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Table 4.3 Existing to Project Cross-Section Correspondence Existing section (River Mile) Project Section (arbitrary) Remarks 1196 . 4 1197.9 1198.1 1199.1 1200.5 175.1 175.2 175.3 175.4 175.5 Gage & highway bridge @ La Junta 4.2 Development of Sediment Data The average measured suspended sediment load of the Arkansas River at the Las Animas gaging station for the 22-year period 1940 through 1962 is 1,646 acre-feet per year. The particle size analysis of the suspended sediment samples gathered at this station indicates an average of 15 percent sand, 66 percent silt, and 19 percent clay. Bed material samples from the Arkansas Ri ver near Las Animas are composed of an average of 12 percent gravel, 78 percent sand and 10 percent silt-sized materials. The average annual flow of the Arkansas River at Las Animas is about one-third of the average annual flow at Pueblo, while the suspended sediment load in the river at Las Animas is about four times that carried by the river at Pueblo. The decrease in the flow of the river between Pueblo and Las Animas is due mostly to irrigation withdrawals. The increase in sediment load is produced from tributaries draining areas with high sediment production rates. This has resulted in a slow aggradation of the Arkansas River in the vicinity of Las Animas and La Junta. As shown on Figures 4.2 and 4.3, the rating curves at Las Animas and La Junta shifted upwards by 4 ft. and 1 ft. respectively between about 1950 and 1970. 4.2.1 Bed Material Bed material gradations for the study reach for 1946 were obtained from the SWA report "Arkansas River and Tributaries above John Martin", Vol II. It was decided to use the single, average gradation presented on Plate 56 of that report (reproduced herein on Fig. 4.4) for the HEC-6 simulations. As individual bed material size distributions are available for various locations along the reach, incorporation of this more detailed data could be accomplished at a later date if deemed appropriate. The average gradation curve was broken into 10 grain size classifications ranging from silt to coarse gravel. It was thought that sampling of the current bed material might be necessary for calibration and use in the predictive simulations. However, as the bed material observed during the 10-13 Jan 83 field inspection conformed qualitatively to that presented in the J. Martin report, and preliminary model results appeared reasonable, it was decided at the 23 Karch 1983 data review meeting not to obtain additional bed material samples for the purposes of this study. 15