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<br /> <br />Introduction <br /> <br /> <br />Dams and river regulation have become an integral part of our twentieth.century <br />landscape and livelihood. Although untamed rivers are part of the cultural heritage of the <br />United States. virtually every river in the lower 48 states is now regulated by dams. locks. <br />or diversions. These regulated rivers have afforded society many benefits - cheap elec. <br />tricity. navigable streams. absence of devastating floods. decreased threat of drought. But <br />regulated rivers are fundamentally different ecological and physical entities from untamed <br />rivers. Natural cycles of flooding and sediment transport have been eliminated from many <br />of these rivers. Channel shape. riverine vegetation. and in.stream aquatic communities <br />have. in many cases. changed as a result of flow regulation. <br />Because of the scale of dam construction that has taken place in the United States. <br />society now has before it a set of choices regarding the kind of river characteristics we <br />desire. Like it or not. we control the destiny of these streams. Traditionally. river managers <br />have focused on issues of engineering efficiency. sometimes to the neglect of in.stream <br />environmental values. The engineering matters remain a focus of management. but our <br />society must also choose whether or not to manage rivers for their intrinsic environmental <br />values. We can consciously choose to manage our rivers for certain anticipated environ. <br />mental consequences. or we can intentionally choose to accept the environmental <br />responses as they haphazardly occur. The purpose ofthis Circular is to illustrate the <br />downstream environmental consequences of dams. and to explain the basis on which <br />rivers can be scientifically managed. <br />Egyptians were building dams upstream from Cairo 5,000 years ago. Western Europe. <br />ans constructed dams to power water wheels during the late Middle Ages (Smith. 1971). <br />Eight hundred years ago. the Anasazi built small check dams on Mesa Verde in Colorado to <br />hold storm runofffor later use on their crops (Ortiz. 1979). As early as AD 833. the Chi. <br />nese used human and animal power to build a 90.foot-high dam on the Abang Xi River <br />(Petts. 1984). Twelve centuries later. this dam is still used for irrigation diversion although <br />the reservoir has filled with sediment. <br />