The surficial sediment along a 70-mile reach in Lake Mead was examined as to texture and mineralogical composition in order to determine possible relations to diagenesis and deposition. Samples were taken at six locations from Grand Wash at the upstream end of the lake to the Intake Towers at Hoover Dam. Differences in texture and clay composition were noted between samples upstream and downstream from Virgin Basin. The median diameter of laboratory-deflocculated sediment samples are between 4.2 and 4.6 microns on the downstream side of Virgin Basin and less than 2 microns upstream. Mineralogically, montmorin-type clays dominate the dispersed clay fraction upstream whereas illite is predominant downstream. The quantity of water in Lake Mead represents a calcareous, flocculating environment for responsive clays and it is proposed that particle-size distribution in the native environment is different from the dispersed state, probably due to montmorin-bound flocs existing in the silt-size range in the lake water. An alternative hypothesis for the differential distribution of texture and composition is the possible "drag" exerted on the surface of sediment near the dam by the outflow of water through the gates, resulting in winnowing of the fines from the surficial material.

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