Dams and Other Hydraulic Structures
Of all the river flow across the continents, the greater proportion crosses regions where it is impracticable to abstract water or there is no need to do so. Thus most of the huge flow that crosses the Siberian plains (2000km3 per year) and the Amazon basin (6500km3 per year) runs freely to the sea. About 580km3 per year flows through the Mississippi basin but, although there are vast areas of fertile soil, the flatness of the riverine terrain makes flood protection a higher priority than river water abstraction. However diversion works on the Indus River and its Punjab tributaries mean that a large proportion of the annual flow of 210km3 per year is withdrawn for irrigation projects in Punjab, Rajasthan and Sind in both India and Pakistan. In the Aral Sea basin in Central Asia so much water was abstracted from the Syr Darya and Amu Darya rivers that the flow of 100-120km3 per year that emerges from the mountains was reduced in some years to no flow at all into the diminished inland sea.