Differential transport of sand grains by unidirectional water flow over a cohesionless bed material having superimposed ripples or dunes results in hydraulic sorting. Hydraulic sorting occurs on many scales in natural fluvial systems, e.g., flood plain vs. channel deposits, bed vs. load in transit, various parts of cross-bed sets or individual laminae, etc. The mechanisms of hydraulic sorting include differential entrainment at the source, differential deposition and differential transport rates which are sensitive to prevailing bed configurations. By comparing transport rates of individual grain sizes calculated from size analysis of bed material and load under ripple and dune conditions, it is shown that the sampled load may be differentiated into two subpopulations of rains moved by different transport mechanisms, namely, those particles confined to transport within bed forms, and intermittent suspension load. Due to practical limitations in the flume, this study is limited to the two coarsest subpopulations observed in typical samples of river sediment. Theory and results of flume experiments suggest the following approximate hydraulic criterion for separating the two mechanisms of sediment transport: the point where the relative difference in transport rates between size fractions increases abruptly. This criterion is confirmed by flume experiments for flow conditions spanning the stability fields for ripples and dunes.

End_of_Article - Last_Page 1682------------