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after the peak in the hydrograph than it did before the peak, or (ii) the grain size of the sand supply is becoming coarser with time, thus not as much sand is being carried in suspension, nor is it being carried as high in the flow; some of the coarser sand could instead be moving as bed load. It is not possible to distinguish between these effects without specific data characterizing the evolution.of the grain size distribution of the suspended load over time. Whatever the case, is not uncommon for the size distribution of the suspended load to change over time as finer or coarser sediment becomes available. For example, suspended sediment measurements taken on the Colorado River in Grand Canyon, before the construction of Glen Canyon dam, show that the median grain size of the suspended load generally increased on the receding limb of the annual hydrograph [Topping et al., 2000]. Similarly, measurements of the bed material and suspended bed-material load taken in sand-bed reaches of the Rio Grande in the late 1950s likewise showed that the bed material became coarser over the period of the hydrograph [Nordin and Beverage, 1965]. These observations are relevant to the present study because, if there is a natural tendency for the grain size distribution of the suspended sediment load in the Colorado River to coarsen with time, then any further reduction in peak flows could lead to a significant reduction in the total mass of sand carried. It is important to maintain the mass balance of all sediment sizes in order to maintain the existing channel capacity, as well as the existing suite of in-channel habitats. The USGS data indicate that about 20% of the suspended load is sand-sized sediment, which is approximately equal to the proportion of sand found in the substrate (bulk bed material). The similarity in percentages suggests that the sand in transport in the Colorado River exchanges in equal proportions with the sand stored in the bed, consistent with contemporary theories for equilibrium transport in gravel-bed channels [Parker et al., 1982; Parker and Toro-Escobar, 2002]. 37