Such artificial barriers were intended, not (as Strabo believed) for the protection of the towns upon the river from a hostile fleet, but to raise the level of the stream, in order that its water might flow off into canals on one bank or the other, whence they could be spread by means of minor channels over large tracts of territory.

The canals themselves have in most cases been gradually filled up.

In one instance, however, owing either to the peculiar nature of the soil or to some unexplained cause, we are still able to trace the course of an Assyrian work of this class and to observe the manner and principles of its construction.
[Illustration: PLATE 134] In the tract of land lying between the lower course of the Great Zab River and the Tigris, in which was situated the important town of Calah (now Nimrud), a tract which is partly alluvial, but more generally of secondary formation, hard gravel, sandstone, or conglomerate, are the remains of a canal undoubtedly Assyrian, which was carried for a distance of more than five-and-twenty miles from a point on the Khazr or Ghazr Su, a tributary of the Zab, to the south-eastern corner of the Nimrud ruins.

[PLATE CXXXIV., Fig.

1.] Originally the canal seems to have been derived from the Zab itself, the water of which was drawn off, on its northern bank, through a short tunnel--the modern Negoub--and then conducted along a cutting, first by the side of the Zab, and afterwards in a tortuous course across the undulating plain, into the ravine formed by the Shor-Derreh torrent.