Nile - longest river in the world, c.4,160 mi (6,695 km) long from its remotest headstream, the Luvironza River in Burundi, central Africa, to its delta on the Mediterranean Sea, NE Egypt. The Nile flows northward and drains c.1,100,000 sq mi (2,850,000 sq km), about one tenth of Africa, including parts of Egypt, Sudan, Ethiopia, Eritrea, Kenya, Uganda, Rwanda, Burundi, Tanzania, and Congo (Kinshasa). Its waters support practically all agriculture in the most densely populated parts of Egypt, furnish water for more than 20% of Sudan's total crop area, and are widely used throughout the basin for navigation and hydroelectric power.

Course and Navigability

The trunk stream of the the Nile is formed at Khartoum, Sudan, 1,857 mi (2,988 km) from the sea, by the junction of the Blue Nile (c.1,000 mi/1,610 km long) and the White Nile (c.2,300 mi/3,700 km long). The Blue Nile rises in the headwaters of Lake Tana, NW Ethiopia, a region of heavy summer rains, and is the source of floodwaters that reach Egypt in September; the Blue Nile contributes more than half of all Nile waters throughout the year. During floodtime it also carries great quantities of silt from the highlands of Ethiopia; these now collect in Lake Nasser behind the Aswan High Dam, but for centuries they were left on the floodplain after the floods and helped replenish the fertility of Egypt's soils. The White Nile (known in various sections as the Bahr-el-Abiad, Bahr-el-Jebel, Albert Nile, and Victoria Nile) rises in the headwaters of Lake Victoria in a region of heavy, year-round rainfall; unlike the Blue Nile, it has a constant flow, owing in part to its source area and in part to the regulating effects of its passage through lakes Victoria and Albert and the Sudd swamps. Other important tributaries of the Nile are the Atbara and Sobat rivers. The Gezira, or "island," formed between the Blue Nile and the White Nile as they come together at Khartoum is Sudan's principal agricultural area and the only large tract of land outside Egypt irrigated with Nile waters.

From Khartoum to the Egyptian border at Wadi Halfa (now submerged) and on to Aswan in Egypt, the Nile occupies a narrow entrenched valley with little floodplain for cultivation; in this stretch it is interrupted by six cataracts (rapids). From Aswan the river flows north 550 mi (885 km) to Cairo, bordered by a floodplain that gradually widens to c.12 mi (20 km); irrigated by the river, this intensively cultivated valley contrasts with the barren desert on either side. North of Cairo is the great Nile delta (c.100 mi/160 km long and up to 115 mi/185 km wide), which contains 60% of Egypt's cultivated land and extensive areas of swamps and shallow lakes. Two distributaries, the Dumyat (Damietta) on the east and the Rashid (Rosetta) on the west, each c.150 mi (240 km) long, carry the river's remaining water (after irrigation) to the Mediterranean Sea. Regular steamship service is maintained on the Nile between Alexandria (reached by canal) and Aswan; the Blue Nile is navigable June through December from Suki (above Sennar Dam) to Roseires Dam; the White Nile is navigable all year between Khartoum and Juba in Sudan and between Nimule and Kabalega (formerly Murchison) Falls on the Victoria Nile.

Irrigation along the Nile

The use of the Nile for irrigation, now regulated by the Nile Waters Treaty of 1959, dates back to at least 4000 b.c. in Egypt. The traditional system of basin irrigation—in which Nile floods were trapped in shallow basins and a cool-season crop of wheat or barley was grown in soaked and silt-replenished soil—has been replaced since the mid-1800s by a system of perennial irrigation and the production of two or three crops a year, including cotton, sugarcane, and peanuts. The delta barrages, just below Cairo, channel water into a system of feeder canals for the delta, and other barrages at Isna, Asyut, and Nag Hammadi keep the level of the Nile high enough all year for perennial irrigation in the valley of Upper Egypt; the Idfina Barrage on the Rashid prevents infiltration by the sea at low water. Nile water is also used for irrigation in the Faiyum Basin.

The Aswan Dam (completed 1902 and raised twice since then) was the first dam built on the Nile to store part of the autumn flood for later use; it has a storage capacity of 5 billion cu m and is now supplemented by the Aswan High Dam (completed 1971), 5 mi (8 km) upstream, with a storage capacity of 48 billion cu m, sufficient (with existing dams) to hold back the entire flood for later use. Construction of the Aswan High Dam has added c.1,800,000 acres (728,500 hectares) of irrigated land to Egypt's cultivable area and converted c.730,000 acres (295,400 hectares) from basin to perennial irrigation. Lake Nasser, created by the Aswan High Dam, has experienced problems with silting. There has been a reduction of soil replenishment downstream and a reduction of nutrients that once fed the E Mediterranean Sea. Other important storage dams, all outside Egypt, but built with Egypt's help or cooperation, are the Owen Falls Dam (1954) and Jabal Awliya Dam (1937) on the White Nile; the Sennar (1927) and Roseires (1966) on the Blue Nile; and the Kashm-el-Girba Dam (1964) on the Atbara River.

The Search for the Nile's Source

The source of the Nile and its life-giving floods was a mystery for centuries. Ptolemy held that the source was the "Mountains of the Moon," and the search for these and for the origin of the Nile attracted much attention in the 18th and 19th cent. James Bruce, the Scottish explorer, identified (1770) Lake Tana as the source of the Blue Nile, and John Speke, the British explorer, is credited with the identification (1861–62) of Lake Victoria and Ripon Falls as the source of the White Nile.

Bibliography

See B. Brander, The River Nile (2d ed. 1968); J. Waterbury, Hydropolitics of the Nile Valley (1979); A. Moorhead, The White Nile (rev. ed. 1983).

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