1200, however, the Arabian geographers mention a tributary, the Tharthar, navigable in flood time, which flowed from the Jaghigagh branch of the Khabur, a tributary of the Euphrates, to the Tigris.
On the eastern side of the river, on the other hand, there are several important tributaries descending from the Persian mountains: the Khabur, a little north of 37° N., navigable for rafts; the Great Zab, at 36° N., just below Nimrud, the ancient Calah; the Little Zab, about 35° 15' N.; the 'Adhem at 34° N.
Anciently the country on both sides of the Euphrates was habitable as far as the river Khabur; at the present time it is all desert from Birejik downward, the camping ground of Bedouin Arabs, the great tribe of Anazeh occupying esh-Sham, the right bank, and the Shammar the left bank, Mesopotamia of the Romans, now called elJezireh or the island.
Here the roads from Damascus, by way of Palmyra, and from Mosul, by way of the Khabur, reach the Euphrates, and here there must always have been a town of considerable commercial and strategic importance.
A little below Deir the river is joined by the Khabur (Khaboras, Biblical Khabor), the frontier of the Roman empire from Diocletian's time, which rises in the Karaja Dagh, and, with its tributary, the Jaghijagh (Mygdonius; Arab.