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REPORT NO.3 HISTORICAL PATTERNS OF VEGETATION CHANGE IN GRAND CANYON BASED ON A GIS ANALYSIS G.L. Waring School of Forestry, NAU Flagstaff, AZ Introduction Several studies have shown the dynamic history of riparian vegetation in Grand Canyon's river corridor (Turner and Karpiscak, 1980; Carothers and Aitchison, 1976; Martin, 1971). The current use of GIS analysis and photogrammetry is in establishing a permanent, quantitative, georeferenced record of vegetation change along the Colorado River. The current effort is unique in its ability to assess the impact of river regulation on riparian vegetation, including the initial damming of the river, post-dam flooding, and a more recent interim flow regime. Our approach considers community-level and species-specific vegetation responses to regulation, which will provide an understanding of large-scale processes and elements that comprise them. The permanent nature of this analysis will permit future riparian vegetation research within GIS reaches in Grand Canyon to consider an historical perspective while studying the present. Although some data are available on vegetation responses to impoundment (see Waring, 1993), GIS analysis of vegetation change following river regulation is unavailable for most of the world's dammed rivers because of a lack of historical photographic records. The extensive aerial photography record of the Colorado River in Grand Canyon provides the opportunity to identify detailed vegetation responses to initial impoundment and subsequent dam operations. Objectives Our objectives are to describe the response of riparian vegetation along the Colorado River in Grand Canyon to river regulation, from the initial damming phase through the present (1965 to 1992). Our efforts will correlate azimuth, different depositional environments (e.g., above and below major tributaries), narrow and wide reaches and distance from the dam with vegetation cover, and will estimate their influence on vegetation responses to river regulation. These data will be archived within the GCES/GIS data base. Discussion We primarily consider two broad categories of Colorado River riparian vegetation, OHWZ (old high water zone) vegetation, which includes mesquite and acacia that were established high above the river before the dam, and NHWZ (new high water zone) vegetation, including native coyote willow and exotic tamarisk, that has become established in close proximity to the river following damming. Individual species we are studying include mesquite, which are migrating down into the NHWZ, pre-dam tamarisk in the OHWZ, and Goodding willow, a tree, willow which appears to be negatively affected by river regulation. Minimal vegetation occurred along the river prior to impoundment, mainly Goodding willow and the nonnative tamarisk, which occurred almost exclusively on separation bars. Separation bars may have been relatively stable habitats in the pre-dam environment, and frequent flooding prevented long-term colonization of vegetation in more exposed habitats. 27