In Egypt the one and only valley was that of the Nile, and the one and only stream that which had formed it, and flowed along it, at a lower or higher level, ceaselessly. It might perhaps have been possible for Egyptian engineering skill to have blocked the valley at Silsilis, or at the Gebelein, and to have thus turned Upper Egypt into a huge reservoir always full, and always capable of supplying Lower Egypt with enough water to eke out a deficient inundation. But this could only have been done by an enormous work, very difficult to construct, and at the sacrifice of several hundred square miles of fertile territory, thickly inhabited, which would have been covered permanently by the artificial lake. Moreover, the Egyptians would have known that such an embankment can under no circumstances be absolutely secure, and may have foreseen that its rupture would spread destruction over the whole of the lower country. Amenemhat, at any rate, did not venture to adopt so bold a design. He sought for a natural depression, and found one in the Libyan range of hills to the west of the Nile valley, about a degree south of the latitude of Memphis--a depression of great depth and of ample expanse, fifty miles or more in length by thirty in breadth, and containing an area of six or seven hundred square miles. It was separated from the Nile valley by a narrow ridge of hills about two hundred feet high, through which ran from south-east to north-west a narrow rocky gorge, giving access to the depression. It is possible that in very high floods some of the water of the inundation passed naturally into the basin through this gorge; but whether this were so or no, it was plain that by the employment of no very large amount of labour a canal or cutting might be carried along the gorge, and the Nile water given free access into the depression, not only in very high floods, but annually when the inundation reached a certain moderate height. This is, accordingly, what Amenemhat did. He dug a canal from the western branch of the Nile--the modern Bahr Yousuf--leaving it at El-Lahoun, carried his canal through the gorge, in places cutting deep into its rocky bottom, and by a system of sluices and flood-gates retained such an absolute control over the water that he could either admit or exclude the inundation at his will, as it rose; and when it fell, could either allow the water that had flowed in to return, or imprison it and keep it back. Within the gorge he had thus at all times a copious store of the invaluable fluid, banked up to the height of high Nile, and capable of being applied to purposes of cultivation both within and without the depression by the opening and shutting of the sluices.