Its flood is, however, very irregular, owing to the nature of its banks, and the general inclination of the plain, whereof mention was made above.

If care is taken, the inundation may be pretty equally distrib uted on either side of the stream; but if the river banks are neglected, it is sure to flow mainly to the west, rendering the whole country on that side the river a swamp, and leaving the territory on the left bank almost without water.

This state of things may be traced historically from the age of Alexander to the present day, and has probably prevailed more or less since the time when Chaldaea received its first inhabitants.
The floods of the Tigris and Euphrates combine with the ordinary action of their streams upon their banks to produce a constant variation in their courses, which in a long period of time might amount to something very considerable.

It is impossible to say, with respect to any portion of the alluvial plain, that it may not at some former period have been the bed of one or the other river.

Still it would seem that, on the whole, a law of compensation prevails, with the result that the general position of the streams in the valley is not very different now from what it was 4000 years ago.