The reason is to be found in some past neglect of the ancient dams that
kept the water on a high level, so that it could flow by means of
artificial canals at a greater height (and consequently at a slower
rate) than the rivers themselves. The Tigris and Euphrates are rivers
fed by the melting snow in the mountains of Armenia. The hotter the
season and the more necessary a plentiful supply of water, the greater
is the amount brought down. The rivers, however, when they reach the
flat alluvial plain between the region round about Baghdad and the
Persian Gulf, when left to themselves are always bringing down a
deposit and choking themselves up and then breaking out in a new
direction, causing swamps and turning much of the land into useless
marsh. Consequent also upon this silting-up process the banks of the
rivers are higher than the surrounding country, and there is a gentle
drop in the level of the land as it recedes from the river.

[Illustration: MUD HOUSES ON THE TIGRIS]

The object of the ancient irrigationists was to tap the rivers at the
higher part of this plain, and then, by means of great canals, lead the
water where they wanted it. Large reservoirs and lakes for storing
surplus water were made, and thus the uneven delivery of water by the
rivers was checked and a more regular and manageable supply maintained.

The greatest of these ancient channels was the Nahrwân. A regulator, the
ruins which are still traceable in the bed of the Tigris, turned
sufficient water into this high-level river at Dura. It stretched
southwards for about 250 miles along the left bank of the Tigris. It was
the neglect of this canal that led to a fearful catastrophe which must
have been responsible for the death of millions; a catastrophe which
turned some 20,000 square miles of fruitful land, teeming with populous