SOBAT, a river of N.E. 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 S.W. of the Abyssinian highlands and N.W. 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 460 m. The Baro rises in about 36 10' E., 7 50' N. at an altitude of some 7000 ft. It has a general W. direction with a slight N. tendency. It is joined by numerous other streams which also rise on the Abyssinian plateau. These mountain torrents descend the escarpment of the plateau between great walls of rock, the Baro dropping 3000 ft. in 45 m. It then flows through a narrow gorge at an altitude of about 2000 ft., the mountains on either side towering 3000 to 4000 ft. above the river bed. Just east of 35' E. the Birbir, descending from the plateau, joins the Baro and brings with it a large volume of water. Some 40 m. lower down the hills are left behind, the rocks and rapids in the bed of the Baro cease, and the river flows W. across a vast plain with many windings and several divergent channels. From Gambela, a town on its N. 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. From the N. the Baro is joined by two considerable rivers which also rise in the rampart of hills that separates Abyssinia from the Sudan, but its chief tributaries are from the S. In about 33 20' E., 8 30' 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 and flows N.E. and N. It is joined from the E. and S. by various streams having their sources on the W. slopes of 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 30' N., 35 30' E., and after leaving the mountains flows N.W. through flat swampy tracts. 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 100 yds. 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 N.W. trend, passing for some distance through a region of swamps. Just 'beyond the swamps and some 40 m. below the confluence, is the fortified post of Nasser. From this point the ground on either side of the river gradually rises, though on the S. it is liable to inundation during flood time. From Nasser to the junction of the Sobat with the Nile the river has a course of about 180 m. 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. Several khors join it from N. and S., some being simply spill channels. These channels or " loops " are a characteristic feature of the river. The Sobat enters the Nile almost at right angles in 9 22' N., 31 31' E. It is 400 ft. wide at its mouth and has a depth of 18 to 20 ft. at low water and of 30 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. 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 (see above) 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 the 15th of May 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 AngloEgyptian Sudan and Abyssinia.
See NILE, SUDAN and ABYSSINIA. (W. E. G. ; F. R. C.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)