Another flood strategy is construction of dams, such as those of the Tennessee Valley Authority. They have been used to store water during times of heavy buildup and for gradual release during dry periods. China’s Xiaolangdi Multipurpose Dam Project is expected to mitigate the flooding of the Yellow River, which has been called “China’s Sorrow.”

Dams offer other advantages. They are most commonly used to generate electricity through water power, to charge water-supply systems, and to create artificial lakes for recreation.

However, dams can harm the environment. They interfere with fish migration and habitat for other wildlife by reducing water flow.

Downstream human communities also may suffer from diminished water resources. Worldwide, more than 200 rivers flow through two or more countries. Friction over water resources constantly threatens conflict in the Middle East. No country is so dependent on a single lifeline as Egypt is on the Nile—whose source is in Ethiopia. Eighty percent of Iraq’s water originates outside its borders. Headwaters of the Tigris and Euphrates, which flow through Syria and Iraq, are largely controlled by Turkey. The two downstream countries have complained about massive Turkish dam-building programs.

In seeking to control flooding by manipulating water flow, people create new sets of problems.