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<br />Observations of Environmental Change in Grand <br />Canyon, Arizona <br /> <br />By Robert H. Webb, Theodore S. Melis, and Richard A. Valdez <br /> <br />ABSTRACT <br /> <br />Few scientific data have been collected on pre-dam conditions of the Colorado River corridor through <br />Grand Canyon National Park. Using historical diaries, interviews with pre-dam river runners (referred to <br />as the "Old Timers"), and historical scientific data and observations, we compiled anecdotal information <br />on environmental change in Grand Canyon. The most significant changes are the: lowering of water <br />temperature in the river, near-elimination of heavily sediment-laden flows, erosion of sand bars, invasion <br />of non-native tamarisk trees, reduction in driftwood, development of marshes, increase in non-native fish <br />at the expense of native fishes, and increase in water bird populations. In addition, few debris flows were <br />observed before closure of Glen Canyon Dam, which might suggests that the frequency of debris flows in <br />Grand Canyon has increased. Other possible changes include decreases in bat populations and increases <br />in swallow and bighorn sheep populations, although the evidence is anecdotal and inconclusive. These <br />results provide a perspective on managing the Colorado River that may allow differentiation of the effects <br />of Glen Canyon Dam from other processes of change. <br /> <br />INTRODUCTION <br /> <br />The Glen Canyon Environmental Studies <br />(GCES) program, which existed between 1982 and <br />1996, amassed considerable data about the current <br />ecology and geomorphology of Grand Canyon (U.S. <br />Department of the Interior, 1988; 1995). Most of the <br />research conducted during this program was related to <br />the decade following the large flood of 1983, in Grand <br />Canyon or the controlled flood of 1996 (Webb and <br />others, 1999a). The work of GCES has continued since <br />the establishment of the Grand Canyon Monitoring and <br />Research Center (GCMRC) in 1996. Until recently, <br />research on the effects of Glen Canyon Dam (located <br />15.5 mile upstream of Lee's Ferry) has mostly focused <br />on processes related to the post-dam Colorado River <br />(Fig. 1). This concentration of effort meets many of the <br />management needs of regulatory agencies but ignores <br />some questions of scientific interest. On-going changes <br />in the regulated river were inherited from a free- <br />flowing river that very few people, and fewer <br />scientists, have experienced. <br />Initially, our project focused on specific <br />geomorphic questions about the pre-dam river, <br />particularly the frequency of debris flows and their <br />effects on the Colorado River. Debris flows are slurries <br /> <br />of water and sediment that transport large boulders into <br />the Colorado River, forming the rapids (Webb and <br />others, 1989, 1999b, 2000) and constraining the sites of <br />eddies (Melis, 1997; Melis and others, 1994). In <br />attempting to understand debris-flow frequency in <br />Grand Canyon, we relied on historic photographs taken <br />between 1872 and the 1970s; in matching them (Melis, <br />1997; Melis and others, 1994; Webb, 1996), we learned <br />where and when debris flows occurred. The old <br />photographs opened a Pandora's box, yielding far more <br />than just information on debris flows. Insights into <br />longevity and stability of desert plants, invasion of <br />riparian vegetation, effects of burro grazing, changes in <br />rapids, and stability of sand bars were gained from <br />replicated photographs (Webb and Bowers, 1992; <br />Webb and Bowers, 1993; Bowers and others, 1995; <br />Schmidt and others, 1995; Webb, 1996). <br />Although photographs are an excellent source of <br />unbiased information about the environment of Grand <br />Canyon, they have limitations in terms of spatial <br />coverage and content. For example, photographs are an <br />excellent source for evaluating the frequency of debris <br />flows, but are a poor source of information about <br />wildlife populations or the hydraulics of a particular <br />rapid. To gain a broader perspective, we began to <br />discuss the changes we thought had occurred with river <br /> <br />Abstract 1 <br />