as 10 metres (35 feet).
It was not until the late 18th century that Europeans made systematic attempts to find the source, direction, and outlet of the Niger. In 1795 Mungo Park, a Scottish explorer, traveled overland from the Gambia region and reached the Niger near Ségou, where in July 1796 he established that the river flowed eastward. In 1805 Park sailed more than 2,400 km (1,500 miles) down the river, seeking to reach its mouth, but he and his party were drowned in the rapids at Bussa (now covered by Lake Kainji). Another Scottish explorer, Alexander G. Laing, determined but did not visit the source of the river in 1822. Two English explorers, John and Richard Lander, established the lower course of the Niger in 1830 by canoeing down the river from Yauri (now also covered by Lake Kainji), to the Atlantic Ocean, via the Nun River passage. In the second half of the 19th century two German explorers, Heinrich Barth and Eduard R. Flegel, in separate travels established the course of the Benue from its source to its confluence with the Niger.
The Nile River (Arabic: Baḥr al-Nīl or Nahr al-Nīl) is 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 6,650 km (4,132 miles) and drains an area estimated at 3,349,000 square km (1,293,000 square miles). 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-Baḥr al-Azraq; Amharic: Abay) and the Atbara (Arabic: Nahr ‘Aṭbarah), which flow from the highlands of Ethiopia, and the White Nile (Arabic: Al-Baḥr 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 naḥal, meaning a valley or river valley, and hence, by an extension of the meaning, a river. The fact that the Nile—unlike other great rivers known to them—flowed 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 BCE), 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 Al-Nīl, Al-Bahr, and Bahr al-Nīl or Nahr al-Nīl.
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-Kabīr 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 feasible (e.g., during the flood season). Improvements in air, rail, and highway facilities in the 20th century, however, greatly reduced dependency on the waterway.
It is thought that approximately 30 million years ago the early Nile, then a much shorter stream, had its sources about latitude 18° to 20° N. Its main headstream may then have been the present Atbara River. To the south lay the vast enclosed drainage system containing the large Lake Sudd. According to one theory on the evolution of the Nile system, about 25,000 years ago the East African drainage to Lake Victoria developed an outlet to the north, which sent its water into Lake Sudd. With the accumulation of sediments over a long period, the water level of this lake rose gradually. As a result of the overflow, the lake was drained, spilling over to the north. The overflow waters of Lake Sudd, rapidly forming a riverbed, linked the two major parts of the Nile system, thus unifying the drainage from Lake Victoria to the Mediterranean Sea.
The basin of the present-day Nile falls naturally into seven major regions: the Lake Plateau of East Africa, the Al-Jabal (El-Jebel), the White Nile, the Blue Nile, the Atbara, the Nile north of Khartoum in the Sudan and Egypt, and the Nile delta.
The Lake Plateau region of East Africa produces a number of headstreams and lakes that feed the White Nile. It is generally agreed that the Nile has several sources rather than one. The furthest headstream may be regarded as the Kagera River, which rises in the highlands of Burundi near the northern tip of Lake Tanganyika and then flows into Lake Victoria. The Nile proper, however, rises from Lake Victoria, the second largest freshwater lake in the world, which has an area of more than 69,400 square km (26,800 square miles) and forms a huge but shallow lake. The Nile begins near Jinja, Uganda, on the north shore of the lake, flowing northward over Ripon Falls, which has been submerged since the completion of the Owen Falls Dam (now the Nalubaale Dam) in 1954. The northward stretch of the river, known as the Victoria Nile, enters the shallow Lake Kyoga (Kioga) and, passing through its swamp vegetation, flows out in a westerly direction, descending into the East African Rift System over Murchison (Kabalega) Falls before entering the northern end of Lake Albert. Unlike Lake Victoria, Lake Albert is a deep, narrow lake with mountainous sides. There the waters of the Victoria Nile unite with the lake waters, passing northward as the Albert Nile—a portion of the river, somewhat wider and slower, that is fringed with swampy growth and is navigable for steamers.
The Nile enters the Sudan at Nimule, and from there to Jūbā—a distance of some 193 km (120 miles)—it is called the Al-Jabal River (Mountain Nile). This section of the river descends through narrow gorges and over a series of rapids, including the Fula (Fola) Rapids, and receives additional water from short tributaries on both banks. It is not commercially navigable. Below Jūbā the river flows over a large and extremely level clay plain, which extends through a narrow valley with hill country on either side, lying some 370 to 460 metres (1,200 to 1,500 feet) above sea level, and through the centre of which flows the mainstream. Because the gradient of the Nile there is only 1:13,000, the great volume of additional water that arrives during the rainy season cannot be accommodated by the river. And as a result, during those months almost the entire plain becomes inundated. This circumstance promotes the growth of enormous quantities of aquatic vegetation—including tall grasses and sedges (notably papyrus)—that collectively is called sudd, literally meaning “barrier.” The region is known as Al-Sudd (or also As-Sudd). These great masses of vegetation, the growth of which is exacerbated by the gentle flow of the water, break off and float downstream, effectively choking the mainstream and blocking the navigable channels. Channels have become further choked since the 1950s by the rapid spread of the South American water hyacinth.
This basin receives drainage from numerous other streams. The Al-Ghazāl (Gazelle) River flows in from the southwestern Sudan, joining the Al-Jabal at Lake No, a large lagoon where the mainstream takes an easterly direction. The waters of the Al-Ghazāl undergo extensive loss through evaporation, and only a small proportion of them ever reach the Nile. A short distance above Malakāl the mainstream is joined by the Sobat (Baro in Ethiopia), and downstream from there the river is called the White Nile. The regime of the Sobat is quite different from the steady flowing Al-Jabal, with a maximum flow occurring between July and December. The annual flow of the Sobat is about equal to the water lost through evaporation in Al-Sudd marshes.
The White Nile, about 800 km (500 miles) in length, supplies some 15 percent of the total volume entering Lake Nasser (called Lake Nubia in the Sudan) downstream. It begins at Malakāl and joins the Blue Nile at Khartoum, receiving no tributaries of importance. Throughout this stretch the White Nile is a wide, placid stream, often having a narrow fringe of swamps. The valley is wide and shallow, however, thus causing a considerable loss of water by both evaporation and seepage.
The Blue Nile drains from the lofty Ethiopian Plateau, where it descends in a north–northwesterly direction from a height of about 1,830 metres (6,000 feet) above sea level. Its reputed source is a spring, considered holy by the Ethiopian Orthodox Church, from which a small stream, the Abay, flows down to Lake Tana (T’ana), a fairly shallow lake with an area of about 3,625 km (1,400 square miles). The Abay leaves Lake Tana in a southeasterly direction, flowing through a series of rapids and plunging through a deep gorge. It is estimated that the lake supplies the river with only about 7 percent of its total flow, but this water is important because it is silt-free. The river then flows west and northwest through the Sudan to join the White Nile at Khartoum. In the greater part of its course from Lake Tana down to the Sudanese plains, it runs in a canyon that in places is 1,220 metres (4,000 feet) below the general level of the plateau. All of its tributaries also run in deep ravines. While the White Nile at Khartoum is a river of almost constant volume, the Blue Nile has a pronounced flood season (late July to October) caused by the summer monsoon rains over the Ethiopian Plateau and the rapid runoff from its numerous tributaries; historically, it was this surge that contributed most to the annual Nile floods in Egypt.
The Atbara River, the last tributary of the Nile, flows into the mainstream nearly 320 km (200 miles) north of Khartoum. It rises in Ethiopia at heights of 1,830 to 3,050 metres (6,000 to 10,000 feet) above sea level, not far from Gonder, to the north of Lake Tana. The two principal tributaries that feed the Atbara are the Angereb (Arabic: Baḥr al-Salam) and the Tekezo (Amharic: “Terrible”; Arabic: Nahr Satīt). The Tekezo is the most important of these, having a basin more than double the area of the Atbara itself. It rises among the high peaks of the Ethiopian highlands and flows north through a spectacular gorge to join the Atbara in the Sudan. For most of its course in the Sudan, the Atbara is well below the general level of the plains. Between the plains and the river, the ground is eroded and cut into by gullies formed by water running off the plains after rainfall. The Atbara rises and falls rapidly, like the Blue Nile. In flood it becomes a large, muddy river, and in the dry season it is a string of pools. The Atbara contributes more than 10 percent of the total annual flow of the Nile, but almost all of this comes in the period of July to October.
Along the stretch of the Nile north of Khartoum, which is sometimes called the United Nile, two parts can be distinguished. The first part, which stretches from Khartoum to Lake Nasser, is about 1,340 km (830 miles) in length. There the river flows through a desert region where rainfall is negligible, although some irrigation takes place along its banks. The second part includes Lake Nasser—which contains the water held back by the Aswan High Dam in Egypt—and below the dam the irrigated Nile valley and delta region.
Below Khartoum, the Nile flows 80 km (50 miles) northward until it reaches Sablūkah (Sababka), the site of the sixth and highest cataract. There the river cuts through hills for a distance of 12.9 km (8 miles). Flowing northward at Barbar, the river takes an S-bend, in the middle of which, from Abū HḤamad to Kūrtī and Al-Dabbah (Debba), the river flows southwestward for about 275 km (170 miles); the fourth cataract is in the middle of this stretch. At the end of this bend, at Dunqulah, it again resumes a northerly direction, crossing the third cataract and flowing into Lake Nasser.
For the roughly 1,300 km (800 miles) from the sixth cataract to Lake Nasser, the riverbed alternates between gentle stretches and series of rapids. Outcropping crystalline rocks that cross the course of the Nile cause the five famous cataracts. Because of these cataracts, the river is not completely navigable, although sections between the cataracts are navigable by sailing vessels and by river steamers.
Lake Nasser, the second largest man-made lake in the world, has a potential maximum area of about 6,740 square km (2,600 square miles); it inundates more than 480 km (300 miles) of the Nile’s course, including the second cataract near the border between Egypt and the Sudan. Immediately below the high dam is the first cataract, which was once an area of rock-strewn rapids that partially obstructed the flow of the river. From the first cataract to Cairo—a distance of about 800 km (500 miles)—the Nile flows northward in a relatively narrow, flat-bottomed groove, sinuous in outline and generally incised into the underlying limestone plateau, which averages 16 to 23 km (10 to 14 miles) in width and is enclosed by scarps that rise in places to heights of about 460 metres (1,500 feet) above the river level. For the last 320 km (200 miles) of its course before reaching Cairo, the Nile shows a strong tendency to hug the eastern edge of the valley floor, so that the greater part of the cultivated land is found on the left bank.
North of Cairo the Nile enters the delta region, a level, triangular-shaped lowland. In the 1st century CE, the Greek geographer Strabo recorded the Nile as fanning out into seven delta distributaries. The flow has since been controlled and redirected, so that the river now flows across the delta to the sea through two main distributaries, the Rosetta and the Dumyātṭ (Damietta) branches.
The Nile delta, the prototype of all deltas, comprises a gulf of the prehistoric Mediterranean Sea that has been filled in. It is composed of silt brought mainly from the Ethiopian Plateau. The silt varies in its thickness from 15 to 23 metres (50 to 75 feet) and comprises the most fertile soil in Africa. It forms a monotonous plain that extends 160 km (100 miles) from north to south, its greatest east–west extent being 250 km (155 miles) between Alexandria and Port Said. Altogether it covers an area twice that of the Nile valley in Upper Egypt. The land surface slopes gently to the sea, falling some 16 metres (52 feet) from Cairo in a gentle gradient. In the north, on the seaward border, are a number of shallow brackish lagoons and salt marshes: Lake Marout (Buḥayrat Maryūṭ), Lake Edku (Buḥayrat Idkū), Lake Burullus (Buḥayrat al-Burullus), and Lake Manzala (Buḥayrat al-Manzilah).
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 rain—more than 152 cm (60 inches)—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 16 to 27 °C (60 to 80 °F) 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 127 cm (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 exceptionally 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 41 °C (105 °F) in Khartoum. Only about 25 cm (10 inches) of rainfall occurs annually in the Al-Jazīrah area (between the White and Blue Nile rivers), as compared with more than 53 cm (21 inches) at Dakar, Senegal, which is at the same latitude. North of Khartoum less than 13 cm (5 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 38 °C (100 °F); in Aswān, for example, the average daily maximum in June is 47 °C (117 °F). 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 20 to 24 °C (68 to 75 °F) and the night minimum is about 10 °C (50 °F). The rainfall in Egypt is of Mediterranean origin and falls mostly in the winter, with the amount decreasing toward the south. From 20 cm (8 inches) on the coast, it falls gradually to a little over 2.54 cm (1 inch) in Cairo and to less than 2.54 cm (1 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 Aswān, 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 flood—particularly a sequence of such years—resulted 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 lake—some 23 billion cubic metres (812 billion cubic feet)—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 26 billion cubic metres (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 Al-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 Malakāl 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 Al-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 320 km (200 miles) of plains through which the river passes, thus delaying the arrival of the rainwater in its lower reaches until November-December. 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 Sudan—the Ar-Rahad and the Ad-Dindar—both 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 flood–waters 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 6 metres (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-Awliyā’ 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 710.8 million cubic metres (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 45.3 million cubic metres (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 capacity—more than 168 cubic km (40 cubic miles)—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.
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 BCE, traveled up the Nile as far as the first cataract (Aswān). About the second century BCE 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 BCE 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 66 CE, during the reign of the emperor Nero, was impeded by the Al-Sudd, and the attempt was therefore abandoned. Ptolemy, the Greek astronomer and geographer who lived in Alexandria, wrote in 150 CE 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 Páez, 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.
Modern exploration of the Nile basin began with the conquest of the northern and central Sudan by the Ottoman viceroy of Egypt, Muḥammad ‘Alī, 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 32 km (20 miles) beyond the present port of Jūbā, 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 160 km (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 Jūbā. 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 particularly 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 Chaillé-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 unsuccessful, but he marched to Lake Tanganyika and traveled down the Congo River to the sea. In another memorable journey in 1889, taken to relieve the German traveler Mehmed Emin Paşa (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.
