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42 The wide range of grain sizes leads to two important features of bed heterogeneity, surface armoring and bimodal grain size distribu- tions. Figure 21 reveals the armor nature of the surface bed material. Figure 20 shows a bimodal distribution in the subsurface substrate sizes. Transport modes depend on surface armoring and the availability of the finer sizes. The Helley-Smith load is significantly coarser than the suspended sand load, but constitutes only 0.4% of the total load (Figure 22). Only the size fractions smaller than 1.0 mm were suspended high enough above the bed to be captured by the suspended sampler. The sediment particles greater than 1.0 mm (coarse sand) could be captured only by the Helley-Smith sampler and were not suspended higher than 3 inches above the bed. The median particle size captured in the sampler was 0.42. This is approximately one hundred-seventy times smaller the median bed material size indicating that all the transported sand is wash load. The Helley-Smith load median diameter is also seventy times smaller than the subsurface substrate. The Helley-Smith sampler, from the peak flow (May 31) through June 24, collected gravels and cobbles moving on the bed. A total of 42 coarse gravels greater than 19 mm and five small cobbles were captured by the sampler. Most of the cobbles were disc-shaped. Recent work by Andrews (1983) and Parker (1982) indicate that particles as large as the median diameter of the river bed substrate are entrained by discharges which equal or exceed the bankfull discharge. Entrainment of a given size particle has been shown to be a function of the bed material size distributions. Differential entrainment of the coarse bed fraction occurs in a relatively narrow range of shear stress (Andrews, 1983). From observations at Mathers Hole, the bedload consisted of relatively few large gravels and cobbles at the peak discharge, but included some of all particle sizes smaller than the median substrate size. General motion of the entire bed was not observed at Mathers Hole and changes in the cross section configuration could not be detected. Using the entrainment criteria developed by Andrews (1983) for naturally sorted riverbed material, it has been determined that the critical dimensionless shear stress is approximately equal to 0.03 for the median substrate material (D5 = 75 mm). This is the same criteria used in last year's analysis regerencing Parker's work (1982). The ratio of the threshold particle diameter to the median particle diameter of the subsurface bed material is the basis for determining the incipient motion criteria in Andrews' analysis. Mobilization of the median bed material particle will ensue at approximately 21,500 cfs. All bed particles in the cobble reach, except the very largest, will be entrained at this discharge which corresponds to the bankfull discharge. Entrainment of the imbricated particles may require a larger critical shear stress. There is no evidence to support any prediction of large scale channel configuration change or erosion below bankfull discharge which is required to create a mobile-bed phenomenon. Evidence shows that even in a 'mobile bed' of coarse particles, movement is limited to short leaps and hops, the bed load at any instant still be relatively small (Butler, 1977).