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Rob'ett Dolan Alan Howard Arthur Gallenson Man-made landforms are found throughout the United States; how- ever, some of the most extensive and persistent scars of large-scale environmental modification are found in the American Southwest. The sparse vegetation and generally slow pace of geomorphic processes leave the landscape much as man . leaves it-here he is a significant agent of geologic change. Agriculture, mining, highway con- struction, and earthmoving associ- ated with rapid urban growth con- tribute measurably to erosion and siltation. The large reservoirs on the major rivers of the Southwest serve as sediment sinks for much of the eroded material, disrupting the sediment budgets of the drainage systems below the dams. In this Robert Dolan u;ill be remembered as the se- nior author of "Man's Impact on the Barrier Islands of North Carolina," which appeared in American Scientist in March 1973. Alan Howard, who is also in the Department of Environmental Sciences at the University of Virginia, received his doctorate in 1970 from The Johns Hopkins University. His research interests are in fluvial geomorphology and .' arid-zone landforms. Arthur Gallenson has been with Grand Canyon Expeditions, Kanab, Utah, for seven years. Before be- coming a Colorado River Guide, he com- pleted graduate degree work in geology. This research was supported by the Office of Natural Sciences, National Park Service, Washington, D.C. The authors wish to ac- knowledge the valuable discussions of their colleagues during the course of a float trip in 1973: Yates Borden (project leader), Fred Borden, Jack Rogers, Charles Strauss, Brian Turner, Harmer Weeden, and Roy Johnson, to whom they are especially indebted for providing information on common riverfront species and for discussions about vegeta- . tional responses to the regulation of the river. Address for Drs. Dolan and Howard: Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903. OJ7 0.3 :J..:,ci;J~ -=-----,~,~ Man's Impact on the Colorado River in the Grand Canyon The Grand Canyon is being affected both by the vastly changed Colorado River and by the increased presence of man paper we will describe the environ- mental impact that a recently com- pleted dam-reservoir system is hav- ing on one of the largest rivers in the Southwest, the Colorado. The reach of the Colorado that we in- vestigated extends 280 miles from the Glen Canyon Dam to Lake Mead-an area known as the Grand Canyon (Fig. 2). The problem When Major J. W. Powell made his historic trip down the Grand Can- yon in 1869, the Colorado River system was one of the last unex- plored regions in the United States. In fact, the area was considered so remote and primitive that J. C. Ives, another early explorer of the Southwest, described it in these terms: Ours has been the first, and will doubtless be the last, party of whites to visit this profitless locality. It seems intended by nature that the Colorado River, along the greater portion of its lonely and majestic way, shall be for- ever unvisited and undisturbed [IJ. Ives's prediction was off considera- bly: the National Park Service re- ports that the forty-millionth visi- tor will enter Grand Canyon Na- tional Park sometime during the present decade. Even disregarding the environmen- tal changes caused by forty million visitors, the Colorado River today is vastly different from the river Pow- ell explored in the mid-180Gs. Pow- ell's Colorado remained fundamen- tally unchanged until Hoover Dam was completed in 1935; and, al- though the Hoover Dam reservoir (Lake Mead) extended into the lower reaches of the Grand Canyon, the upper reaches of the canyon re- mained in an essentially natural state until the Glen Canyon Dam reservoir (Lake Powell) was com- pleted in 1963. Since then, the flow of the Colorado in Marble Canyon and Grand Canyon has been almost completely dependent upon the re- lease of water from Lake Powell (Fig. 2). Virtually all the sediment that formerly passed through these canyons is now trapped in the res- ervoir, and the frequent high water (flash flood) that is associated with the rivers of the Southwest is now to- tally controlled. The environmental responses to these changes have been both rapid and significant. Before Glen Canyon Dam existed, the river gained volume from the spring snowmelt in the headwaters, reached maxiIl'um flow in Mayor June, and then receded during the remainder of the year. Flash flood- ing in the late summer often result- ed in a second peak. During periods of high water, when the river had the greatest transport capability, large quantities of sand. and silt were carried through the canyon, scouring the channel. As the water receded in the summer, the river lost both competence and capacity and deposited much of its silt and sand load along the channel. The river bars and terraces (colloquially Figure 1. This steep beach face near Nanko- weap Rapids in Marble Canyon was produced by rapid undercutting of fine-grained terrace deposits. Such examples of very rapid ero- sion are uncommon; elsewhere the wide- spread removal of pre-dam fluvial deposits by backcutting becomes apparent only through photographic comparison of the present riverfront with pre-dam conditions. (Photo by Alan Howard.) Repinted from AME_RICAN SCIE!'>iTIS'I"Vol. 62, No.4, Ju!y-August, 1974, pp. 392-401