[1] The present study examines variations in the reference shear stress for bed load transport (t r ) using coupled measurements of flow and bed load transport in 45 gravel-bed streams and rivers. The study streams encompass a wide range in bank-full discharge (1-2600 m 3 /s), average channel gradient (0.0003-0.05), and median surface grain size (0.027-0.21 m). A bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate W* versus the reach-average dimensionless shear stress t*. The reference dimensionless shear stress t* r was then estimated by selecting the value of t* corresponding to a reference transport rate of W* = 0.002. The results indicate that the discharge corresponding to t* r averages 67% of the bank-full discharge, with the variation independent of reach-scale morphologic and sediment properties. However, values of t* r increase systematically with average channel gradient, ranging from 0.025-0.035 at sites with slopes of 0.001-0.006 to values greater than 0.10 at sites with slopes greater than 0.02. A corresponding relation for the bank-full dimensionless shear stress t* bf , formulated with data from 159 sites in North America and England, mirrors the relation between t* r and channel gradient, suggesting that the bank-full channel geometry of gravel-and cobble-bedded streams is adjusted to a relatively constant excess shear stress, t* bf À t* r , across a wide range of slopes.

[1] Quantifying landscape-scale variations in sediment supply to streams and rivers is fundamental to our understanding of both denudational processes and stream channel morpho-dynamics. Previous studies have linked a variety of sediment-supply proxies to climatic, topographic, or geologic factors, but few have connected these directly to the characteristics of fluvial systems draining these landscapes. Here we correlate landscape controls on sediment supply to observed sedimentology and channel patterns through direct measurements of water and sediment fluxes in over 80 drainage basins ranging in area from 1.4 to 35,000 km 2 in the northern Rocky Mountains, USA. These data show that the relative sediment supply, defined by the bankfull sediment concentration, is dominated by basin lithology, while exhibiting little correlation to factors such as relief, mean basin slope, and drainage density. Bankfull sediment concentrations (bed load and suspended load) increase as much as 100-fold as basin lithology becomes dominated by softer sedimentary and volcanic rocks, relative to basins with more resistant lithologies. As bed load concentrations increase, stream beds become less armored, and bed load grain size coarsens. At very high sediment concentrations, bed surface, subsurface, and bed load grain sizes converge and a transition from single-thread to to braided channel patterns is commonly observed.Citation: Mueller, E. R., and J. Pitlick (2013), Sediment supply and channel morphology in mountain river systems: 1. Relative importance of lithology, topography, and climate,

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...

The area burned annually by wildfires is expected to increase worldwide due to climate change. Burned areas increase soil erosion rates within watersheds, which can increase sedimentation in downstream rivers and reservoirs. However, which watersheds will be impacted by future wildfires is largely unknown. Using an ensemble of climate, fire, and erosion models, we show that postfire sedimentation is projected to increase for nearly nine tenths of watersheds by >10% and for more than one third of watersheds by >100% by the 2041 to 2050 decade in the western USA. The projected increases are statistically significant for more than eight tenths of the watersheds. In the western USA, many human communities rely on water from rivers and reservoirs that originates in watersheds where sedimentation is projected to increase. Increased sedimentation could negatively impact water supply and quality for some communities, in addition to affecting stream channel stability and aquatic ecosystems.