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 Merowe Dam, under construction below the fourth cataract in Sudan, will also capture the silt, though the dam there is designed to facilitate the flushing of sediment. 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, Sudan, and Juba, South Sudan, and between Nimule, South Sudan, on the White Nile, and Kabalega (formerly Murchison) Falls, Uganda, on the Victoria Nile.