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I I I I I I I I I I I I I I I I I I I 28 Although a low flow year like 1994 generally causes only a small amount of deposition at the mouths of these backwaters, it is still unclear how several consecutive low flow year would affect this habitat. If deposition from consecutive low flow years is additive, it is likely that access to these backwaters would become limited. EV ALVA TION OF COARSE-SEDIMENT TRANSPORT The channel of the Colorado River in the Grand Valley and Ruby- Horsethief Canyon is formed by gravel- and cobble-sized sediment. Any future recommendations for habitat improvement for endangered fish will need to consider what flows are required to move the coarse substrate of the upper Colorado River. By definition, complex or multi-thread channel reaches are formed because of bank erosion and bar deposition. For either of these processes to occur, the bed material must be in motion, and thus, it is necessary to define the conditions under which sediment transport is initiated. In natural rivers, this task is complicated by effects associated with the variation in sediment properties and bed topography. We have tried to address some of these complications by selecting a number of different sites and using techniques that account for the spatial variability in bed topography and flow. In the absence of direct observations of tagged-particle movement or bed-load transport, the only practical means for estimating the threshold for sediment transport (or critical shear stress, rd is to use an empirical relation. The most common approach is to use Shields' parameter * 'tc 't = c (Ps - p) g D (1) where r*c is the critical dimensionless shear (Shields) stress, Ps and p are the density of sediment and water respectively, g is the gravitational acceleration,