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sucker (Xyrauchen texanus). Riparian vegeta- tion occurs in two distinct zones: one relict of the pre-dam hydrologic conditions and a denser assemblage at lower elevation that is an artifact of the post-dam flow regime. Both vegetation zones constitute habitat for birds and other fauna. The stream flow that maintains this riverine ecosystem and is the focus of so many people's recreational experience is determined by water releases from Glen Canyon Dam. Today's floods are much smaller and today's base flows are much larger than those which occurred prior to the dam. The existence of the dam also blocks all sediment delivered from the Colorado River upstream from Lake Powell reservoir. When the reservoir is full, stream flow temperature averages 80C. These changes to water flow, sediment transport, and water temperature have trans- formed the geomorphology and ecology of the Colorado River and its alluvial valley (Carothers and Brown, 1991; National Re- search Council, 1991; U.S. Department of the Interior, 1995; Webb, 1996). Among the geomorphic features affected by the dam's existence and operations are fine-sediment deposits, the size and abundance of which are indicators of the degree to which the post-dam ecosystem has been altered from the pre-dam condition (National Research Council, 1996). The purpose of this paper is to describe the physical transformation of the channel and alluvial deposits of the Colorado River during the twentieth century in Glen, Marble, and upper Grand Canyons, with emphasis on fine sediment (Fig. 1). The upstream end of the study area is Glen Canyon Dam, located at River Mile -151. The downstream end of the study area is River Mile 87, which is the location ofD. S. Geological Survey (USGS) gaging station 09402500 (Colorado River near 1 Locations in the study area are described in terms of the distance, in river miles, from Lees Ferry. Distance downstream is positive, and distance upstream is negative. Locations are cited to the nearest 0.1 mile, based on the location system of the Grand Canyon Monitoring and Research Center (GCMRC). '"~ ~'.: ~ f, .... j 11 f . 81 . '.' 1.iWe Colorado River ,.' ,> ; -. ~':. . J':", . .-.., { " , . I": ' . .... .. - . '.,0 .' . i I I . !'112~ 10. .20 ., , km . '"~,,,' . . ~ .i . _ _ . . . ..N. . Figure 1. Map showing the 4 river segments (in capital letters) that comprise the study area. Five study reaches where historical aerial photographs were analyzed are shown in boxes: (1) Lees Ferry Reach, (2) Redwall Gorge Reach, (3) Point Hansborough Reach, (4) Tapeats Gorge Reach, and (5) Big Bend Reach. The sixth study reach was all of Glen Canyon. NAU study sites are indicated by *,with location in River Mile. . 2 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor...