Page:Africa by Élisée Reclus, Volume 1.djvu/105

 The two zones of alluvial land skirting the Nile are intersected by numerous irrigation canals, which distribute the fecundating waters far and wide. Like those of other streams flowing through alluvial plains, the banks of the river are higher than the surrounding country. A cutting running transversely to the direction of the valley would show that from cliff to cliff the plain presents the form of a convex curve, so that at high water the stream occupies the most elevated level between the ranges of hills bordering both sides of its valley. From this central elevation the surface of the current inclines right and left, and the slope is continued in both directions across the riverain plains. This disposition of the ground is due to the greater quantity of sedimentary matter deposited along the banks of the stream. The waters have thus a double incline, that is, according to the general direction of the river valley, and according to the lateral slope of its banks. If they met with no obstacle in the irrigating canals, they would flow at once to the lowest level on either side, and convert the whole depression into a vast lake. Hence they have to be retained at the higher elevation by means of a transverse dyke, which is opened only when the upper levels have been sufficiently submerged. The overflow is then arrested in a second section also confined by embankment works, and in this way the water is distributed to every part of the surrounding plains by a system of canals disposed at successive levels.

Nevertheless the normal incline of the land has in many places been modified by the local alluvial deposits, and by the action of opposing currents in the lateral channels. The shifting sands brought by the winds from the neighbouring escarpments have also here and there raised the low-lying plains to a level with, and even higher than, the banks of the Nile, thus obliging the cultivators to change the whole plan of their irrigation works. Formerly, when the Nile was inhabited by five different species of the crocodile, the rising flood was preceded and heralded by the suk, a small and harmless variety, which was accordingly welcomed with much ceremony by the villagers, and even honoured with divine worship in many towns far removed from the Nile. Temples were dedicated to them, where they were kept alive, decked with armlets and pendants, and fed on the flesh of victims. But none of these saurians are now seen in the Egyptian Nile, even as high up as Thebes, although the canals intersecting Cairo were still infested by them at the beginning of the present century. None appear to be met below Omboe, south of Asuan, and this region of the cataracts is also inhabited by electric fish. But the hippopotamus has retreated still higher up to the neighbourhood of the Atbara confluence.

When the flood begins to subside, the water in the higher canals would at once flow back to the main stream were it not retained by sluices, and thus stored to meet the requirements of the following spring and summer. During the subsidence the level of the overflow is still maintained in the plains some 18 or 20 feet above the bed of the main stream. The peasantry also utilise the waters which filtrate laterally into the ground to a distance of some miles, but so slowly that the effect of the inundations is not felt for weeks and even months after the normal period of the rising. Even within 300 or 400 feet of the Nile the water in