This study investigates trends in bed surface and substrate grain sizes in relation to reachscale hydraulics using data from more than 100 gravel-bed stream reaches in Colorado and Utah. Collocated measurements of surface and substrate sediment, bankfull channel geometry and channel slope are used to examine relations between reach-average shear stress and bed sediment grain size. Slopes at the study sites range from 0·0003 to 0·07; bankfull depths range from 0·2 to 5 m and bankfull widths range from 2 to 200 m. The data show that there is much less variation in the median grain size of the substrate, D 50s , than there is in the median grain size of the surface, D 50 ; the ratio of D 50 to D 50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress. Similar trends are observed in an independent data set obtained from measurements in gravel-bed streams in Idaho. A conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system. The results of the transport model suggest that downstream trends in total bed load flux may vary appreciably, depending on the assumed relation between surface and substrate grain sizes. Figure 1. Changes in bed load transport rate and median grain size for two rivers in Idaho, USA.load transport equations, differences in mobility of small and large particles are accounted for by a so-called hiding function, f(D i /D 50 ), where D i is an individual grain size and D 50 is the median grain size. Let us assume for the moment that D i and D 50 are characteristic grain sizes of the bed load and bed surface, respectively, and that D i < D 50 , as noted above. Let us also assume that the substrate is the primary source of the bed load, thus D i is equivalent to the substrate median grain size, D 50s . In a channel network, both D 50 and D 50s should become finer downstream due to selective transport, deposition and/or abrasion. If the two sizes fine at the same rate, then the ratio of D 50 to D 50s is constant, and the effects of hiding and exposure stay the same in a relative sense. Alternatively, if the two sizes fine at different rates, the hiding-exposure effects may offset changes in shear stress and limit (or enhance) the mobility of the bed load as it moves through the network. Both hypotheses are reasonable; however, conditions favoring one versus the other have not been explored, nor have the implications with respect to models of downstream fining or drainage basin evolution.In this paper, we examine interactions between reach-scale flow properties and trends in surface and substrate grain sizes. We have amassed a large data set from field studies of gravel-and cobble-bed rivers in Colorado and Utah that allows us to examine relations between shear stress, armoring and bed load transport intensity over a broad range of scales. Additional data from studies conducted elsewhere in the USA are included to assess the a...

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A two-dimensional hydrodynamic model was applied to seven study reaches in the Colorado River within Grand Canyon to examine how operation of Glen Canyon Dam has affected availability of suitable shoreline habitat and dispersal of juvenile humpback chub (Gila cypha). Suitable shoreline habitat typically declined with increasing discharges above 226-425 m 3 / s, although the response varied among modelled reaches and was strongly dependent on local morphology. The area of suitable shoreline habitat over cover types that are preferred by juvenile humpback chub, however, stayed constant, and in some reaches, actually increased with discharge.

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