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fl <br />n <br />Pi <br />1 <br />rj <br />[+ <br />1 <br />1 <br />In plan view, macroforms are situated obliquely, at about 30' to the <br />direction of flow (fig. 5). Width of macroforms is about 0.6 times the <br />channel width, and length is about 1.9 times the channel width. The <br />height of macroforms generally does not exceed 2 meters. A steep slipface <br />forms the downstream end of a macroform. Small channels, oriented <br />about perpendicular to the slipface, occur near the downstream end of <br />macroforms... Macroforms move only during the highest flows when they <br />are submerged... Downstream migration rates of 1.0 to 1.5 m/hr (meters <br />per hour) were measured in the Platte River during 1980 high flows. <br />Long -term rates of migration, computed from measurements on aerial <br />photographs, varied between about 10 and 24 m/yr (meters per year). <br />The figure referenced in this citation is reproduced as Figure 2.19 since it provides a good <br />visualization of macroforms on the Platte River. <br />2.4.5 Sediment Trapping <br />Information was compiled by Simons & Associates (1990d) documenting the quantity of <br />sediment trapped in vari ous reservoirs in the basin. This information was based on a <br />number of reservoir sedimentation surveys that have been conducted over time. Table <br />2.3 (after Simons & Associates, 1990d) summarizes the results of these surveys. The <br />table presents the quantity of sediment trapped by each reservoir, for the time periods <br />indicated, both in terms of tons per square mile per year as well as the total tonnage per <br />year. These figures allow comparison of the quantity of sediment production from the <br />upstream or contributing watershed trapped in each of the reservoirs. <br />Table 2.3 Summary of Reservoir Sedimentation Survey Results <br />Reservoir <br />Survey Time Period <br />Total Sediment <br />Total Sediment <br />Trapped <br />Trapped <br />(tons /mil /yr) <br />(million tons /yr) <br />Pathfinder (North <br />1909 -1950 <br />138 <br />0.46 <br />Platte) <br />Seminoe (North <br />1939 -1950 <br />153 <br />1.12 <br />Platte) <br />Guernsey (North <br />1944 -1947 <br />243 <br />0.14 <br />Platt e) <br />1947 -1957 <br />107 <br />0.06 <br />Lake McConaughy <br />1941 -1986 <br />149 <br />1.28 <br />(North Platte) <br />Cheesman (South <br />1900 -1931 <br />37 <br />0.05 <br />Platte) <br />21 <br />