It is formed originally of three main branches: the Diarbekr stream, or true Tigris, the Myafarekin River, and the Bitlis Chai, or Centrites of Xenophon, which carries a greater body than either of the other two.

From its entry on the low country near Jezireh to the termination of its course at Kurnah, it is continually receiving from the left a series of most important additions.

The chain of Zagros, which, running parallel to the two main springs, shuts in the Mesopotamian plain upon the east, abounds with springs, which are well supplied during the whole summer from its snows, and these when collected form rivers of large size and most refreshing coolness.

The principal are, the eastern Khabour, which joins the Tigris in lat.

37 deg.