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The second thing to note in these figures is that there was very little sediment coarser than sand (D > 2 mm) caught in the traps in years when the peak discharge did not exceed -300 m3/s. This is approximately the flow level that was recommended for producing initial motion of the bed material [Pitlick and Cress, 2000]. The presence of pebbles and fine gravel in samples taken near the peak in 1998 indicates that portions of the bed surface were indeed mobilized during the period of high flow that year. These sizes are not as common in samples collected in subsequent years, suggesting that, at flows less than -300 mils, most of the bed surface remains immobile, as predicted. However, in addition, the data clearly show that even during periods of low flow, the Colorado River continues to transport fine-medium sand (0.1 < D < 0.5 mm). In a general long-term sense, this has probably always been the case; however, with streamflows now regulated, there are concerns that the build up of fine sediment on the bed of Colorado River will impair biological productivity [Osmundson'et al., 2002]. Thus, in addition to moving coarse sediment on the bed surface, another management goal might be to augment flows on the recessional limb of the hydrograph to keep fine-medium sand in suspension over the most productive and important habitats (riffles). The criterion for suspension is based on an empirical relation for estimating the settling velocity, ws , of natural particles in water as a function of grain size and grain shape [Dietrich, 1982]. When the local fluid shear velocity, u> _ (ghS,)"', exceeds the settling velocity of a given particle size, u< > ws, then those sizes should be transported in suspension; otherwise they should move as bed load. Using Dietrich's [1982] relations for quartz-density sediment with a shape factor or 0.7, the fall velocity for medium sand, D = 0.5 mm, is calculated to be ws = 7 cm/s. Based on results from flow modeling in the reach near RM 176 (discussed in the next section), a discharge of 125 m3/s should be sufficient to keep particles finer than 0.5 mm in suspension over riffles. 44