Rapid growth in water requirements makes it necessary to increase the amount of water drawn from rivers. The dams necessary for capturing river water have to be built to resist damage when large floods occur, and an idea of the possible destructive power of floods is given by the front photograph. The need for protection results in thick sill structures fitted with gates, and "upstream" and "downstream" cut-off walls. Sediment transported by rivers settles forming deposits behind dams, where flow velocities decrease. On the other hand, where flow velocities are high through hydraulic...