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<br /> <br />I ntroauction <br /> <br />rules for answering this question involve taking some long hard looks at costs and benefits. <br />What benefits do we value, what costs are we willing to bear? What new values -like <br />downstream ecological impact - have been brought to bear in this accounting since the <br />dam was designed and built? Arriving at a meaningful answer requires setting aside <br />political and personal biases long enough to honestly say what we want from a dam and its <br />river. and to accurately evaluate all the dimensions of impact that a dam can have. Society <br />must decide what it wants; scientists can help to show what we are likely to get under <br />different management strategies. <br />Viewed in one carefully chosen dimension. many dams have been worthwhile - this <br />dam prevented flooding, that dam generated a lot of electricity. But with time. we have <br />also come to realize that the adverse environmental effects of a dam may extend in circles <br />far wider than had been appreciated in the past. For decades, people have known and <br />argued about the more obvious effects of dams: flooded valleys and displaced farmers; <br />fish migrations blocked or disrupted; one state taking water needed by another state <br />downstream; water quality improved or impaired. We did not spend a lot of time thinking <br />about the issue of downstream effects when conceiving dams during the first half of the <br />twentieth century. But in the past twenty years, scientists and the public have begun to <br />appreciate an additional effect of dams: changes to the downstream river environment. <br />The river emerging from a dam is not the same river that entered its reservoir. That <br />new river may be hotter or colder. Its daily discharge may vary wildly. while its seasonal <br />pattern of high spring floods and low winter flow may be inhibited beyond recognition. <br />Suddenly starved of its sediment load, the clear waters of a river below a dam may scour its <br />bed and banks. An entirely new succession of riparian plants and animals may move into <br />the river and valley below a dam. Native fishes may die or be severely stressed. <br /> <br /> <br />Oxbow Dam, part of the Hells Canyon Complex, on the Snake River; Idaho <br /> <br />7 <br />