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Similarities and differences in gross shapes of fluvial quartz sand grains contain information useful for interpretation of sediment transport history. The shapes of sand grains in a given river depend on the source, or sources, of sand within the drainage basin and on the abrasion and shape sorting that has occurred during transport. It is highly unlikely that 2 major streams will carry precisely similar-shaped grain suites. Therefore, when 2 streams join, the resulting sand can be recognized, on the basis of shape, as being mixtures of the 2 input streams.
The multiple rotations method of quantitative shape analysis characterizes sets of grain shapes with 5 or 6 numerical factor loadings, and individual grains are described by 5 or 6 factor scores. Trends of shape changes, such as those that occur along the length of a river, show up well on bivariate-factor score plots. These trends are interrupted and offset by mixing of sands contributed by tributary streams. Shapes of sands obtained from the Missouri River above the junction with the Platte River in Nebraska are different from those from below the Platte; but when Platte River sand shapes are subtracted, the remaining differences are insignificant. Farther downstream, sands from the Kansas River show the same relationships. The relative contributions of sand from a river and its tri utaries, and the rates of mixing of the sands, can be estimated from quantitative shape analysis of several samples.
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