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ted that cross sections could be used to provide an assessment of the sand storage in the channel. the storage conditions for possible bar-building high releases. and the effectiveness of those releases in scouring stored sand and rebuilding sandbars. Purpose and Scope The purpose of this report is to document locations of the 131 monumented cross sections, the measnre- ment dates during the study period June 1992 to September 1999, and the methods of data collection and processing, and to present a general assessment of the variability in cross-sectional area. A subset of the data, referrt:d to as the primary data set, was lIsed to illustrate the temporal and spatial variability in cross-sectional area. In addition, a subset of the primary data set consisting of data frolll measurements made on or nearly on the same day at sections downstream from the Paria and Little Colorado Rivers, referred to as the matching-date data set. was used to further explore the effects of hydro-logic conditions on the changes in cross-sectional area. All cross-section measurement data are availahle electronically as ASCII tiles from the Arizona District Oftice of the USGS (address on back of title page). Acknowledgments The authors thank the many people who provided assistance throughout the life of this project. In panic- ular. we acknowledge Julia B. Graf. USGS (retired), for her guidance and leadership in establishing the foundations upon which this project was developed. We also thank Dave Anning, Greg Fisk, and Gail Cordy of the USGS for their help in the preparation of this report. Description of the Study Area The study area includes the Colorado River between Glen Canyon Dam and Lava Falls Rapid, which is about 195 miles downstream from Glen Canyon Dam in nmthern Arizona. The study area encompasses parts of Glen Canyon National Recreational Area and Grand Canyon National Park (fig. I). In this report. distances along the Colorado River are expressed in river miles. and all other measurements are expressed in metric units. Physical and Hydrological Conditions For much of its length through the study area, the Colorado River is confined by bedrock, by large blocks of talus, or by alluvial debris fans that are composed of sediment too coarse to be transported except during high flows. Flow depths in the thalweg of the river range from about 1.5 to 32 m at a discharge of 708 m3!s. Depths can change abruptly, and there are near-venical drops of as much as 15 m in gorges formed in the metamorphic and igneous rock (Wilson, 1986). Between Lees FetTY (river mile 0) and Diamond Creek (river mile 226), the elevation of the river decreases about 542 m. Much of the decrease in elevation occurs in short, steep reaches, many of which are the well-known rapids of the Colorado River. Although the average gradient between Lees Ferry and Diamond Creek is 0.0015, the gradient of many of the short reaches exceeds 0.01 (Schmidt and Graf, 1990). Many of the reaches between rapids have gradients less than 0.0005 (Birdseye, 1923). In the study area, tributaries of the Colorado River can be categorized as those having headwaters above the rim and those having headwaters below the rim. The Pari a and Colorado Rivers are tributaries having headwaters above the rim: their drainage basins include large plateau areas, and they supply most of the suspended sediment to the Colorado River within the study area. Tributaries having headwaters below the rim have drainage basins largely within the inner canyon and can transport fine sediment as well as large boulders in debris flows (Cooley and others, 1977: Webb and others, I 989l. The debris flows from these smaller tributaries detine the longitudinal protile and control the geomorphic framework of the Colorado River by forming rapids (Webb, 1996). In addition, the debris fans directly control the fonnation and stability of most sandbars (Schmidt, 1990: Schmidt and Graf. 1990: Schmidt and Rubin, 1995). In general. the channel pattern of the Colorado River through the study area is dominated by an alternating rapid-pool se4uence (Howard and Dolan, 1981 ). Flow through the constrictions created by the debris-flow deposits at the mouths of tributaries forms the rapids. Flow through rapids ends in pools (Dolan 4 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid. Northern Arizona, 1992-99