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<br />Observations of Environmental Change in <br />Grand Canyon, Arizona <br /> <br />, <br /> <br />. <br /> <br />By Robert H. Webb, Theodore S. Melis, and Richard A. Valdez <br /> <br />u.s. GEOLOGICAL SURVEY <br /> <br />Water-Resources Investigations Report 02-4080 <br /> <br />Prepared in cooperation with <br /> <br />GRAND CANYON MONITORING AND RESEARCH CENTER <br /> <br />. <br /> <br />Tucson, Arizona <br />2002 <br />