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<br />.. <br /> <br />stream from the Mud Creek confluence (5 miles downstream from John Martin Dam) the <br />median grain size was about 1.5 mm, Four miles downstream from the Lamar bridge (26 <br />miles below John Martin Dam) the median grain size was about 1.2 mm. At the Highway 385 <br />bridge (44 miles downstream from John Martin Dam) the median grain size was about 0.75 <br />mm. It can be concluded that the bed of the Arkansas River is coarser due to sediment storage <br />in John Martin Dam, <br /> <br />Hydraulic and Sedimentation Analysis <br /> <br />12. Regulation of Arkansas River flows may have significantly effected the channel- <br />forming discharge and thus the stable bankfull width of the channel. The channel-forming <br />discharge can be assumed to be the same as the effective discharge. The effective discharge <br />can be determined by integrating the average-annual flow-duration curve with a bed-material <br />sediment discharge rating curve and then identifying the discharge increment which conveys <br />the most bed-material sediment load. <br /> <br />13. A preliminary analysis was made for this study to assess the significance of <br />changing the channel-fonning discharge. The USGS streamgage Arkansas River at Lamar was <br />chosen to make the comparison. 83 years of mean daily flow records are available at this <br />gage. Average-annual flow-duration curves were developed from these data for the years 1913 <br />-1943, representing pre-diun conditions, and 1948-1996, representing post-dam flow <br />characteristics. The two flow duration curves are displayed in figure 6. Range 8 which is <br />located about 12 miles downstream from John Martin Dam was chosen as a representative <br />river cross-section for the preliminary analysis, A more detailed analysis would include <br />analyses of additional ranges. Hydraulic parameters were calculated for the surveyed <br />geometries in 1945 and 1987. A roughness coefficient of 0.035 was assigned to the channel. <br />In the 1987 cross-section a roughness coefficient of 0.050 was assigned to the channel benn or <br />bar. The appropriateness of this assumption should be continned in the field. Also to be <br />continned in the field is whether or not the 1987 benn is capable of transporting sediment or if <br />it is just a sediment deposition feature. In this analysis it was assumed to not transport <br />sediment. Bed-material sediment transport rating curves were developed for 1945 and 1987 <br />conditions using the Yang sediment transport equation. An average bed-material gradation <br />determined from the two samples collected upstream from John Martin Reservoir was used for <br />the 1945 condition. The bed-material gradation taken downstream from the Mud Creek <br />confluence was used for the 1987 condition. The integrated results are shown in Figure 7 and <br />show that significant channel-fonning flows have noticeably decreased in magnitude. The <br />preliminary analysis indicates that the channel-forming discharge prior to construction of the <br /> <br />Arkansas River, Colorado <br />Geomorphological Assessment <br /> <br />11 <br /> <br />August 8, 1997 <br /> <br />- <br />