Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 110. The natural flow of almost every river in the United States has been modified to meet various socioeconomic goals—navigation, irrigation, power generation and flood control. The success of the dams and reservoirs built to achieve these goals has been accompanied by changes in the status of riverine resources downstream, a cause of growing environmental and ecological concern. For example, before Glen Canyon Dam was completed, the Colorado River transported large quantities of sediment in floods as large as 8500 m3/s. After the dam was closed in 1963, dam releases typically were less than the powerplant capacity of 890 m3/s and exhibited large daily flow fluctuations. The river carried little sediment. The daily fluctuations in flow eroded sand bars, and the smaller, controlled flow did not redeposit them. The clear, cold water resulted in increased aquatic productivity such that rainbow trout and other nonnative fishes thrived while most native species were lost or endangered.