"العراق والخليج الفارسي" [ظ‎‎٤‎١] (٨٦٢/٨٧)

46 GEOLOGY AND DESCRIPTION OF THE LAND
Babylonian times must be largely conjectural. Opis is believed to be
identical or connected closely with Akshak, an earlier Akkadian town,
which there is some evidence to locate at the Khafaje mounds on the
Diyala, lomiles east of Baghdad (photo. 95). It seems possible there
fore that in the second or first millennium B.c. the Tigris took a more
easterly course than it does to-day, approximately along the line of
the later Abu-l-Jund and Nahrwan canals to the Diyala near Khafaje
(fig. 13). Then, still in this early period, the river occupied the present
channel for an unknown distance below Qadisiya long enough to
erode its bed through the conglomerate sill and to scour its bed above.
In order to raise its level and prevent further scour a great dam was
erected across the river, the waters of which were diverted south
wards over the harder rock, where it cut a new channel west of the
older and modern courses, while a smaller channel fed the older
course from above the dam. If this interpretation is correct it would
account for the origin of the upper part of the Nahrwan canal which
was subsequently straightened, embanked, and furnished with a new
intake near Daur by the Sassanid kings.
The westerly course of the Tigris was maintained during late
Sassanid times, and for most of the Abbasid period (6th-i2th cen
turies). Prosperous towns were built on its banks, the most important
of which in the tenth century, Alth and Ukbara, are recognizable to
day, the first by a small village and mound, the second by an extensive
mound, Tell Akbar. The fertile district to the west was watered by
the Dujail canal, the course of which is plainly marked to-day by the
embankments which successively enclosed it, by its numerous ruined
off-takes, and by the great stone bridge near Harba which was built
by the Caliph Mustansir in a.d. 1232 within ten years of the river
bursting back into its older bed below Qadisiya. But whatever course
it took to the east in earlier times, it no longer followed the Nahrwan
canal to the Diyala after 1242, but left it just east of the Adhaim con
fluence and took a more southerly course, more or less as at present,
re-entering its former western bed near the village of Baradan, 13
miles due north of Baghdad, which had been founded by the Caliph
Mansur about 500 years earlier (a.d. 762) on the west bank of the
Tigris (p. 243).
At the present time there are no towns and few large villages on
the Tigris between Samarra and Baghdad; near the village of Balad,
which stands on the left bank of the western channel and near the
right bank of the modern river, the Tigris is to-day joined by the
Shatt al Adhaim. This stream drains a comparatively small basin of