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<br />The correlations and graphical representations indicate an apparent change in the <br />suspended sediment transport characteristics of the Arkansas River as a result of John Martin <br />Dam. This is expected, since one of the benefits of a reservoir often is retention of sediment. <br />The Sedimentation in John Martin Reservoir, Arkansas River Basin, Colorado, Resurvey of <br />March 1972 report (USACE 1973) states that the reservoir has a calculated trap efficiency of <br />87.5%. The change, however, is more complex than a simple reduction of suspended sediment. <br />While the aggregate data and linear representations are relatively well behaved and consistent <br />for the Arkansas River at Las Animas (and the Purgatoire River, as well), there is considerably <br />more scatter for the Arkansas River below the dam. The correlation coefficient for the <br />Arkansas River below the dam is considerably less, 0.69, compared to the Arkansas River at <br />Las Animas, 0.85, and the Purgatoire River at Las Animas, 0.86 for the total data sets. <br /> <br />For the monthly analyses, the Arkansas and Purgatoire Rivers at Las Animas show <br />reasonable curve fits, with correlation coefficients of 0.65 to 0.95 and 0.77 to 0.90, <br />respectively, while the Arkansas River below John Martin Dam values range from 0.00 to 0.78. <br />The graphs further illustrate this latter point - for the months of May through September the <br />fitted lines show a marked difference in slope, with May and July sloping in the opposite <br />direction from the others (Q" decreasing for increasing Q) and September showing a near- <br />horizontal line. Part of this is likely due to the limited data sets, but it would also seem to <br />indicate that the suspended load is being dispersed in a different manner during the summer <br />months. <br /> <br />It is worth noting that one method of calculating sediment discharge for sands, Colby's <br />relations, adjusts the base calculated value upward for increased suspended sediment <br />concentration (Vanoni 1975). This implies a correlation between suspended sediment <br />concentration and bed material transport capacity. A change in the wash load component of the <br />Arkansas River downstream of the dam could be having an impact on the sands carried by the <br />river. This, coupled with a reduction in the peak discharges in the river as a result of operation <br />of the dam for flood control, would be expected to cause deposition without an equalizing <br />change in the river hydraulics (e.g., channel geometry) or bed material load. It is assumed that <br />the reservoir also traps a significant quantity of bed material, but it is not readily apparent <br />whether the reduced peak discharges or reduced sediment load is dominant. Furthermore, the <br />scour, which generally occurs immediately downstream of a dam, often causes deposition at <br />some point farther downstream. <br /> <br />These factors indicate that the river has been placed in a state of non-equilibrium as a <br />result of John Martin Dam, although there are many other factors that have also contributed to <br />this condition. <br /> <br />Degradation Range Surveys were analyzed from nine survey periods ranging from <br />December 1943-February 1944 to March 1987 (USACE 1965, 1973, 1981, 1987). The cross- <br />sectional areas were compared by choosing an arbitrary elevation above all of the section points <br />and calculating the areas between this line and the ground surface. These were then compared <br />to get changes in area. The resulting areas were also multiplied by the channel distances <br /> <br />15 <br />