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The Nile River of northeastern Africa, the longest river in the world, flows north for 6,650 km (4,132 mi) from the most distant headwaters of the White Nile in Burundi to the Mediterranean Sea, crossing Rwanda, Uganda, Sudan, and Egypt. Major tributaries, such as the Blue Nile, rise in the highlands of western Ethiopia, and smaller watercourses in Kenya and Tanzania also drain into the Nile. The drainage basin thus encompasses about 3,349,000 sq km (1,293,000 sq mi). More than 3,200 km (2,000 mi) of the Nile are navigable.
The word Nile is probably derived from the Semitic root nahal, meaning "river valley," which later took the forms Neilos in Greek and Nilus in Latin. Although references to the Nile appear in the earliest records from the eastern Mediterranean region, the Nile's tributaries remained unknown in Europe and the Muslim world until the modern era. In 1613, Pedro Paez, a Spanish Jesuit, visited the headwaters of the Blue Nile; in 1860, the English explorer John Speke identified the Kagera River in present-day Burundi as the southernmost source of the White Nile.
The River's Course
Relative to its great length, the Nile basin is narrow, reaching only 1,930 km (1,200 mi) at its widest point. The course of the Nile and its tributaries may be divided into eight major geographical regions from south to north. In the far south the lake region of East Africa gives rise to streams that ultimately form the headwaters of the White Nile. Some of these streams flow into Lake Victoria, Africa's largest lake; some flow into Lake Albert; they then unite at Kabalega Falls in Uganda. The second zone begins near the Sudan-Uganda border where the stream called the Bahr al-Jebel ("river of the mountain") descends rapidly over rough terrain, eventually flowing into the third zone, a low, broad, marshy plain known as the Sudd about 160 km (100 mi) farther north. North of the Sudd the stream is called the White Nile; about 800 km (500 mi) long, the White Nile furnishes 29% of the Nile's total volume. The fourth and fifth zones are the Blue Nile and the Atbara rivers, the Nile's eastern tributaries, which drain the Ethiopian highlands and Lake Tana and provide the greatest amount of water to the main river. The sixth zone begins at Khartoum (the capital city of Sudan), where the White Nile and the Blue Nile join, and continues to Aswan in southern Egypt. Along this stretch of the Nile, about 1,800 km (1,100 mi) long, the river makes a great S-shaped turn and is broken by the six cataracts, stretches of rapids and waterfalls. Aswan, located at the first cataract, is also the site of the Aswan High Dam (completed 1971), which created Lake Nasser, one of the world's largest artificial lakes. In the seventh zone, between Aswan and the Nile Delta, the river flows for about 725 km (450 mi), irrigating lands that are cultivated to the shoreline. The eighth zone, north of Cairo, is the 160-km-long (100-mi) delta, formed by thousands of years of silt deposits. Here the Nile branches into many small streams and two major ones--the Rosetta and Damietta--which meander to the Mediterranean Sea.
Environment
Temperatures vary according to the season and the region. At Khartoum temperatures reach 34 degrees C (93 degrees F) in June and drop to about 4 degrees C (39 degrees F) in January. The East African lakes region enjoys a temperature range of 16 degrees C to 27 degrees C (60 degrees F to 80 degrees F). Cairo experiences a winter low of 11 degrees C (52 degrees F) and a summer high of 27 degrees C (81 degrees F).
Most of the Nile watershed receives no precipitation from November to March. An average of 200 mm (8 in) of rain, mostly of Mediterranean origin, falls on the Mediterranean coast in the north, but Cairo receives only 25 mm (1 in) of precipitation annually. Precipitation at Khartoum amounts to only 150 mm (6 in) annually; 1,270 mm (50 in) of precipitation falls in the far south. The recently completed Aswan High Dam has altered the environment of the Nile in Egypt. Before the dam's completion, heavy seasonal rainfall in the southern part of the basin during April and May caused annual floods in Egypt, usually in October. These inundations replenished the floodplain with fertile alluvial soil; the dam, however, now permits more precise flood control.
In areas unaffected by irrigation, vegetation varies according to rainfall. The highlands of Ethiopia, East Africa, and the Nile-Congo divide are covered by tropical forest along with great stretches of lightly wooded savanna. To the north the plains adjacent to the Sudd support high grasses; papyrus, water lettuce, and water hyacinth grow in and near the stream. The Sahel, a drier zone of acacia and scrub, extends from about latitude 10 degrees north to Khartoum, and true desert prevails farther north except in the heavily irrigated Nile valley. The river harbors fish, snakes, turtles, crocodiles, and lizards; hippopotamuses are found in the south.
History and Economy
The Nile basin did not always look as it does today. About 30 million years ago the Atbara may have been the main headstream. At that time streams in the southern portion of the drainage system flowed into the Sudd, which formed a large inland lake independent of the Nile. Much later, only 20,000 to 25,000 years ago, the East African lake system began draining north toward the Sudd, causing it to spill over and combine its watershed with that of the northern Nile. The region sheltered not only the ancient Egyptian civilization (see Egypt, ancient), which arose about 3100 BC, but also Meroe, the capital (656 BC-AD 320) of Cush and an early iron-working center; early Christian and Muslim states in southern Egypt and the northern Sudan; and kingdoms such as Buganda, established during the 17th century in East Africa. Many smaller social and political units also unified peoples of the Nile basin.
People living along the Nile and its tributaries include fishers and cultivators in the irrigated north and wetter south, nomads who raise cattle and camels in the northern Sudan, and cattle herders in the White Nile, Bahr al-Jebel, and East African lakes regions. Nearly 99% of Egypt's population lives in the Nile valley and delta.
In the arid parts of the northern Nile basin, the economy is closely linked to the river. Irrigation began in the time of the ancient Egyptians, but the first dams were constructed only in 1861. The Aswan High Dam increased the amount of land under irrigation by about 20% and produces substantial electricity, although some fear that the dam endangers the fertility of the soil in the delta by increasing the salinity of the river and by capturing some of the rich soil carried by the Nile at the bottom of Lake Nasser. Other dams operate on the Nile in Sudan and Uganda to provide hydroelectric power and surplus water during the dry season. Work on the Jonglei Canal, designed to divert the flow of the White Nile around the Sudd and provide additional water to Egypt and northern Sudan, was halted by civil war in 1984. More recently, drought in the upper Nile basin has threatened power and water shortages downstream in Egypt.