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466 Topographic Response of a Oar 'J .. 24 26 150 300 METERS I I Contour Int_. 1 Met. Fig. 2. Channel topography or the Ouray reach. 160% of the bankfull value, unvegetated, sandy channel bars beoomeemergent and \give the river a braided pattern within the bankfull channel. ~. In order to investigate the topographic response of channel bars to variations in discharge, we selected a study reach about 10 channel widths _in lengths, located within the Ouray National Wildlife Refuge 6.5 km upstream from the mouth of the Duchesne River. Ten cross sections were established within the study reach and resurveyed 5 times between July 1986 and Sept. 25, 1987. In addition, the water discharge, suspended sediment concentration, and size and bed-material size were determined each time the channel cross sections were surveyed. A contour map of the study reach is shown in Figure 2. This topography was generated from the initial survey on July 15-17, 1986, at a discharge of ",280 m3/s, on the descending limb of a hydrograph which peaked at 620 m3/s 38 days previously. A larger, primary channel lies on the left side of the bar and a smaller, secondary channel lies to the right of the bar. The secondary channel is shallower than the primary channel. Flow through the secondary channel ceases at a discharge of 200 m3/s, when the water suiface is ",1.5 m below the bankfull stage. The top of the bar lies about 1.2 m below the bankfull elevation and is free of any vegetation. Furthermore, the bar surface is generally clear of large debris, such as logs or tree stumps, as well as coarse bed-material. Thus, large obstructions in the flow do not appear to influence the stability of the bar. A comparison of available aerial photographs shows that this bar has existed since 1962 with roughly the same configuration and location. The channel of the Green River in the vicinity of the study reach appears to be typical of alluvial reaches between the Jensen gage and the mouth of the Duchesne River. Although the study reach is located 14 km upstream from the Ouray gage, daily water discharge and suspended sediment concentration determined at the Jensen gage, 90 km upstream, provide a much more accurate description of flow and sediment transport in the study reach than does the record of the Ouray gage. Tributary inflow to the reach between the Jensen gage and the study reach is very small. The Duchesne River, drainage area ",11,000 km2, and White River, drainage area ",13,300 km2, represent 93 percent of the difference in drainage area between the Jensen and Ouray gage, and join the Green River downstream from the study reach. Therefore, the record of the Jensen gage will be used to describe the variations of flow and sediment transport which have occurred in the study reach over the past 40 years. Andrews [19861 provided a detailed description of the effects of reservoir regulations upon ffow and sediment transport in the Green River basin, including