Relation between flow, surface-layer armoring and sediment transport in gravel-bed rivers

This study investigates trends in bed surface and substrate grain sizes in relation to reach-scale 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. Copyright (C) 2007 John Wiley & Sons, Ltd.