ABSTRACT
Major floods are generated by atmospheric concentrations of water and energy from areas vastly greater than those of watersheds. By destroying hillside forests and misusing the cleared areas so that soil stability is degraded, humans contribute directly to flood hazard. Although field techniques for rehabilitation of upper watersheds are known, successful restoration of a damaged watershed only reinstates the former natural measure of flood control. As development in lowland areas proceeds, the growth in investments vulnerable to damage by floods increases the insurance value of upstream protective structures. In tropical conditions, the strong seasonal contrasts increase the importance of upstream storage for flood control. If flood-controlling dams on all major tributaries are linked by modern telecommunications, engineers can delay or release flows from tributaries so that they pass down the main channel in succession. Flood-control measures have thus begun, historically, at the wrong end of the rivers.
