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Tributary streamflow and suspended-sediment discharge measured at the USGS streamflow-gaging stations Paria River at Lees Ferry (09382000) and the Lillie Colorado River near Cameron (09402000; tig. I) are shown in figure 3. and annual statistical summaries are shown in tables 2 and 3. Because sediment input from the Paria and Little Colorado Rivers is only significant during stonns, suspended-sediment samples were collected only during storms; consequently. annual daily maximums and minimums for suspended- sediment discharge are not available. These samples were collected using automated pump samples or single-stage samplers, or were collected as equal- width-increment or dip samples. On the basis of sediment-discharge rating curves. discharges greater than 30 m3/s in the Paria River and 60 m3/s in the Little Colorado River generally can contribute large quantities of sediment to the mainstem of the Colorado River, (G.G. Fisk. U.S. Geological Survey. oral commun.. 2000). During the study period, the largest annual suspended-sediment contribution from the Pari a River as the result of storms. 4.4 million metric tons, occurred in water year 1997 (table 2). The largest annual suspended-sediment contribution from the Little Colorado River as the result of stonns, 18.8 million metric tons. occurred in water year 1993 (table 3). Most of this sediment was input from a storm in January 1993 that has a IS-year recurrence interval "''''.LlImith and others. 1998). ..r Sediment-Monitoring Network A total of 131 monumented cross sections were established to measure bed elevation on the Colorado River between Glen Canyon Dam and Lava Falls Rapid (fig. I) between August 1992 and April 1998. Cross sections were established at locations judged to be favorable for sand storage on the basis of channel geometry. character of bank and visible bed materials. and presence of sand waves on depth-sounder charts (Graf and others. 1995). An emphasis was placed on locating sections immediately downstream from major tributaries, the Pari a and Little Colorado Rivers, because of the large sediment inputs from these tribu- taries and for tlow- and sediment-model verification purpuses. The fuur cross sections between Glen Canyon Dam and Lees Ferry (fig. 4) were established and first measured in April 1998. Munitoring down- stream from the Paria River began in August 1992. Thirty-four cross sections were established between the confluence of the Paria and Colorado Rivers and Badger Creek Rapid at river mile 8 (fig. 5). Twenty-six cross sections were established between Badger Creek Rapid and the mouth of the Little Colorado River at river mile 61.3 (fig. 6). Most of these cross sections were established and first measured in January 1994. Monitoring downstream from the Little Colorado River (fig. 7) began in June and July 1992 when the 15 cross sections at the upstream end were established and first measured. The remaining 17 cross sections at the downstream end were established and first measured in January and February 1993. Thirty-five cross sections were established between river mile 87 (just above Bright Angel Creek) and Lava Falls Rapid at river mile 179.5 (fig. 8). A list of all measurement dates by cross-section name in downstream order and the subsets of the data, primary and matching-date data sets, used in this report is given in table 4. Location information for the monumented cross sections is available from the Arizona District Office of the USGS. Beginning with the fall 1997 measurement trip, bed-material samples were collected at the cross sections being measured at the time (table 4). Because of the small number of samples collected, these data are not considered in this report. METHODS A sonic-depth sounder was used to measure the depth of the river bed below the water surface. and the graphical record produced by the sounder was digitized to create the digital record used to calculate changes in cross-sectional area. The data-collection and processing steps. and the steps used to calculate the change in area between two measurements for a cross section. are shown in figure 9 and described below. Data Collection For each measurement. a kevlar line with flags at 6.08-m (20-ft.) intervals was strung across the river between the cross-section end points. Where feasible, the zero point on the line was positioned on the left- bank side. The positions of each monument and the edge of water on each bank were noted as distances along the line from the zero point. A boat equipped with a sonic-depth sounder was driven back and forth under the line. 8 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and lava Falls Rapid. Northern Arizona. 1992-99