Analyses of gravel samples from the Sixes River in southwestern Oregon show that the relative abundance of individual rock types in these samples varies with the mean size of the particles. At localities where fluvial processes have sorted gravels into groups of different mean grain size, pebble counts reveal that sandstone and argillite are consistently more abundant in the finer gravel population than in the coarser, whereas conglomerate, igneous, and metamorphic rock types are generally more abundant in the coarser population. These variations result largely from the relative availability of specific sizes of each rock type at a given locality. Relative availability is mainly a function of the size of the material weathered from bedrock, and of the susceptibility of this material to further breakdown during transport.

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