The Zab, when this part of the work was constructed, ran deep along its northern bank, and, sending a portion of its waters into the tunnel, maintained a constant stream in the canal.

But after awhile the river abandoned its north bank for the opposite shore; and, water ceasing to flow through the Negoub tunnel, it became necessary to obtain it in some other way.

Accordingly the canal was extended northwards, partly by cutting and partly by tunnelling to the Ghazr Su at about two miles above its mouth, and a permanent supply was thenceforth obtained from that stream.

The work may have been intended in part to supply Calah with mountain water; but the remains of dams and sluices along its course sufficiently show that it was a canal for irrigation also.

From it water was probably derived to fertilize the whole triangle lying south of Nimrud between the two streams, a tract containing nearly thirty square miles of territory, mostly very fertile, and with careful cultivation well capable of supporting the almost metropolitan city on which it abutted.
In Assyria it must have been seldom that the Babylonian system of irrigation could have been found applicable, and the water simply derived from the rivers by side-cuts, leading it off from the natural channel.