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e121t Observations of Environmental Change in Grand Canyon, Arizona By Robert H. Webb, Theodore S. Melis, and Richard A. Valdez ABSTRACT Few scientific data have been collected on pre-dam conditions of the Colorado River corridor through Grand Canyon National Park. Using historical diaries, interviews with pre-dam river runners (referred to as the "Old Timers"), and historical scientific data and observations, we compiled anecdotal information on environmental change in Grand Canyon. The most significant changes are the: lowering of water temperature in the river, near-elimination of hcavily sediment-laden flows. erosion of sand bars. invasion of non-native tamarisk trees, reduction in driftwood. development of marshes, increase in non-native fish at the expense of native fishes, and increase in water bird populations. In addition, few debris flows were observed before closure of Glen Canyon Dam, which might suggests that the frequency of debris flows in Grand Canyon has increased. Other possible changes include decreases in bat populations and increases in swallow and bighorn sheep populations, although the evidence is anecdotal and inconclusive. These results provide a perspective on managing the Colorado River that may allow differentiation of the effects of Glen Canyon Dam from olher processes of change. INTRODUCTION The Glen Canyon Environmental Studies (GCES) program. which existed between 1982 and 1996. amassed considerable data about the current ecology and geomorphology of Grand Canyon (U.S. Depanment of the Interior, 1988; 1995). Most of the research conducted during this program was related to the decade following the large flood of 1983, in Grand Canyon or the controlled flood of 1996 (Webb and others. I 999a). The work ofGCES has continued since the establishment of the Grand Canyon Monitoring and Research Center (GCMRC) in 1996. Until recently. research on the effects of Glen Canyon Dam (located 15.5 mile upstream of Lee's Ferry) has mostly focused on proces>es related to the post-dam Colorado River (Fig. I). This concentration of effort meets many of the management needs of regulatory agencies but ignores some queslions of scientific interest. On-going changes in the regulated river were inherited from a free- flowing river Ihat very few people, and fewer scientists. have experienced. Initially. our project focused on specific geomorphic questions about the pre-dam river, panicularly the frequency of debris flows and their effects on the Colorado River. Debris flows are slurries of water and sediment that transpon large boulders into the Colorado River, forming the rapids (Webb and others, 1989. I 999b, 2000) and constraining the sites of eddies (Melis. 1997: Melis and others. 1994). In attempting to understand debris-flow frequency in Grand Canyon, we relied on historic photographs taken between 1872 and the 1970s: in matching them (Melis. 1997; Melis and others. 1994: Webb. 1996), we learned where and when debris flows occurred. The old photographs opened a Pandora's box, yielding far more than just infornlation on debris flows. Insights into longevity and stability of desen plants, invasion of riparian vegetation. effects of burro grazing. changes in rapids. and stabi lity of sand bars were gained from replicated photographs (Webb and Bowers, 1992: Webb and Bowers. 1993: Bowers and others. 1995: Schmidt and others. 1995: Webb. 1996). Although photographs are an excellent source of unbiased information about the environment of Grand Canyon, they have limitations in terms of spatial coverage and content. For example. photographs are an excellent source for evaluating the frequency of debris flows. but are a poor source of information about wildlife populations or the hydraulics of a panicular rapid. To gain a broader perspective. we began to discuss the changes we thought had occurred with river Abstract 1