Allowing for a certain amount of patriotic exaggeration, the exclamation at least shows at what a high degree of excellence the irrigation system of Mesopotamia was maintained in the 10th century A.D.  Yet Mesopotamia is to-day a desert except for the regions in the immediate vicinity of the rivers.  You can go westwards from Baghdad to the Euphrates, and every mile or so you will have to cross earthworks, not unlike irregular railway embankments, showing a vast system of irrigation channels both great and small.  But there is not a drop of water near and not a tree and no sign of any life.  How came the change and how can such a network of channels have ceased to work entirely?

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 Nahrwan.  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 cities, into a dismal swamp.

The intake from the Tigris of this and other canals evidently silted up, and thus enormous volumes of water, usually carried off by them in times of flood, helped to swell this river till, bursting its banks, it inundated the whole country.  The result remains to-day—­a vast tract of swampy land, barren and almost useless, except to a few wandering tribes of Arabs.