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2009 stream from the Mud Creek confluence (5 miles downstream from John Martin Dam) the median grain size was about 1.5 mm. Four miles downstream from the Lamar bridge (26 miles below John Martin Dam) the median grain size was about 1.2 mm. At the Highway 385 bridge (44 miles downstream from John Martin Dam) the median grain size was about 0.75 mm, It can be concluded that the bed of the Arkansas River is coarser due to sediment storage in John Martin Dam. Hydraulic and Sedimentation Analysis 12. Regulation of Arkansas River flows may have significantly effected the channel- fonning discharge and thus the stable bankfull width of the channel. The channel-fonning discharge can be assumed to be the same as the effective discharge. The effective discharge can be detennined by integrating the average-annual flow-duration curve with a bed-material sediment discharge rating curve and then identifying the discharge increment which conveys the most bed-material sediment load. 13. A preliminary analysis was made for this study to assess the significance of changing the channel-fonning discharge. The USGS streamgage Arkansas River ar Lamar was chosen to make the comparison. 83 years of mean daily flow records are available at this gage. Average-annual flow-duration curves were developed from these data for the years 1913 -1943, representing pre-dlun conditions, and 1948-1996, representing post-dam flow characteristics. The two flow duration curves are displayed in figure 6. Range 8 which is located about 12 miles downstream from John Martin Dam was chosen as a representative river cross-section for the preliminary analysis. A more detailed analysis would include analyses of additional ranges. Hydraulic parameters were calculated for the surveyed geometries in 1945 and 1987. A roughness coefficient of 0.035 was assigned to the channel. In the 1987 cross-section a roughness coefficient of 0.050 was assigned to the channel benn or bar. The appropriateness of this assumption should be confinned in the field. Also to be confinned in the field is whether or not the 1987 benn is capable of transporting sediment or if it is just a sediment deposition feature. In this analysis it was assumed to not transport sediment. Bed-material sediment transport rating curves were developed for 1945 and 1987 conditions using the Yang sediment transport equation. An average bed-material gradation detennined from the two samples collected upstream from John Martin Reservoir was used for the 1945 condition. The bed-material gradation taken downstream from the Mud Creek confluence was used for the 1987 condition. The integrated results are shown in Figure 7 and show that significant channel-fonning flows have noticeably decreased in magnitude. The preliminary analysis indicates that the channe1-fonning discharge prior to construction of the Arkansas River, Colorado Geomorphological Assessment 11 August 8, 1997