SOBAT, a river of north-east Africa, the most southerly of the great eastern affluents of the Nile. It is formed by the junction of various streams which rise in the south-west of the Abyssinian highlands and north-west of Lake Rudolf. The length of the Sobat, reckoning from the source of the Baro, the chief upper stream, to the confluence with the Nile is about 46o miles. The Baro rises in about 36° 1o' E., 7° 5o' N. at an altitude of some 7,000 ft. The Baro descends the escarpment of the plateau between great walls of rock, dropping 3,00o ft. in 45 miles. It then flows through a narrow gorge at an altitude of about 2,000 ft., the mountains on either side towering 3,00o to 4,000 ft. above the river bed. Just east of 35' E. the Birbir joins the Baro. Some 40 m. lower down the hills are left behind and the river flows west across a vast plain. From Gambela, a town on its north bank 20 m. below the Birbir junction, the river is navigable by steamers during flood time (June—December) to the point of confluence with the White Nile. In about 33° 20' E., 8° 3o' N., it is joined by the Pibor. This river issues from the swamp region east of Bor on the Bahr-el-Jebel stretch of the Nile. It is joined from the east and south by various streams from the Kaffa plateau. Of these the chief are the Gelo—which breaks through a gap in the mountains in a series of magnificent cascades—and the Akobo. The Akobo rises in about 6° 3o' N., 35° 3o' E. The whole region of the lower Pibor and Baro is one of swamps, caused by the rivers overflowing their banks in the rainy season. At its junction with the Baro the Pibor is over ioo yd. wide, with a depth of 8 ft. and a speed of 2.3 ft. per second.
Below the confluence of the Pibor and Baro the united stream, now known as the Sobat, takes a decided north-west trend, passing for some distance through a region of swamps. Just beyond the swamps and some 4o m. below the confluence, is the fortified post of Nasser. From Nasser to the junction of the Sobat with the
Nile the river has a course of about 180 miles. As it approaches the Nile the Sobat flows in a well-defined channel cut in the alluvial plains through which it passes. The banks become steep, the slope rapid and the current strong. The Sobat enters the Nile almost at right angles in 9° 2 2' N., 31' E. It is 400 ft. wide at its mouth and has a depth of 18 to 20 ft. at low water and of 3o ft. when in flood. The colour of the water when in moderate flood is that of milk, and it is from this circumstance that the Nile gets its name of Bahr-el-Abiad, i.e., White river. In full flood the colour of the Sobat is a pale brick red. The amount of alluvium brought down is considerable. The average volume of water dis charged varies from ioo cu.m. per second at low stage in April to 770 in October and November when it is in full flood. For the part played by the Sobat in the annual rise of the Nile see NILE.
The Sobat was ascended for some distance in 1841 by the Egyptian expedition despatched in the previous year to explore the upper Nile. The post of Nasser was founded in 1874 by General C. G. Gordon when governor of the equatorial provinces of Egypt, and it was visited in 1876 by Dr. W. Junker, the German explorer. The exploration of the river system above Nasser was carried out in the last decade of the 19th century by the Italian explorer V. Bottego; by Colonel (then Captain) Marchand, of the French army, who, on his way from Fashoda to France, navigated the Baro up to the foot of the mountains; and by Captain M. S. Wellby, Majors H. H. Austin and R. G. T. Bright, of the British army, and others. By the agreement of May 15, 1902, between Great Britain and Abyssinia the lower courses of the Pibor and Baro rivers to their point of confluence form the frontier between the Anglo-Egyptian Sudan and Abyssinia. (See NILE, SUDAN and ABYSSINIA.)