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river in the proper sense, but merely a khor or watercourse, which is becoming yearly less navigable, and already inaccessible to boats except for a short time during the floods. The whole low-lying region at present intersected by the Bahr-el-Jebel, the Zaraf and all their countless affluents, channels, and branches was evidently at one time a vast lake, that has been gradually filled up by the alluvia of these rivers. Its northern margin is indicated by the abrupt change in the course of the Nile at the confluence of the Bahr-el-Ghazal, or "Gazelle River." At this point the whole system of waters is collected in a single channel, which is deflected eastwards along the escarpment of the upland Kordofan plains. A cavity of the old depression is still flooded by a remnant of the lake called the No, Nu, or Birket-el-Ghazal, which, however, under the action of the currents and periodical floods, is continually overflowing its marshy banks, shifting its place and modifying its outlines. Fig. 11—From Dufilé to Lado.
Scale 1 : 1,500,000.
Nowhere else is the Nile more obstructed by vegetable refuse as along this section of its course. The floating islands drifting with the current being arrested by the abrupt winding of the stream are collected together, and stretch at some points right across the channel, which thus becomes displaced. But the new channel is soon blocked by fresh masses of sedd, as it is called, which in many places covers a space of twelve miles. This sedd often acquires great consistency, supporting a dense growth of papyrus, and even of arborescent vegetation, beneath which the main stream continues its sluggish course. Numerous families of the Nuer tribe pitch their tents on the verdant surface, living exclusively on fish caught by piercing the foundations of their dwellings, and on the grain of various species of nymphæaceæ. In certain places along the banks of the river and surrounding swamps are seen myriads of earth-mounds, all raised above the highest level of the inundations by their architects, the termites, who ascend and descend from story to story with the flowing and ebbing stream. One of the most