Mobility of bed material in Harris Creek

The mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable. The bed surface remained intact throughout the freshet, and scour was limited to small areas of the bed. Observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction. Fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature. At low flows, no size fraction was fully mobile. At intermediate flows (shear stress >27 Pa) sand was fully mobile while larger material remained partially mobile. At high flows (>43 Pa) the division between the full and partial mobility regimes occurred around 16 mm, which remains finer than the median size of the subsurface material. The largest material scarcely moves at mean annual flood. Magnetic tracers confirmed results obtained from the pit traps. Three distinct zones of mobility can be defined in fractional transport plots: partial mobility, full mobility, and overpassing/suspended. Critical shear stress for incipient motion varied over an order of magnitude; a qualitatively similar result for bimodal sediment was reported by Wilcock and McArdell [1993]. Incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface, and a lower envelope that corresponds with our (nonsimilar) entrainment criterion.