<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
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