"العراق والخليج الفارسي" [ظ‎‎٤‎٤] (٨٦٢/٩٣)

50 GEOLOGY AND DESCRIPTION OF THE LAND
approximately in the position of Kut al Imara and that in Abbasid
times the Nahrwan canal rejoined the Tigris close to it. The river
then flowed approximately along the course of the present Shatt al
Gharraf, past the city of Wasit into the Great Swamp (p. 31).
Arab geographers and historians record that the Tigris took this
course as its main channel about a.d. 635, having abandoned a more
easterly bed, and that it was able to maintain this new course in the
confused years following the Arab conquest. Wasit was founded in
a.d. 703 on its banks and remained throughout the Abbasid period
one of the most important cities of Iraq. The Tigris reached the
Great Swamp by five navigable waterways below the city, and there
was a navigable channel through a succession of lagoons to the Shatt
al Arab. Unfortunately, the exact site of Wasit is not known, though
it was probably in the neighbourhood of Kut al Hai. The Tigris
is known to have again changed its main channel during the first
part of the sixteenth century, and to have then taken its present
course to the east.
It seems therefore that the chief channel in pre-Abbasid times
must have been the Shatt Dujaila, and it would be interesting to
know the exact site of Madharaya, and who built the high Sinn
canal banks near the first bend of the Shatt Dujaila. Are they the
terminals of the Nahrwan canal? How long did the Shatt Dujaila
form the principal channel of the Tigris? It must be emphasized
that when a river changes its main course in delta lands it does not
immediately take a defined bed, unless it breaks into an existing
channel, natural or artificial; on the contrary, it spreads over the flat
land and forms marshes, which are gradually drained to lower ground
by channels working upwards and cutting beds into the soft alluvium.
This process has already been mentioned when describing the river
changes on the Euphrates below Musaiyib (p. 36), and a glance at
the map (fig. 9) shows how it has operated here on the Tigris.
The Euphrates channel for most of the way from Nasiriya to Samawa
is probably an illustration of the same process on a grand scale, the
Great Swamp having been drained southwards during the last 500
years, after the Tigris at Kut broke into its present eastern course
and ceased to supply the Great Swamp except in high flood. Since
that time the Tigris waters have been dispersed in the low ground on
either side of its new course, an operation that is now taking place
near and below Amara (p. 52).
The Tigris at Kut al Imara is only 39 - 6 feet (la^oS m.) above sea-
level in the low-water month of October, but it rises in April to a