"كتيّب بلاد الرافدين. المجلد II. ١٩١٧" [‎٤‎٤‎٧] (٥٤٢/٤٥٨)

CONTROL OF TIGRIS WATER 447
Fourthly, if the floods were caused merely by the destruction of
banks and breaching of hunds, they could not be detrimental to the
navigation of the river down-stream: they would only be obstacles
to movement by land. Such floods would make no difference to the
navigation of the Tigris in low supply, and would make navigation
in flood-time somewhat easier. In order to prevent navigation
down-stream of the diversion it would be necessary to dam the
river.
Fifthly, no preventive measures against a flooding of the country
by an enemy could be taken by a force on the Tigris down-stream of
the enemy position, and not within striking distance of the point where
they chose to divert the water. All that could be done would be to pro
tect limited areas by means of hunds, and under certain circumstances
to provide escapes for at least part of the water into areas where it
would cause less inconvenience. The only way to make sure against
such flooding would be to hold the head of the Tigris delta, and also
to hold the Euphrates up to Hit.
Sixthly, with regard to damming the river: (1) it would not be
impossible, but under the present war conditions would be exceed
ingly difficult. It would need much preparation and labour. The
news that it was being attempted would spread like wildfire among
the natives, for the down-stream Arabs would be ruined by such
a work. Early intelligence of it would, therefore, almost certainly
be received. It would probably be difficult to get Arab labour for
the purpose.
(2) The most favourable time for the building of a dam on one of
the Mesopotamian rivers would be in the months when the water is
falling. The reason, of course, is that as the dam progresses and the
channel available for the water decreases, it is of great advantage that
the quantity of water in the channel should also decrease. (This
holds good both for a stone dam and for one of earth and brushwood.)
(3) No hard material (stone) is readily available below Baghdad, and
the method of construction would probably be the one usually adopted
in Mesopotamia, namely, with earth and brushwood ; a method well
known to the Arabs. A dam of this kind is carried out from one
or both banks, and its crest must be continually kept above water
level.
(4) It would probably be best to begin such a dam at the time
when the river is beginning to subside (say, early June). Provided
that sufficient preparations had been made in the previous months,
a dam then begun could be closed in low-water season (say, October).
(5) The difficulty of damming would, in the present circumstances,
be great even in the favourable season, June-October. The difficulty