Towards the north and east it is provided by nature with an ample supply of water, rills everywhere flowing from the Armenian and Kurdish ranges, which soon collect into rapid and abundant rivers.

The central, southern, and western regions are, however, less bountifully supplied; for though the Euphrates washes the whole western and south-western frontier, it spreads fertility only along its banks; and though Mount Masius sends down upon the Mesopotamian plain a considerable number of streams, they form in the space of 200 miles between Balls and Mosul but two rivers, leaving thus large tracts to languish for want of the precious fluid.

The vicinity of the Arabian and Syrian deserts is likewise felt in these regions, which, left to themselves, tend to acquire the desert character, and have occasionally been regarded as actual parts of Arabia.
The chief natural division of the country is that made by the Tigris, which, having a course nearly from north to south, between Til and Samarah, separates Assyria into a western and an eastern district.

Of these two, the eastern or that upon the left bank of the Tigris, although considerably the smaller, has always been the more important region.

Comparatively narrow at first, it broadens as the course of the river is descended, till it attains about the thirty-fifth parallel a width of 130 or 140 miles.