Effect of bed load supply on sediment transport in mountain streams

Mountain streams with their tributary torrents build the upper part of the fluvial network. They are important regarding the transfer of sediment from headwaters to lower basins. Channels are typically steep with wide grain size distributions, ranging from fine sand up to large boulders, and a stabilized bed surface. Mountain streams often are supply-limited with respect to mobile bed load, which needs to be addressed when bed load transport equations are applied to such streams. To better understand supply limitation, laboratory experiments highlighting the effect of bed load supply on incipient motion and bed load transport rate are discussed. Experimental tests were done in which fine bed load was supplied to a previously armored channel bed, with flow conditions ranging from one-third to twice the critical dis-charge for the bed surface. At flows not exceeding the critical discharge, the time series of the bed load transport rate at the downstream model boundary featured consistent patterns which are attributed to distinct phases: (i) a temporal lag, (ii) an equilibrium state, and (iii) a post-supply phase. Bed load transport occurred even at flows distinctly below that for incipient motion of the bed surface. But, with the mass of total bed load outflow approaching the supply amount, the mass did not exclusively consist of supplied grains. The coarser the supplied bed load, the more sediment was mobilized from the bed surface. At higher flows, processes differed. Total bed load outflow exceeded the supply amount and the break-up of the armor layer caused a refining of the bed surface.     

Patterns in stream longitudinal profiles and implications for hyporheic exchange flow at the H.J. Andrews Experimental Forest,Oregon, USA

There is a need to identify measurable characteristics of stream channel morphology that vary predictably throughout stream networks and that influence patterns of hyporheic exchange flow in mountain streams. In this paper we characterize stream longitudinal profiles according to channel unit spacing and the concavity of the water surface profile. We demonstrate that: (1) the spacing between zones of upwelling and downwelling in the beds of mountain streams is closely related to channel unit spacing; (2) the magnitude of the vertical hydraulic gradients (VHGs) driving hyporheic exchange flow increase with increasing water surface concavity, measured at specific points along the longitudinal profile; (3) channel unit spacing and water surface concavity are useful metrics for predicting how patterns in hyporheic exchange vary amongst headwater and mid‐order streams. We use regression models to describe changes in channel unit spacing and concavity in longitudinal profiles for 12 randomly selected stream reaches spanning 62 km² in the H.J. Andrews Experimental Forest in Oregon. Channel unit spacing increased significantly, whereas average water surface concavity (AWSC) decreased significantly with increasing basin area. Piezometer transects installed longitudinally in a subset of stream reaches were used to measure VHG in the hyporheic zone, and to determine the location of upwelling and downwelling zones. Predictions for median pool length and median distance between steps in piezometer reaches bracketed the median distance separating zones of upwelling in the stream bed. VHG in individual piezometers increased with increasing water surface concavity at individual points in the longitudinal profile along piezometer transects. Absolute values of VHG, averaged throughout piezometer transects, increased with increasing AWSC, indicating increased potential for hyporheic exchange flow. These findings suggest that average hyporheic flow path lengths increase—and the potential for hyporheic exchange flow in stream reaches decreases—along the continuum from headwater to mid‐order mountain streams. Copyright © 2005 John Wiley & Sons, Ltd.     

Impact of wastewater discharge on the channel morphology of ephemeral streams

The impact of wastewater flow on the channel bed morphology was evaluated in four ephemeral streams in Israel and the Palestinian Territories: Nahal Og, Nahal Kidron, Nahal Qeult and Nahal Hebron. Channel changes before, during and after the halting of wastewater flow were monitored. The wastewater flow causes a shift from a dry ephemeral channel with intermittent floods to a continuous flow pattern similar to that of humid areas. Within a few months, nutrient‐rich wastewater flow leads to rapid development of vegetation along channel and bars. The colonization of part of the active channel by vegetation increases flow resistance as well as bank and bed stability, and limits sediment availability from bars and other sediment stores along the channels. In some cases the established vegetation covers the entire channel width and halts the transport of bed material along the channel. During low and medium size flood events, bars remain stable and the vegetation intact. Extreme events destroy the vegetation and activate the bars. The wastewater flow results in the development of new small bars, which are usually destroyed by flood flows. Due to the vegetation establishment, the active channel width decreases by up to 700 per cent. The deposition of fine sediment and organic material changed the sediment texture within the stable bar surface and the whole bed surface texture in Nahal Hebron. The recovery of Nahal Og after the halting of the wastewater flow was relatively fast; within two flood seasons the channel almost returned to pre‐wastewater characteristics. The results of the study could be used to indicate what would happen if wastewater flows were introduced along natural desert streams. Also, the results could be used to predict the consequences of vegetation removal as a result of human intervention within the active channel of humid streams. Copyright © 2001 John Wiley & Sons, Ltd.     

Flow behaviour,suspended sediment transport and transmission losses in a small (sub‐bank‐full) flow event in an Australian desert stream

The behaviour of a discrete sub‐bank‐full flow event in a small desert stream in western NSW, Australia, is analysed from direct observation and sediment sampling during the flow event and from later channel surveys. The flow event, the result of an isolated afternoon thunderstorm, had a peak discharge of 9 m³/s at an upstream station. Transmission loss totally consumed the flow over the following 7·6 km. Suspended sediment concentration was highest at the flow front (not the discharge peak) and declined linearly with the log of time since passage of the flow front, regardless of discharge variation. The transmission loss responsible for the waning and eventual cessation of flow occurred at a mean rate of 13.2% per km. This is quite rapid, and is more than twice the corresponding figure for bank‐full flows estimated by Dunkerley (1992) on the same stream system. It is proposed that transmission losses in ephemeral streams of the kind studied may be minimized in flows near bank‐full stage, and be higher in both sub‐bank‐full and overbank flows. Factors contributing to enhanced flow loss in the sub‐bank‐full flow studied included abstractions of flow to pools, scour holes and other low points along the channel, and overflow abstractions into channel filaments that did not rejoin the main flow. On the other hand, losses were curtailed by the shallow depth of banks wetted and by extensive mud drapes that were set down over sand bars and other porous channel materials during the flow. Thus, in contrast with the relatively regular pattern of transmission loss inferred from large floods, losses from low flows exhibit marked spatial variability and depend to a considerable extent on streamwise variations in channel geometry, in addition to the depth and porosity of channel perimeter sediments. Copyright © 1999 John Wiley & Sons, Ltd.     

Slope-induced changes in channel character along a gravel-bed stream: The Allt Dubhaig,Scotland

Changes in channel character along a small river in the Scottish Highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0.02 to 0.00015 because of local baselevel control. This decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed. The channel pattern alters progressively rather than abruptly, in the sequence (1) near-braided, (2) meandering with active point-bar chutes, (3) meandering with active outer-bank talweg, (4) stable equiwidth sinuous. The changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical concepts than by purely empirical discriminant or trend equations. Measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload. It is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions, are typical of many streams in mountainous areas and can be understood in terms of slope-induced changes in hydraulic properties.     

The triggering of debris flow due to channel‐bed failure in some alpine headwater basins of the Dolomites: analyses of critical runoff

The debris deposits at the bottom of very steep natural channels and streams in high mountain areas can be mobilized by runoff, triggering a water–sediment mixture flow known as debris flow. The routing of debris flow through human settlements can cause damage to civil structures and loss of human lives. The prediction of such an event, or the runoff discharge that triggers it, assumes an interest in risk analyses and the planning of defence measures. The object of this study is to find a method to determine the critical runoff value that triggers debris flow as a result of channel‐bed failure. Historical and rainfall data on 30 debris flows that occurred in six watersheds of the Dolomites (north‐eastern Italian Alps) were collected from different sources. Field investigations at the six sites, together with the hydrologic response to the rainfalls that triggered the events, were performed to obtain a realistic scenario of the formation of the debris flow there occurred. Field observations include a survey along the channel of the triggering reach of debris flow, with measurements of the channel slope and cross‐section and sampling of debris deposits for grain size distribution. Simulated runoff discharge values based on the rainfall recorded by pluviometers were then compared with values obtained through experimental criteria on the initiation and formation of debris flow by bed failure. The results are discussed to provide a plausible physical‐based method for the prediction of the triggering of debris flow by channel‐bed failure. Copyright © 2007 John Wiley & Sons, Ltd.     

Sensitivity of bed shear stress estimated from vertical velocity profiles: the problem of sampling resolution

Bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile. However, most measuring devices used in the field do not allow for flow velocity to be measured very close to the bed. The lack of near-bed measurements is a critical loss of information which may affect bed shear stress estimates. Detailed velocity profiles obtained from a field acoustic Doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near-bed points is critical for the estimation of bed shear stress in a shallow river environment. Moreover, the results indicate that using the full flow depth instead of the bottom 20 per cent of the profile generates an underestimation of the shear stress when flow is uniform. © 1998 John Wiley & Sons, Ltd.     

The impact of exceptional events on erosion,bedload transport and channel stability in a step‐pool channel

Sediment transport in the Erlenbach, a small stream with step‐pool morphology in the canton of Schwyz, Switzerland, has been monitored for more than 20 years. During this time three exceptional events (events with high sediment yield and long return times that have a large effect on channel morphology) have impacted the stream and partly or completely rearranged the existing step‐pool morphology. In the aftermath of the events, sediment transport rates at a given discharge and total sediment yield remained elevated for about a year or longer. For the last event, dated on the 20 June 2007, observations of boulder mobility and step destruction were used to interpret channel stability. Boulders with median diameters of up to 135 cm and estimated weights of more than 2·5 tons have moved during the 2007 event. Using hydraulic observations and shear stress calculations boulders up to 65 cm in diameter were predicted to have been fully mobile in peak conditions, even if form resistance and increased critical stresses needed for the initiation of motion in steep streams were taken into account. For two of the events, estimated peak shear stresses at the bed exceeded 1000 Pa, calculated both from observations of the flow hydraulics and from boulder mobility. This suggests that highly energetic flows occur relatively frequently in small, steep streams and that large boulders can be transported by fluvial processes in such streams. The observations have potential significance for hazard risk mitigation, stream engineering and restoration. Copyright © 2009 John Wiley & Sons, Ltd.     

Infiltration-recharge through wadi beds in arid regions

Abstract

Groundwater recharge in arid regions is intermittent and usually occurs as a result of flood flow transmission losses in dry wadi channels. Hydrograph characteristics play a dominant role in determining the amount of channel abstraction in relation to the width of the wetted perimeter and the time of inundation, and the subsequent groundwater recharge. Large variations in the magnitude of channel losses result mainly from the diversity in inflow volumes. The magnitude of groundwater recharge in relation to bed transmission losses is dependent on flood volume and duration, soil moisture content and physical soil profile characteristics. Runoff volume and duration are the dominant factors influencing the cumulative infiltrated volume and recharge to shallow water tables. Taking into consideration the influence of various hydrological and channel characteristics, several regression equations are suggested to estimate the transmission losses from a wadi bed and the groundwater recharge.     

The influence of lithology on channel geometry and bed sediment organization in mountainous hillslope-coupled streams

Sediment transport and channel morphology in mountainous hillslope-coupled streams reflect a mixture of hillslope and channel processes. However, the influence of lithology on channel form and adjustment and sediment transport remains poorly understood. Patterns of channel form, grain size, and transport capacity were investigated in two gravel-bed streams with contrasting lithology (basalt and sandstone) in the Oregon Coast Range, USA, in a region in which widespread landslides and debris flows occurred in 1996. This information was used to evaluate threshold channel conditions and channel bed adjustment since 1996. Channel geometry, slope, and valley width were measured or extracted from LiDAR and sediment textures were measured in the surface and subsurface. Similar coarsening patterns in the first few kilometres of both streams indicated strong hillslope influences, but subsequent downstream fining was lithology-dependent. Despite these differences, surface grain size was strongly related to shear stress, such that the ratio of available to critical shear stress for motion of the median surface grain size at bankfull stage was around one over most of the surveyed lengths. This indicated hydraulic sorting of supplied sediment, independent of lithology. We infer a cycle of adjustment to sediment delivered during the 1996 flooding, from threshold conditions, to non-alluvial characteristics, to threshold conditions in both basins. The sandstone basin can also experience complete depletion of the gravel-size alluvium to sand size, leading to bedrock exposure because of high diminution rates. Although debris flows being more frequent in a basalt basin, this system will likely display threshold-like characteristics over a longer period, indicating that the lithologic control on channel adjustment is driven by differences in rock competence that control grain size and available gravel for bed load transport. © 2020 John Wiley & Sons, Ltd.     

Patterns of bedload entrainment and transport in forested headwater streams of the Columbia Mountains,Canada

We monitor bedload transport and water discharge at six stations in two forested headwater streams of the Columbia Mountains, Canada. The nested monitoring network is designed to examine the effects of channel bed texture, and the influence of alluvial (i.e. step pools and riffle pools) and semialluvial morphologies (i.e. boulder cascades and forced step pools) on bedload entrainment and transport. Results indicate that dynamics of bedload entrainment are influenced by differences in flow resistance attributable to morphology. Scaled fractional analysis shows that in reaches with high form resistance most bedload transport occurs in partial mobility fashion relative to the available bed material, while calibers finer than 16 mm attain full mobility during bankfull flows. Equal mobility transport for a wider range of grain sizes is achieved in reaches exhibiting reduced form resistance. Our findings confirm that the Shields value for mobilization of the median surface grain size depends on channel gradient and relative submergence; however, we also find that these relations vary considerably for cobble and gravel bed channels due to proportionality between dimensionless shear stress and grain size. Exponents of bedload rating curves across sites correlate most with the D_90s of the mobile bed, however, where grain effects are controlled (i.e. along individual streams), differences in form resistance across morphologies exert a primary control on bedload transport dynamics. Application of empirical formulae developed for use in steep alpine channels present variable success in predicting transport rates in forested snowmelt streams. Formulae that explicitly account for reductions in mobile bed area and high morphological resistance associated with woody debris provide the best approximation to observed empirical data. Copyright © 2014 John Wiley & Sons, Ltd.     

Sediment transport in high‐speed flows over a fixed bed: 1. Particle dynamics

Particle dynamics are investigated experimentally in supercritical high‐speed open channel flow over a fixed planar bed of low relative roughness height simulating flows in high‐gradient non‐alluvial mountain streams and hydraulic structures. Non‐dimensional equations were developed for transport mode, particle velocity, hop length and hop height accounting for a wide range of literature data encompassing sub‐ and supercritical flow conditions as well as planar and alluvial bed configurations. Particles were dominantly transported in saltation and particle trajectories on planar beds were rather flat and long compared with alluvial bed data due to (1) increased lift forces by spinning motion, (2) strongly downward directed secondary currents, and (3) a planar flume bed where variation in particle reflection and damping effects were minor. The analysis of particle saltation trajectories revealed that the rising and falling limbs were almost symmetrical contradicting alluvial bed data. Furthermore, no or negligible effect of particle size and shape on particle dynamics were found. Implications of experimental findings for mechanistic saltation‐abrasion models are briefly discussed. Copyright © 2017 John Wiley & Sons, Ltd.     

FLOW FIELD IN SCOURED ZONE OF CHANNEL CONTRACTIONS

Experiments were conducted in a laboratory flume to measure the two-dimensional turbulent flow field in the scoured zone of channel contractions under a clear-water scour condition. The Acoustic Doppler Velocimeter (ADV) was used to detect the flow field at different vertical lines along the centerline of uncontracted (main channel) and contracted zones of the channel. The distributions of time-averaged velocity components, turbulent intensity, turbulent kinetic energy, and Reynolds stresses are presented in nondimensional graphical form. The bed shear stresses are computed from the measured Reynolds stresses being in threshold condition within the zone of contraction where bed was scoured. The data presented in this paper would be useful to the investigators for the development of kinematic flow model and morphological model of scour at a channel or river contraction.     

The response of headwater stream channels to urbanization in the humid tropics

Analysis of the bankfull cross-sections of headwater streams in Ado-Ekiti region of Southwestern Nigeria and their comparison with data from other tropical environments and temperate latitudes reveal that the channel capacities of streams in the humid tropics are relatively smaller than those of temperate regions, averaging 1.51 m² with a coefficient of variation of 87 per cent. This is attributed to the small stream discharge, the predominantly low and highly seasonal flows of the streams, the low shear stress of stream load, and the stabilizing and protective influence of riparian vegetation and surface incrustations. The chanel capacities of the urban streams (mean = 1.13m²) are about 47 per cent smaller than those of the natural streams (mean = 2.12 m²) in the same ecological zone. In terms of hydraulic efficiency, the urban streams also have relatively inefficient cross-sections and larger width/depth ratios than their rural or natural counterparts. Resurveys of seventeen monumented cross-sections reveal that while channel shoulder width increased by only 6 per cent over a one-year period, channel depth and capacity decreased by 16 per cent and 4 per cent respectively; the observed decrease in channel size occurs entirely in the channel depth dimension. Thus the response of stream channels to the urbanization of small headwater catchments in the humid tropics is probably more of vertical accretion of channel bed and reduction in channel capacity rather than the widely-reported anomalous enlargement of urban streams through channel widening. The rapid rate of channel aggradation is attributed to excessive rates of sediment production and delivery to streams in urbanized catchments in the humid tropics, rapid deposition of sediments during small runoff events and on the falling stage of storm hydrographs, and the inability of the streams to evacuate the sediments delivered to them despite the increased discharge and peak flow associated with urbanization. The low competence of the urban streams is attributed to the predominance of low flows, very gentle bed slopes, and most importantly the widespread dumping of refuse into the channels thereby reducing flow velocity and promoting backwater flooding, ponding, and sedimentation. The correlations between drainage basin area, a surrogate for stream discharge, and channel capacity are very strong for the rural watersheds, and the regression analysis indicates a tendency towards a steady-state isometric relationship. Urban channels are, to a large extent, in disequilibrium with the urban hydrological state. However, spatial variations in the degree of urbanization of the catchments, and, therefore in runoff volume and velocity, exercise strong control on channel width, depth, and size. A model of the sequence of stream channel adjustment to the urbanization of small headwater catchments in the humid tropics is presented.     

Diel stream geochemistry,Taylor Valley,Antarctica

Unlike temperate and polythermal proglacial streams, the proglacial streams in Taylor Valley (TV), Antarctica, are derived primarily from glacier surface melt with no subglacial or groundwater additions. Solute responses to flow reflect only the interaction of glacial meltwater with the valley floor surrounding the stream channel. We have investigated the major, minor and trace element 24‐h variations of two proglacial melt streams, Andersen Creek and Canada Stream, originating from the Canada Glacier in TV, Antarctica. Both streams exhibited diel mid‐austral summer diurnal flow variation, with maximum flow being more than 50 times the minimum flow. Dissolved (< 0.4 µm) major, minor and trace solute behaviors through diel periods were strongly controlled by the availability of readily solubilized material on the valley floor and hyporheic‐biological exchanges. Anderson Creek had generally greater solute concentrations than Canada Stream because of its greater receipt of eolian sediment. Andersen Creek also acquired greater solute concentrations in the rising limb of the hydrograph than the falling limb because of dissolution of eolian material at the surface of the stream channel coupled with minimal hyporheic‐biological exchange. Conversely, Canada Stream had less available eolian sediment, but a greater hyporheic‐biological exchange, which preferentially removed trace and major solutes in the rising limb and released them in the falling limb. Given the dynamic nature of discharge, eolian, and hyporheic‐biological processes, solute loads in TV streams are difficult to predict. Copyright © 2012 John Wiley & Sons, Ltd.     

Hyporheic and total transient storage in small,sand‐bed streams

Key processes in stream ecosystems are linked to hydraulic retention, which is the departure of stream flow from ideal ‘plug flow’, and reflects fluid movement through surface and hyporheic storage zones. Most existing information about hyporheic exchange is based on flume studies or field measurements in relatively steep streams with beds coarser than sand. Stream tracer studies may be used to quantify overall hydraulic retention, but disaggregation of surface and hyporheic retention remains difficult. A stream tracer approach was used to compute the rates at which stream water is exchanged with water in storage zones (total storage) in short reaches of two small, sand‐bed streams under free and obstructed flow conditions. Tracer curves were fit to the one‐dimensional transport with inflow storage model OTIS‐P. Networks of piezometers were used to measure specific discharge between the stream and the groundwater. In the sand‐bed streams studied, parameters describing total retention were in the upper 50% of data compiled from the literature, most of which represented streams with beds coarser than sand. However, hyporheic storage was an insignificant component of total hydraulic retention, representing only 0·01–0·49% of total exchange, and this fraction did not increase after installation of flow obstructions. Total retention did not vary systematically with bed material size, but increased 50–100% following flow obstruction. Removal of roughness elements, such as large wood and debris dams, is detrimental to processes dependent upon transient storage in small, sand‐bed streams. Copyright © 2007 John Wiley & Sons, Ltd.     

Energy losses in compound open channels

This paper investigates energy losses in compound channel under non-uniform flow conditions. Using the first law of thermodynamics, the concepts of energy loss and head loss are first distinguished. They are found to be different within one sub-section (main channel or floodplain). Experimental measurements of the head within the main channel and the floodplain are then analyzed for geometries with constant or variable channel width. Results show that head loss differs from one sub-section to another: the classical 1D hypothesis of unique head loss gradient appears to be erroneous. Using a model that couple 1D momentum equations, called “Independent Sub-sections Method (ISM)”, head losses are resolved. The relative weights of head losses related to bed friction, turbulent exchanges and mass transfers between sub-sections are estimated. It is shown that water level and the discharge distribution across the channel are influenced by turbulent exchanges for (a) developing flows in straight channels, but only when the flow tends to uniformity; (b) flows in skewed floodplains and symmetrical converging floodplains for small relative flow depth; (c) flows in symmetrical diverging floodplains for small and medium relative depth. Flow parameters are influenced by the momentum flux due to mass exchanges in all non-prismatic geometries for small and medium relative depth, while this flux is negligible for developing flows in straight geometry. The role of an explicit modeling of mass conservation between sub-sections is eventually investigated.     

Groundwater cation concentrations in the riparian zone of a forested headwater stream

Groundwater cation concentrations in relation to hydrologic flow paths were studied in the riparian forest zone of a small headwater catchment near Toronto, Ontario. Groundwater entering the riparian zone from uplands showed significant differences in cation concentrations between slope-foot and near-stream locations. Mean Ca, Mg, K, and Na concentrations in shallow groundwater at the upland perimeter of the riparian forest were 65-0, 11-2, 0-7, and 1-8 mg L^?1 respectively. Mean Ca, Mg, K, and Na concentrations in deep groundwater flowing upwards through glacial sands beneath the riparian zone were 52-1, 15-1, 1-3, and 2-6 mg L^?1 respectively. Shallow groundwater emerged as slope-foot springs producing surface rivulets which crossed the riparian zone to the streams. Deep groundwater flowed upward through organic soils into the rivulets and also discharged directly to the streams as bed and bank seepage. Springs had higher Ca concentrations and lower Mg, K, and Na values than rivulets entering the streams. Conversely, Mg, K, and Na concentrations were higher and Ca concentrations were lower in bank seeps in comparison to rivulets. These results suggest that differences in cation concentrations in groundwater entering the streams result from initial contrast in the chemistry of shallow and deep groundwater rather than from the effects of riparian soils and vegetation.     

Bedform development and its effect on bed stabilization and sediment transport based on a flume experiment with non-uniform sediment

Non-uniform sediment deposited in a confined, steep mountain channel can alter the bed surface composition. This study evaluates the contribution of geometric and resistance parameters to bed sta-bilization and the reduction in sediment transport. Flume experiments were done under various hydraulic conditions with non-uniform bed material and no sediment supply from upstream. Results indicate that flume channels respond in a sequence of coarsening and with the formation of bedform-roughness features such as rapids, cascades, and steps. A bedform development coefficient is introduced and is shown to increase (i.e. vertical sinuosity develops) in response to increasing shear stress during the organization process. The bedform development coefficient also is positively correlated with the critical Shields number and Manning's roughness coefficient, suggesting the evolution of flow resistance with increasing bedform development. The sediment transport rate decreases with increasing bed shear stress and bedform development, further illustrating the effect of bed stabilization. An empirical sedi-ment transport model for an equilibrium condition is proposed that uses the bedform development coefficient, relative particle submergence (i.e. the ratio of mean water depth and maximum sediment diameter), modified bed slope, and discharge. The model suggests bedform development can play a primary role in reducing sediment transport (increasing bed stabilization). The model is an extension of Lane's (1955) relation specifically adapted for mountain streams. These results explain the significance of bedform development in heightening flow resistance, stabilizing the bed, and reducing sediment transport in coarse, steep channels.