'IRAQ AND THE PERSIAN GULF' [‎39v] (83/862)

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GEOLOGY AND DESCRIPTION OF THE LAND
Mosul to Qala Sharqat (direct distance 60 miles; by river 73 miles)
Mosul, on the right bank of the Tigris, opposite the site of Nineveh
on the left, has been by far the most important town in northern Iraq
since the time of the Omayyad caliphs when it became the capital of
their Jazira province. It is the main crossing-place of the Tigris in
this area; its old floating bridge, so often renewed, was replaced in
I 933“ I 934 by a new steel structure.
The average low-water height in September is 703 feet above sea-
level, and the river is still within the foothill steppe region, confined
within deep banks; it is thus able to take the normal high-water rise
of 13 feet, and even the greatest recorded rise in the last 25 years
(16-8 ft.), without danger to the countryside. The average discharge
throughout a normal September is 5,300 cubic feet (141 cu.m.) per
second, while the monthly average for April reaches 48,750 cubic feet
(1,380 cu.m.).
Below Mosul the characteristics of the bed and course are similar
to those above, but the hills are much lower, and there is more open
steppe. The river is kept to a general south-south-easterly course by
low hills on its right bank. About 30 miles below the town the Tigris
is joined by the Great Zab, which has drained a large snow-fed moun
tain basin of about 7,700 square miles in Kurdistan (pp. 102-13).
After leaving the mountains this torrential tributary flows in a rocky
or shingly bed to its junction with the Tigris. It is fordable in the
low-water season at many places, but in winter is subject to sudden
rises and fluctuations, and its peak has been exceptionally recorded as
early as h ebruary, because of rain. But its volume normally increases
to a maximum at the end of April, and the discharge remains high
throughout May, when the monthly average at Girdmamukh (28,500
cu. ft. per sec.; 808 cu.m.) reaches a maximum; it falls to 3,000 cubic
feet (87 cu.m.) in September and October. The main body of its
flood waters reaches the Tigris rather later than that of the Tigris
itself, but the two floods overlap, and the effect of the Great Zab is to
increase the discharge below the junction by two-thirds in a normal
year.
Just above the junction are the ruins of Assyrian Calah, or
Nimrud (p. 223), and near them a ‘new town’, Haditha, was built
by the Omayyad caliphs. No town or village of any importance is
here to-day. Below the junction the Tigris flows almost due south
through open steppe country between adequate banks. It is en
cumbered by shingle at many places, particularly in the low-water