—The Gazelle river, which flows into Lake No, has a catchment basin of 240,000 square kilometres, with an annual rainfall of 75 millimetres; and an additional catchment basin of 230 square kilometres with an annual rainfall of 30 millimetres; and yet the river discharges nothing in summer and about 40 cubic metres per second in flood. It is the most extraordinary river in the world, often blocked by sudd and invisible. It has practically no banks in flood or in times of low supply, while the waterway varies in width from 6 metres to 90 and in depth from 2 metres to 6 metres. It makes its way between interminable marshes of papyrus and water grasses. The Gazelle river is a deltaic river in a still more embryonic stage than the Albert Nile north of Ghaba Shambe. It, however, performs one good function. It keeps the swamps of the Sudd region full of water, year in and year out; and without it, the water of the Albert Nile would doubtless be lost by percolation in the Sudd region and the White Nile be left high and dry for 3 months every year.
Schweinfurth was the first to ascend and describe this river and its tributaries. Beginning from the east, its principal tributaries are:—the Rohl, the Dyow, the Tondy, the Kit, the Dyûr, the Dembo, the Humr and the Bahr el Arab. With courses of between 450 and 750 kilometres in length, the tributaries fall from 500 to 700 metres in their upper courses of from 300 to 500 kilometres, and in their remaining courses they traverse swamps. Certain of the tributaries may be discharging 10 cubic metres per second in summer and 250 cubic metres per second in flood, but the main stream of the Gazelle seems never to add more than 40 cubic metres per second to the Albert Nile. The whole of the water is evaporated from the weeds, papyrus, watergrasses and open sheets of water which cover a desolate area of 70,000 square kilometres. Evaporation and rainfall balance each other. If the «Singhara» or Indian water nut could be induced to grow in this waste of waters, some profit might be got out of them.
15. The Bahr Zeraf.
—The Bahr Zeraf is the right hand branch of the Albert Nile which leaves the latter river north of Ghaba Shambe and joins it again midway between Lake No and the Sobat river. It is fed largely by the escape water of the Atem river brought down in a traceable channel and in flood by torrents from the hilly region east of Gondokoro. Beginning with a series of marshes and swamps, it gradually establishes its right to be called a river and finally after a tortuous and meandering course of about 270 kilometres tails into the Albert Nile. In its lower reaches it is about 50 metres wide and from 2 to 4 metres deep in summer and 6 to 8 metres deep in flood. Its discharges vary from 30 cubic metres per second in summer to 160 cubic metres per second in flood. In its lower reaches the banks consist of solid earth, proving that at one time it carried water other than that brought down by the Albert Nile.
16. The Sobat River.
—This river drains 156,000 square kilometres lying between the catchment basins of Lake Rudolf and the Blue Nile. The rainfall in the mountainous region of Gallaland is plentiful and especially heavy in autumn, and were it not for the extensive lakes and marshes in its middle course, it would be a torrent in flood. Regulated and restrained by the lakes and marshes, this river has an extraordinarily even rise and fall, as a reference to [Table 24] will certify. It is unfortunate that the Nasser gauge has been read so interruptedly. The Doleb Hilla gauge is in the back water of the Albert Nile and not very reliable. The discharges of the last four years have varied from 40 cubic metres per second in low supply to 1000 cubic metres in flood, though there have been years when the discharges have fallen to zero in summer and when the flood must have exceeded 1500 cubic metres per second. April is the month of low supply and November of maximum flood.
In its last 50 kilometres, the river has a deep, well defined channel between high banks, which are never topped in the highest floods. The width of waterway is about 110 metres and the depth 7 metres in summer and about 10 to 11 metres in flood.
The principal tributaries of the Sobat are the Baro from the north-east and east, and the Akobo and Pibor from the south-east and south. All the tributaries meet and form extensive swamps from which the Sobat has its origin. The village of Nasser is situated on the Sobat near its origin. A gauge has been erected here.
17. The Sudd region.
—The Sudd region of the Albert Nile lies north of Ghaba Shambe and corresponds to that part of the river where not only do the floods overflow the banks, but the summer supplies can do so in many places. It is the delta of the river in a very embryonic stage. There are two main branches to the river, the Albert Nile proper and the Zeraf, which have both been already described. Both these rivers are liable to be blocked by sudd or blocks of living vegetation. These blocks are sometimes as much as 5 metres thick and capable of turning nearly the whole supply of the river out of its course. They are formed of papyrus, weeds and watergrasses, which grow on the half sandy half peaty banks of the lagoons and marshes traversed by the river, and which, under the double action of a rising flood and strong winds, are torn up and driven into the channels wherever they are confined in width, and there jammed into solid masses of floating weeds, filling the whole width of the river, very nearly the whole depth, and sometimes over a kilometre in length. In addition to the local weeds and grasses, there are always at hand in high floods dense masses of pistea weeds which have come from the upper waters of the Albert Nile south of Dufile. While the sudd floats it is not so bad as when it sinks, as it has done at block No. 15 north of Gaba Shambe, where the Nile has left its course for 37 kilometres owing to sunken sudd. When the sudd sinks, its becomes putrid and especially loathsome.