The Salinity of Rivers

Many ancient civilizations rose by diverting rivers and irrigating arid lands to grow crops. For such projects to succeed human beings had to learn to work cooperatively toward a common objective. The most fruitful of the ancient systems was created at the southeastern end of the Fertile Crescent, the broad valley formed by the Tigris and the Euphrates in what is now Iraq. From there civ il ization spread eastward through present-day Iran, Afghanistan, Pakistan, India and thence into China, wherever rivers disgorged through valleys of recently deposited alluvial soil. At its peak of productivity each irrigated region probably supported well over a million people. All these civilizations ultimately collapsed, and for the same reason: the land became so salty that crops could no longer be grown on it. The salts that were washed out of the soil at higher elevations became concentrated in the irrigated fields as the water evaporated from the surface and transpired through the leaves of the growing crops. Although floods, plagues and wars took their toll, in the end the civilizations based on irrigation faded away because of salination.