Except for a small area in northeastern Sudan where wadis discharge the sporadic runoff into the Red Sea or rivers from Ethiopia flow into shallow, evaporating ponds west of the Red Sea Hills, the entire country is drained by the Nile and its two main tributaries, the Blue Nile (Al Bahr al Azraq) and the White Nile (Al Bahr al Abyad). The longest river in the world, the Nile flows for 6,737 kilometers from its farthest headwaters in central Africa to the Mediterranean. The importance of the Nile has been recognized since biblical times; for centuries the river has been a lifeline for Sudan.

The Blue Nile flows out of the Ethiopian highlands to meet the White Nile at Khartoum. The Blue Nile is the smaller of the two; its flow usually accounts for only one-sixth of the total. In August, however, the rains in the Ethiopian highlands swell the Blue Nile until it accounts for 90 percent of the Nile's total flow. Several dams have been constructed to regulate the river's flow--the Roseires Dam (Ar Rusayris), about 100 kilometers from the Ethiopian border; the Meina al Mak Dam at Sinjah; and the largest, the forty-meter-high Sennar Dam constructed in 1925 at Sannar. The Blue Nile's two main tributaries, the Dindar and the Rahad, have headwaters in the Ethiopian highlands and discharge water into the Blue Nile only during the summer high-water season. For the remainder of the year, their flow is reduced to pools in their sandy riverbeds.

The White Nile flows north from central Africa, draining Lake Victoria and the highland regions of Uganda, Rwanda, and Burundi. At Bor, the great swamp of the Nile, As Sudd begins. The river has no well-defined channel here; the water flows slowly through a labyrinth of small spillways and lakes choked with papyrus and reeds. Much water is lost to evaporation. To provide for water transportation through this region and to speed the river's flow so that less water evaporates, Sudan, with French help, began building the Jonglei Canal (also seen as Junqali Canal) from Bor to a point just upstream from Malakal. However, construction was suspended in 1984 because of security problems caused by the civil war in the south.

South of Khartoum, the British built the Jabal al Auliya Dam in 1937 to store the water of the White Nile and then release it in the fall when the flow from the Blue Nile slackens. Much water from the reservoir has been diverted for irrigation projects in central Sudan, however, or it merely evaporates, so the overall flow released downstream is not great.

The White Nile has several substantial tributaries that drain southern Sudan. In the southwest, the Bahr al Ghazal drains a basin larger in area than France. Although the drainage area is extensive, evaporation takes most of the water from the slowmoving streams in this region, and the discharge of the Bahr al Ghazal into the White Nile is minimal. In southeast Sudan, the Sobat River drains an area of western Ethiopia and the hills near the Sudan-Uganda border. The Sobat's discharge is considerable; at its confluence with the White Nile just south of Malakal, the Sobat accounts for half the White Nile's water.

Above Khartoum, the Nile flows through desert in a large Sshaped pattern to empty into Lake Nasser behind the Aswan High Dam in Egypt. The river flows slowly above Khartoum, dropping little in elevation although five cataracts hinder river transport at times of low water. The Atbarah River, flowing out of Ethiopia, is the only tributary north of Khartoum, and its waters reach the Nile for only the six months between July and December. During the rest of the year, the Atbarah's bed is dry, except for a few pools and ponds.