Measurements in a Braided River chute and lobe: 2. Sorting of bed load during entrainment, transport, and deposition


  • P. J. Ashworth,

  • R. I. Ferguson,

  • P. E. Ashmore,

  • C. Paola,

  • D. M. Powell,

  • K. L. Prestegaards


Grain size distributions of bed material, bed load, and bar head deposition were sampled during the evolution of a chute and lobe in the braided gravel bed Sunwapta River, Alberta. Although bed shear stress and total bed load transport rate varied substantially within the reach and from day to day (Ferguson et al., this issue), the median diameter D50 of bed load was remarkably constant. The bed initially fined from chute to bar head but became more uniform. Sediment deposited on the bar head also had a near constant D50, similar to that of the bed load truncated at the same lower size limit. Both load and deposition were finer on average than the bed, suggesting overall selective transport despite little sign of local sorting. In contrast to median diameters, maximum sizes (Dmax) of sediment in motion and deposited on the bar head varied substantially and showed a weak but significant dependence on shear stress. Inferring selective entrainment from this evidence alone would be dubious because Dmax was also found to increase systematically with the mass of sediment sampled. However, two different methods of analysis of fractional transport rates throughout the size range also showed size-selective entrainment and transport. In the marginal transport conditions of this study, size sorting appears to occur but only weakly and mainly in the coarser fractions.