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Sudan
Hydrology
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.
Data as of June 1991
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