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River channels are maintained by coordination between flow hydraulics, sediment supply, riparian vegetation, and sediment transport. This coordination is challenging to understand in natural flow regimes, where climatic and environmental drivers produce episodic flood and sediment supply events. To better understand the response of channels to flood sequences, we have undertaken laboratory flume experiments on sediment storage and export across a sequence of alternating hydrographs. Our experiments indicate that accumulated sediment storage before floods predicts sediment transport during floods, with sediment storage depletion during floods causing a nonlinear variation of sediment‐transport rates through time. Likewise, sediment storage between floods follows a growth‐to‐saturation pattern, whereby the sediment transport gradually increases toward the sediment feed rate depending on the occupation of available sediment storage zones. To describe these non‐linear variations, we developed a mathematical model which represents sediment transport and storage as a coupled dynamical system. This work highlights the crucial role that within‐channel sediment storage and its history play in determining sediment export in rivers. Plain Language Summary: The amount of sediment moving in a river channel largely determines its stability during floods; therefore, numerous ecological and engineering applications utilize predictions of sediment‐transport rates in rivers. Most prediction models assume that the force of the water and the size of sediment are the main variables needed to predict sediment‐transport rates. This implies that no information about the historical patterns of sedimentation in the channel is required to predict the amount of moving sediment. To test this implication, we conducted laboratory experiments on sediment transport across a sequence of floods. In contrast to existing prediction methods, we determined that the amount of sediment accumulated in a channel is a leading predictor of sediment transport both between and during floods. We characterized this dependence of sediment transport on the history of sediment storage and described it with a mathematical model. This work helps us better understand how floods affect the stability of river channels. Key Points: Sediment‐transport rates across flood sequences are largely predicted by the sediment storage accumulated in periods between floodsCoupling between sediment transport and storage generates non‐linear temporal variations of mean sediment‐transport ratesHistorical sediment transport and supply trends therefore control channel stability in conjunction with present‐time flow and sediment characteristics [ABSTRACT FROM AUTHOR]

This study examines the spatial pattern of fractional sediment mobility and assesses the influence of morphologic units on bedload transport in a small pool‐riffle reach with limited supply. Using a 2D hydraulic model and a subsurface‐based sediment transport model, shear stresses, fractional sediment mobility, and bedload transport were examined for flow events ranging in magnitude between 0.2Qbf and 1.5Qbf. Results reveal that while spatial variations in shear stress decrease as discharge increases, only a small proportion of the bed experiences high transport rates. At the reach scale, riffles are the primary morphological unit contributing to fully mobile sediment for all size fractions. However, at a subunit scale, there is evidence of sediment transport reversal for grains >32 mm at flows near or exceeding bankfull discharge in association with shear stress reversal. These transport reversals are important for maintaining pools despite their infrequent occurrence in the study reach. Sediment transport maps indicate that bed morphology considerably influences sediment transport at low to moderate flows. During these events, the shear stress is sensitive to local bed topography and partial mobility is the dominant transport process. In contrast, variations in bedload transport rates decrease during high flows when the flow is less sensitive to variations in bed topography and the bed becomes fully mobile. Key Points: A scaled version of a subsurface‐based model was utilized to evaluate the impact of morphologic units on fractional sediment mobilityRiffles were found to be the main source of fully mobile sediment of all sizes, with local transport reversal observed for grains >32 mmThe impact of morphologic units on bedload transport is more pronounced during low and moderate flows and diminishes during high flows [ABSTRACT FROM AUTHOR]

Widely distributed in natural rivers and coasts, vegetation interacts with fluid flows and sediments in a variable and complicated manner. Such interactions make it difficult to predict associated drag forces during sediment transport. This paper investigates the drag coefficient for an emergent vegetated patch area under nonuniform flow and mobile bed conditions, based on an analytical model solving the momentum equation following our previous work (Zhang et al., 2020, https://doi.org/10.1029/2020WR027613). Emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different vegetation coverage ∅. Laboratory flume tests were conducted to measure variations in both the water and bed surfaces along a vegetated patch on a sand bed. Based on the experimental and theoretical analyses, a dimensionless drag model integrating both terms of flow properties and bed effects is proposed to predict the drag coefficient Cd over a mobile bed. The calculated values of Cd exhibit two different trends, that is, nonmonotonically or monotonically increasing along the streamwise direction, due to the combined effect of water surface gradient and bed slope. The morphodynamic response of the mobile bed to nonuniform flow manifests as an evolution in the bed slope within the vegetated patch. Ongoing scouring directs the flow's energy toward overcoming the rising Cd and bed slope, leading to a relatively stable stage with a low sediment transport rate. This study advances the existing understanding of the drag coefficient's role over a mobile bed within nonuniform flows. It also enhances the applicability of vegetation drag models in riverine restoration. Plain Language Summary: The drag exerted by vegetation on a riverbed dictates the sediment transport rate with important implications for river morphological evolution. Predicting vegetation drag in nonuniform flow based on the bed characteristics of mobile sand bed conditions poses both theoretical and practical challenges. The implications of this endeavor include the formulation of predictive models for drag and a deeper understanding of the influence of gradually varied flow conditions in rivers. Through both experimental and theoretical investigations, this paper reveals that the drag coefficient exhibits varying patterns along the streamwise direction within the vegetated patch over a mobile sand bed. These patterns manifest in two distinct forms: a steady increase or a parabolic shape, wherein the coefficient initially rises before subsequently decreasing. This contrasts with prior studies on fixed beds, where the drag coefficient consistently follows a parabolic distribution in the streamwise direction. The discrepancy is attributed to the distinct physical contributions of pressure, advection, and bed friction to the drag coefficient. This study provides valuable insights into the importance of flow nonuniformity on vegetation drag, aiding in the prediction of backwater profiles in vegetated flows over a mobile bed. Furthermore, it facilitates modifications to sediment transport within vegetated patches. Key Points: Vegetation drag in nonuniform flow over a mobile sand bed is explored using the momentum equationDrag coefficient in nonuniform flow over a mobile bed exhibits either a parabolic or a monotonic increase along the streamwise directionWater surface gradient and bed slope contribute to the flow nonuniformity, collectively influencing the variability of the drag coefficient [ABSTRACT FROM AUTHOR]

Despite geomorphic processes being increasingly recognized as a vital component of river management projects, evidence suggests that they may not be adequately captured in common river assessment procedures. We reviewed 91 river assessment procedures from around the world to evaluate their effectiveness in capturing geomorphic processes relevant for river management goals. Our objectives were to summarize which common geomorphic indicators are measured and how in different types of river assessments categorized based on their main focus: geomorphic, physical habitat, mixed geomorphic and habitat, and hydromorphology. Our analysis identified differences in the types of geomorphic indicators included and measurement methodologies between the types of assessment procedures. Some geomorphic processes, such as sediment transport, are nearly completely absent from all assessments, despite their importance for geomorphic processes. The variability among assessment procedures suggests that a single procedure is unlikely to capture all geomorphic components required to support every river management programs. Here, we discuss how the strengths and limitations of different assessment types can be used to guide decisions around how to select assessments and geomorphic indicators to support management project goals. A companion paper expands the discussion of how to plan effective river assessment procedures to support unique management goals. [ABSTRACT FROM AUTHOR]

Downstream width variations can generate pool‐riffle morphology under experimental conditions, in numerical simulations and natural river channels. The present understanding of how pool‐riffle morphology varies with sediment supply and caliber, however, is insufficient due to the limited range of sediment supply rates explored in previous experiments and the little attention paid to sand supply and sediment size distribution in the laboratory and in the field. We present a model of river morphodynamics that can account for the spatial variability of channel width, and we validate the model with experimental data. Model validation shows how this one‐dimensional model can capture pool‐riffle formation, growth, and equilibration with errors that are comparable with those of other 1D models of river morphodynamics. We then apply the validated model to study the effects of sediment supply rate and caliber on pool‐riffle morphology. Model results show that pool‐riffle morphology is resilient to the range of tested sediment supply (i.e., five‐fold the sediment amount, 41‐fold the sand amount and coarsening the gravel supply). Bed and water surface slopes are sensitive to all types of change of sediment supply, whereas the sensitivity of bed surface sediment grain size varies with the type of change. These findings support prior research emphasizing the role of downstream width variations for the development/maintenance of pool‐riffle morphology and can help in the restoration and recovery of pool‐riffle gravel‐bed rivers. Key Points: A one‐dimensional hydro‐morphodynamic model of gravel‐bed pool‐riffle sequence is developedThe model can well capture pool‐riffle formation, growth, and equilibrationPool‐riffle morphology is resilient to the change of sediment supply and caliber [ABSTRACT FROM AUTHOR]

The sediment supplied to a stream channel impacts the morphological trends experienced by that channel, and the long‐term trends are important to understand for many riverine applications. We introduce the term 'net morphological displacement' (NMD) to denote the net channel change revealed by the morphological method over multiple sediment transport events to make the concept more explicit for river management and use it to determine equilibrium or disequilibrium states. This study explores the morphological response of East Creek, a small threshold gravel‐bed channel in the Coast Mountains of British Columbia, Canada, to variations in flow and sediment supply at multiple spatial and temporal scales over 15 years. High‐resolution topographic data (HRTD) of the bed were combined with cross‐sectional surveys of the banks to determine the sediment supply that the channel responded to annually. The level of detection used to remove noise associated with HRTD was calibrated using an independent tracer stone dataset. The net effects from multiple floods caused distributions of bed elevation change to generally follow the log‐normal distribution, and mean depths of erosion and deposition were predominantly similar between morphological units. At the reach scale, the various reaches of East Creek responded differently to the same hydrological events due to the impacts from the varying supply conditions on the NMD. Shorter measurement periods would have resulted in inconclusive information that does not show the long‐term morphological trends of the channel. Determination of these trends can take years or decades, depending on the time and space scales of change, but there is generally a lack of long‐term channel monitoring programmes, notably after river restoration. More long‐term channel monitoring programmes are required to assess restoration projects and ensure their long‐term sustainability. [ABSTRACT FROM AUTHOR]

A potential control of downstream channel width variations on the structure and planform of pool–riffle sequence local bed topography is a key to the dynamics of gravel bed rivers. How established pool–riffle sequences respond to time-varying changes in channel width at specific locations, however, is largely unexplored and challenging to address with field-based study. Here, we report results of a flume experiment aimed at building understanding of how statistically steady pool–riffle sequence profiles adjust to spatially prescribed channel width changes. We find that local bed slopes near steady-state conditions inversely correlate with local downstream width gradients when the upstream sediment supply approximates the estimated transport capacity. This result constrains conditions prior to and following the imposed local width changes. Furthermore, this relationship between local channel bed slope and downstream width gradient is consistent with expectations from scaling theory and a broad set of field-based, numerical, and experimental studies (n=88). However, upstream disruptions to coarse sediment supply through actions such as dam removal can result in a transient flipping of the expected inverse correlation between bed slope and width gradient, collectively highlighting that understanding local conditions is critical before typically implemented spatial averaging schemes can be reliably applied. [ABSTRACT FROM AUTHOR]

Structure-from-Motion (SfM) photogrammetry has become a popular solution for three-dimensional topographic data collection in geosciences and can be used for measuring submerged bed surfaces in shallow and clear water systems. However, the performance of through-water SfM photogrammetry has not been fully evaluated for gravel-bed surfaces, which limits its application to the morphodynamics of gravel-bed rivers in both field investigations and flume experiments. In order to evaluate the influence of bed texture, flow rate, ground control point (GCP) layout, and refraction correction (RC) on the measurement quality of through-water SfM photogrammetry, we conducted a series of experiments in a 70 m-long and 7 m-wide flume with a straight artificial channel. Bed surfaces with strongly contrasting textures in two 4 m-long reaches were measured under five constant flow regimes with three GCP layouts, including both dry and underwater GCPs. All the submerged surface models with/without RC were compared with the corresponding dry bed surfaces to quantify their elevation errors. The results illustrated that the poorly sorted gravel-bed led to the better performance of through-water SfM photogrammetry than the bed covered by fine sand. Fine sediment transport caused significant elevation errors, while the static sand dunes and grain clusters did not lead to noticeable errors in the corrected models with dry GCPs. The elevation errors of the submerged models linearly increased with water depth for all the tested conditions of bed textures, GCP layouts, and discharges in the uncorrected models, but the slopes of the increasing relations varied with texture. The use of underwater GCPs made significant improvements to the performance of direct through-water SfM photogrammetry, but counteracted with RC. The corrected models with dry GCPs outperformed the uncorrected ones with underwater GCPs, which could still be used to correct the underestimation in surface elevation caused by RC. Based on the new findings, recommendations for through-water SfM photogrammetry in measuring submerged gravel-bed surfaces were provided. [ABSTRACT FROM AUTHOR]

Large wood (LW) and log jams have important implications for sediment dynamics within river systems. Although many case studies explore this topic, far fewer synthesize these results at the reach and basin scales. In this paper, we present a conceptual model that relates valley setting and landscape history of forested, mountain streams to spatiotemporal patterns of sediment storage and transport via the formation and decay processes of logjams. This conceptual model explores the function of logjams as sediment transport moderators, significantly influencing sediment retention, channel morphology, and lateral and longitudinal connectivity. Detailed wood inventories, channel topographic surveys, and channel morphology from four case studies are used to illustrate the range of channel responses to logjam formation, evolution, and decay in different valley contexts. Valley setting is shown to influence logjam formation and dynamics by determining the mechanisms and timing of sediment and wood recruitment. In steep and narrow (and thus coupled, confined) reaches, colluvial processes are the dominant mechanisms of wood and sediment delivery, and logjams grow vertically as sediment is stored in a wedge behind the jam (longitudinal disconnectivity). In contrast, in unconfined reaches (decoupled hillslopes), wood and sediment recruitment occurs primarily through fluvial processes, and logjams grow laterally as the channel shifts across the valley bottom. These logjams have long lifespans, often over a century, and gradually release sediment as the logjam decomposes. Sediment deposits stabilized by riparian vegetation behind these structures may persist long after logjam decay. This conceptual model is a testable framework and practical tool that offers insight into wood-sediment interaction mechanisms within steep, forested basins. [ABSTRACT FROM AUTHOR]

This study examines channel dynamics and bed load transport through a riffle and pool sequence forced by downstream channel width variations within an experimental flume. The experiment consisted of four runs across, which we compare and contrast local and spatially averaged bed surface texture and topography, sediment transport rates, and sediment mobility at five locations across a pool‐riffle pair. Sediment transport was measured using mini Helley‐Smith samplers and particle tracers seeded in the monitored riffle and pool. In this study, "local" sediment transport rates were highly variable across the five monitoring locations. The lowest sediment transport rate was recorded at the riffle tail whereas the highest rates were measured at the riffle head and the pool center. The texture of the bed surface and transported load do not explain measured bed load transport trends and depending on how the measurements are aggregated differing interpretations are supported. In general, the bed texture in the pool was finer than the texture in the riffle, however, specific grain‐size percentile classes derived from pooled population analysis suggests little to no difference between pool and riffle texture. The combined results highlight the importance of acknowledging and applying analysis techniques to better understand the inherent variability of bed load transport within channel reaches where morphology differs, such as pools and riffles. Key Points: We conducted flume experiments to study sediment transport, bed surface texture, and sediment mobility across pool‐riffle unitDifferences in sediment transport occur within the same location, between sampling locations, during the same run, and between runs"Noise" is an important feature of bed load transport particularly for channel with local variations of width and bed topography [ABSTRACT FROM AUTHOR]

We formulate the bedload sediment flux probability distribution from the Lagrangian dynamics of individual grains. Individual particles obey Langevin equations wherein the stochastic forces driving particle motions are switched on and off by particle entrainment and deposition. The flux is calculated as the rate of many such particles crossing a control surface within a specified observation time. Flux distributions inherit observation time dependence from the on–off motions of particles. At the longest observation times, distributions converge to sharp peaks around classically expected values, but at short times, fluctuations are erratic. We relate this scale dependence of bedload transport rates to the movement characteristics of individual sediment grains. This work provides a statistical mechanics description for the fluctuations and observation-scale dependence of sediment transport rates. [ABSTRACT FROM AUTHOR]

We conducted flume experiments in a step‐pool channel with different sediment supply regimes to explore how the sequencing and the grain‐size of sediment pulses affect channel evolution. Our results show that the sequencing of events is not a primary control as the trends in flow characteristics, bedload transport, sediment storage, step evolution, and step frequency were similar in feed phases with different sequencing of sediment pulses. Channel adjustment and step stability in a short timescale (i.e., in a 4‐h run) were mostly controlled by the magnitude and frequency of sediment pulses. A coarser sediment feed mixture led to a coarser bed surface which increased both sediment storage in the channel and step stability. Our experiments show that channel evolution in a step‐pool channel is primarily controlled by the magnitude and frequency of sediment pulses and the grain size of sediment supply rather than the event sequencing of sediment pulses. [ABSTRACT FROM AUTHOR]

We present results from flume experiments in which an 8% steep channel with longitudinal width variations and step‐pool morphology was subjected to sediment feed pulses of different magnitude and frequency under constant water discharge. The channel response to these pulses included (a) large bedload transport rates, (b) bed aggradation, (c) fining of the bed surface, and (d) continuous formation and collapse of steps. In between pulses, the bed surface coarsened, and bedload transport rates dropped by several orders of magnitude. Steps continuously formed and collapsed during and shortly after the pulses, but their stability increased when the sediment feed was turned off. High pulse magnitude enhanced step formation, while low pulse frequency (i.e., long interpulse period) enhanced step stability. We back‐calculated the threshold for motion based on measured bedload transport rates and bed shear stress. Changes in the threshold for motion were much larger than changes in bed surface slope. By accounting for energy dissipation through the effective slope based on flow resistance partitioning, a better prediction was obtained. The threshold for motion decreased following sediment pulses then increased immediately after and fluctuated until the next sediment pulse. Our results indicate that longitudinal width variations and episodic sediment supply are primary controls on the evolution of step‐pool channels. Sediment feed magnitude affects mostly morphological changes, while sediment feed frequency controls channel stability. Key Points: The magnitude and frequency of sediment pulses control channel morphology and step dynamicsStep formation and stability depend more on local channel width variations than on sediment supplyThe threshold of motion changes in response to bedload transport rate and sediment grain size distribution [ABSTRACT FROM AUTHOR]

With the increasing attention on environmental flow management for the maintenance of habitat diversity and ecosystem health of mountain gravel-bed rivers, much interest has been paid to how inter-flood low flow can affect gravel-bed river morphodynamics during subsequent flood events. Previous research has found that antecedent conditioning flow can lead to an increase in critical shear stress and a reduction in sediment transport rate during a subsequent flood. However, how long this effect can last during the flood event has not been fully discussed. In this paper, a series of flume experiments with various durations of conditioning flow are presented to study this problem. Results show that channel morphology adjusts significantly within the first 15 min of the conditioning flow but becomes rather stable during the remainder of the conditioning flow. The implementation of conditioning flow can indeed lead to a reduction of sediment transport rate during the subsequent hydrograph, but such an effect is limited to within a relatively short time at the beginning of the hydrograph. This indicates that bed reorganization during the conditioning phase, which induces the stress history effect, is likely to be erased with increasing intensity of flow and sediment transport during the subsequent flood event. [ABSTRACT FROM AUTHOR]

In small headwater channels coupled to hillslopes, episodic supplies of sediment from events like landslides can dominate local channel morphodynamics and impact ecosystem health. In this study we investigate the response of a small mountain stream in British Columbia to a sediment input event that was up to 150 times the volume of sediment transported in an average flood year caused by the upstream installation of a road bridge. Using four years of repeated UAV surveys, bedload tracers, and other field monitoring techniques, we examined the spatial and temporal changes in sediment storage along the reach, identified changes to channel morphology, and characterized the interaction between the channel bed and the supplied sediment. Historical monitoring of the reach allowed for a comparison of the response to nearly 20 years of information about bedload transport and morphologic change. We observed that most of the change within the reach occurred during the first flood after bridge installation. The supplied sediment mostly moved across the bed surface with little interaction, as tracers installed in the bed had lower than expected rates of mobility. A narrow, steep segment of the channel proximal to the bridge installation stored little sediment, whereas a wider, less steep segment further downstream absorbed much of the change. This effect was exacerbated by a logjam that formed during the first flood after bridge installation. Despite the relative magnitude of the sediment transport event, there was little change to the channel pattern, with the added sediment increasing the relief of the already existing morphologic units. This suggests river managers should focus their restoration attention on segments likely to respond to a disturbance and that the pool-riffle channel morphology can persist across a wide range of sediment-supply conditions. [ABSTRACT FROM AUTHOR]

Individual grains move through gravel bed rivers in cycles of motion and rest, so tracer grains diffuse as they transport downstream. Tracer experiments demonstrate at least three diffusion ranges as observation times increase, with different spreading rates in each range. Until now, the generating processes of these ranges have remained unclear. In this work, we develop a random walk model of individual bedload trajectories including motion, rest, and burial processes. The model describes three bedload diffusion ranges that terminate in a fourth nondiffusive range when all tracers become buried. Using the model, we attribute three‐range tracer diffusion to the interplay between motion, rest, and burial processes, and we relate the multirange diffusion characteristics to the timescales of these processes. Key Points: Random walk model describes coarse gravel tracers diffusing through a river as they gradually become buriedInterchange of grains between motion, surface rest, and burial states generates three diffusion ranges as the observation time increasesSediment burial dominates long‐time properties and exhumation may develop a fourth "geomorphic" range of tracer diffusion [ABSTRACT FROM AUTHOR]

We describe the joint dynamics of bedload transport and bed elevation changes with a stochastic population model, and we analyze (1) the dependence of bedload flux statistics on local bed elevations and (2) resting time distributions for sediment undergoing burial in the fluctuating sedimentary bed. The model involves entrainment and deposition in a control volume characterized by elevation‐dependent rates, and it exhibits a statistical regulation effect, whereby bed aggradation suppresses the variance of the bedload flux while degradation enhances it. This variance regulation effect is contingent on collective entrainment, whereby moving grains destabilize stationary grains in a positive feedback. When collective entrainment is turned off,bedload transport fluctuations become independent of the bed elevation. Return times from above in the bed elevation time series provide heavy‐tailed power law distributions of resting times with tail behavior characterized by the mean erosion rate and the active layer depth. These results imply bedload statistics measurements on relatively short timescales can be strongly biased by bed elevation changes when collective entrainment occurs, and they support the growing consensus that sediment burial generates heavy‐tailed sediment resting times that ultimately generate anomalous bedload diffusion. Key Points: Two‐species stochastic population model describes fluvial bedload particle activity and local bed elevationsModel predicts elevation‐dependent transport statistics and power law distributed resting times for sediment undergoing burialBed elevation changes modify particle activity fluctuations by up to 90%, and sediment burial generates anomalous bedload diffusion [ABSTRACT FROM AUTHOR]

The movement of sediment through mountain river networks remains difficult to predict, as processes beyond streamflow and particle size are responsible for the entrainment and transport of bedload sediment. In deglaciated catchments, additional complexity arises from glacial impacts on landscape organization. Research to date indicates that the quantity of sediment stored in the channel is an important component of sediment transport in systems which alternate between supply and transport limited states, but limited long‐term field data exist which can capture storage‐transfer dynamics over a timescale encompassing episodic supply typical of mountain streams. We use a 45‐year dataset with annual and decadal‐scale data on sediment storage, channel morphology, and wood loading to investigate the spatial and temporal organization of storage in Carnation Creek, a previously glaciated 11 km2 catchment on Vancouver Island, British Columbia. Sediment is supplied episodically to the channel, including additions from debris flows in the early 1980s just upstream of the studied channel region. Analyzing the spatial and temporal organization of sediment storage along 3.0 km of channel mainstem reveals a characteristic storage wavelength similar to the annual bedload particle travel distance. Over time, two scales of variation in storage are observed: small‐scale fluctuation of 3–10 years corresponding to local erosional and depositional processes, and larger scale response over 25–35 years related to supply of sediment from hillslopes. Complex relationships between storage and sediment transfer (i.e., annual change in storage) are identified, with decadal‐scale hysteresis present in storage‐transfer relations in sites influenced by hillslope sediment and logjams. We propose a conceptual model linking landscape organization to temporal variability in storage and to storage–export cycles. Collectively, our results reaffirm the importance of storage to sediment transport and channel morphology, and highlight the complexity of storage–transport interactions. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Future sediment dynamics may be affected by changing climates or hydrological regimes because of the close link between hydrology and sediment erosion, deposition, and transport. Previously, investigations of these potential changes have been constrained by a combination of limited observational data, hydrological drivers, and appropriate mechanistic models. Additionally, there is often ambiguity regarding how to disentangle the impacts of climate and hydrology from direct human factors such as reservoirs and land‐use change, which often exert more control over sediment dynamics. In this study, we utilize a recently developed, large‐scale, distributed, mechanistic sediment transport model to project future sediment erosion, deposition, and transportation within the Fraser River Basin in British Columbia, Canada—a basin with historical water flux and sediment load observations and limited anthropogenic influences upstream of its delta. The sediment model is driven by synthetic land‐surface hydrology derived from Scenarios A1B, A2, and B1 of the Special Report on Emissions Scenarios, which were provided by the Pacific Climate Impacts Consortium. Resulting simulations of water flux and sediment load from 1965 to 1994 are first validated against observational data then compared with future projections. Future projections show an overall increase in annual hillslope erosion and in‐channel transportation, a shift towards earlier spring peak erosion and transportation, and longer persistence of the sediment signal through the year. These shifts in timing and annual yield may have deleterious effects on spawning sockeye salmon and are insufficient to counteract future coastal retreat caused by sea‐level rise. [ABSTRACT FROM AUTHOR]

Modeling sediment transport through large basins presents a challenging problem. The relation between water flux and sediment load is complex, and substantial erosion and transport can occur over small spatial and temporal scales. Analysis of large‐scale basins often relies on lumped empirical models that do not consider spatial or subannual variability. In this study, we adapt a small‐scale, mechanistic, distributed suspended sediment transport model for application to large basins. The model is integrated into the Terrestrial Hydrology Model with Biochemistry to make use of the Terrestrial Hydrology Model with Biochemistry's dynamic water routing. The coupled model is applied to the 230,000‐km2 Fraser River Basin in British Columbia, Canada, using climatic and hydrological inputs provided by a historical run of the Variable Infiltration Capacity model. Hourly simulations are aggregated into monthly and long‐term averages which are compared against observations. Simulated long‐term lake sedimentation values are within an order of magnitude of observations, and monthly load simulations have an average R2 of 0.70 across the five study stations with available data. Model results indicate that sediment loads from tributaries do not heavily influence dynamics along the main stem and suggest the importance of network connectivity. Sensitivity analysis indicates that models may benefit from characterizing bed load irrespective of its contribution to total sediment load. Historical simulations over the 1965–2004 period reveal important changes in sediment dynamics that could not be captured with a lumped model, including a decrease in basin sediment load interannual variability driven by changes in runoff and load variability within a key subbasin. Key Points: A mechanistic sediment transport routine is integrated into a hydrological model to provide river, lake, and floodplain dynamics at each grid cell across a large basinThe distributed model provides insights into the basin's sediment transport that are not possible with a lumped approachTributaries in the study area, even those in mountainous regions, do not substantially impact basin sediment yield [ABSTRACT FROM AUTHOR]

Abstract: Sediment transport in suspension can represent more than 90% of a river's total annual flux of sediment. In the case of the Yangtze River, more than 99% of the sediment supplied to the sea is suspended load. Suspended sediment is thus an important component of the total sediment load, with implications for channel dynamics, landscape evolution, ecology, and human‐related activities. For hydrological management of large basins such as the Yangtze River, knowledge of the processes governing suspended sediment concentration (SSC) is essential. An analysis of the temporal variation of SSC for the Upper Yangtze basin (defined at Pingshan station) is presented here. For this purpose, a database of 50 years of concurrent discharge and SSC measurements, made by the Yangtze River Commission, is used. The analysis is made using a novel probabilistic data‐driven technique, the Generalized Pareto Uncertainty (GPU). This technique allows for the testing of several strategies of prediction and forecast applied to a time series of SSC and streamflow. Changing between local or seasonal variables to feed these strategies, we inferred that although the main driver of the SSC transport is flow (as reported by previous authors), sediment storage is also a major control. Furthermore, the maximum necessary time lag for forecasts made with the data is on the order of one week, which provides one indication of the time scale of the local processes of SSC transport in the Upper Yangtze. In this paper, limitations and data requirements of the GPU methodology are also discussed. [ABSTRACT FROM AUTHOR]

A new method for the approximation of the explicit probability of entrainment for individual coarse particles is presented. The method is based on the derivation of inertial acceleration measurements, space-state approximation of the dynamics close to entrainment, and the probabilistic approximation of the threshold inertial acceleration that causes incipient motion. Results from flume experiments with a custom-made inertial measurement unit enclosed in an idealized spherical enclosure, under varied flow conditions (achieved through slope change) and two different arrangements (saddle and grain-top positions) are presented to demonstrate the application of the method. The analysis supports the modification of the existing flow velocity–based entrainment criteria so they respect the particle-frame realization of forces during incipient motion. [ABSTRACT FROM AUTHOR]

A flume experiment was conducted to study channel adjustment to episodic sediment supply in mountain streams. The bulk sediment used for the bed and feed included grain sizes 0.5-64 mm with geometric mean D [ABSTRACT FROM AUTHOR]

A new particle-based reduced-complexity model to simulate sediment transport and channel morphology in steep streams in presented. The model CAST (Cellular Automaton Sediment Transport) contains phenomenological parameterizations, deterministic or stochastic, of sediment supply, bed load transport, and particle entrainment and deposition in a cellular-automaton space with uniform grain size. The model reproduces a realistic bed morphology and typical fluctuations in transport rates observed in steep channels. Particle hop distances, from entrainment to deposition, are well fitted by exponential distributions, in agreement with field data. The effect of stochasticity in both the entrainment and the input rate is shown. A stochastic parameterization of the entrainment is essential to create and maintain a realistic channel morphology, while the intermittent transport of grains in CAST shreds the input signal and its stochastic variability. A jamming routine has been added to CAST to simulate the grain-grain and grain-bed interactions that lead to particle jamming and step formation in a step-pool stream. The results show that jamming is effective in generating steps in unsteady conditions. Steps are created during high-flow periods and they survive during low flows only in sediment-starved conditions, in agreement with the jammed-state hypothesis of Church and Zimmermann (2007). Reduced-complexity models like CAST give new insights into the dynamics of complex phenomena such as sediment transport and bedform stability and are a useful complement to fully physically based models to test research hypotheses. [ABSTRACT FROM AUTHOR]

Temporal dynamics of sediment transport in steep channels using two experiments performed in a steep flume (8%) with natural sediment composed of 12 grain sizes are studied. High-resolution (1 s) time series of sediment transport were measured for individual grain-size classes at the outlet of the flume for different combinations of sediment input rates and flow discharges. Our aim in this paper is to quantify (a) the relation of discharge and sediment transport and (b) the nature and strength of memory in grainsize- dependent transport. None of the simple statistical descriptors of sediment transport (mean, extreme values, and quantiles) display a clear relation with water discharge, in fact a large variability between discharge and sediment transport is observed. Instantaneous transport rates have probability density functions with heavy tails. Bed load bursts have a coarser grain-size distribution than that of the entire experiment. We quantify the strength and nature of memory in sediment transport rates by estimating the Hurst exponent and the autocorrelation coefficient of the time series for different grain sizes. Our results show the presence of the Hurst phenomenon in transport rates, indicating long-term memory which is grain-size dependent. The short-term memory in coarse grain transport increases with temporal aggregation and this reveals the importance of the sampling duration of bed load transport rates in natural streams, especially for large fractions. [ABSTRACT FROM AUTHOR]

Female salmonids bury and lay their eggs in streambeds by digging a pit, which is then covered with sediment from a second pit that is dug immediately upstream. The spawning process alters streambed topography, winnows fine sediment, and mixes sediment in the active layer. The resulting egg nests (redds) contain coarser and looser sediments than those of unspawned streambed areas, and display a dune-like shape with an amplitude and length that vary with fish size, substrate conditions, and flow conditions. Redds increase local bed surface roughness (<10−1 channel width, W), but may reduce the size of macro bedforms by eroding reach-scale topography (100-101 W). Research has suggested that spawning may increase flow resistance due to redd form drag, resulting in lower grain shear stress and less particle mobility. Spawning, also prevents streambed armoring by mixing surface and subsurface material, potentially increasing particle mobility. Here we use two-dimensional hydraulic modeling with detailed prespawning and postspawning bathymetries and field observations to test the effect of spawning by small-bodied salmonids on sediment transport. Our results show that topographical roughness from small salmon redds has negligible effects on shear stress at the reach-unit scale, and limited effects at the local scale. Conversely, results indicate sediment mixing reduces armoring and enhances sediment mobility, which increases potential bed load transport by subsequent floods. River restoration in fish-bearing streams should take into consideration the effects of redd excavation on channel stability. This is particularly important for streams that historically supported salmonids and are the focus of habitat restoration actions. [ABSTRACT FROM AUTHOR]

Where female salmon build nests ('redds'), streambed material is mixed, fine sediment is winnowed, and bed material is moved into a tailspill mound resembling the shape of a dune. Completed redd surfaces are coarser and better sorted than unspawned beds, which is thought to increase redd stability because larger grains are heavier and harder to move, and sorting increases friction angles for mobility. However, spawning also loosens sediment and creates topography that accelerates flow, which can increase particle mobility. We address these factors controlling the relative stability of redds and unspawned beds in flume experiments where redds were constructed with a dynamic technique that mimics the nesting behavior of female salmon. Although redds exhibited relatively coarse surfaces, measured entrainment forces indicate particle loosening by spawning lowered grain resistance to motion by 12-37% on average compared to unspawned beds. In addition, for the same discharges, boundary shear stress was 13-41% higher on a redd due to flow convergence on the tailspill. Visual measurements of particle entrainment further indicated redd instability, as bed-average shear stress was 22% lower at incipient motion and 29% lower at the discharge that mobilized all grain sizes on a redd. Overall, results demonstrate that redds are unstable compared to unspawned beds, which increases the risk of scour for buried eggs but may facilitate fine sediment flushing and improve the quality of spawning gravels for future generations of spawners. Therefore, managing salmon returns to increase streambed disturbance may be an effective tool for reducing sedimentation impacts on salmon reproduction. [ABSTRACT FROM AUTHOR]

The role played by the texture of the sediment supply on channel bed adjustments in gravel-bed rivers is poorly understood. To address this issue, an experimental campaign has been designed. Flume experiments lasting 96 h in a 9 m long, 0.60 m wide have been performed with different sand-gravel mixtures as feed textures. The response of the surface texture has been found to be highly dependent on the grain size distribution of the feed. When the feed texture included gravel, the finest fractions of the sediment supply infiltrate beneath the surface. Conversely, sand remains on the surface when the feed texture lacks gravel. This different textural response becomes obscured when water discharge increases. Further, the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture. When a small proportion of gravel is part of the feed texture, bed load transport rate asymptotically approaches the feed rate. However, when a significant fraction of gravel is part of the feed grain size distribution, bed load transport rate approaches the feed rate by following an oscillatory path. These findings have been verified in terms of a one-dimensional numerical model. This modeling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture, the more evident is the nonasymptotic transient trend toward equilibrium. [ABSTRACT FROM AUTHOR]

In this paper, we used a bedload transport data set collected at North Fork Caspar Creek, California, to examine temporal variation in sediment transport rate over a 7-year period. Using a hierarchical mixed-effects model, we examined across and within-event variation to determine whether the bedload-shear stress relation trends over time. The relation between bedload transport and shear stress was modeled using log(Qb)α+β*log(τ)+ε, where α and β are constants and ε is an error term. Depending on the length of observation, α and β can vary over several orders of magnitude, making modeling of transport based on flow challenging and highly inaccurate. We found a higher order yearly relation between bedload and shear stress, indicating systematic changes to the system over time. In the absence of significant additions to the system, α decreases roughly linearly over time, while β does not show any trend. From the systematic decline in α, we infer changes to sediment availability in the stream over time. Mixed-effects models have the potential to be a useful predictive tool in fluvial geomorphology, as they are more powerful at detecting trends in sediment transport rates than individual linear regressions. [ABSTRACT FROM AUTHOR]

Episodic sediment supply, past glaciation, and slow responses to disturbance make small mountain streams transitional alluvial regimes in which nonequilibrium conditions are common. Bed load effective discharge in these streams is on average a low-magnitude, high-frequency event, but is highly variable. Using a two-phase sediment transport model and long-term discharge records, we distinguish between three types of streams; streams in which gravel (sediment > 8 mm diameter) moves frequently and effective discharge occurs during gravel transport (Frequently Mobile Gravel (FMG)), streams in which gravel moves infrequently but effective discharge nonetheless occurs during gravel transport (Infrequently Mobile Gravel (IMG)), and streams in which sand (sediment < 8 mm diameter) moves over largely immobile gravel and effective discharge occurs frequently during sand-phase transport (Sand over Immobile Gravel (SG)). Using only effective discharge frequency or magnitude to characterize a stream, without information on mobile sediment type, is insufficient to distinguish between FMG and SG streams. Only the IMG streams have large, rare effective discharges that approximate the bankfull discharge; in FMG and SG streams the effective discharge is much more frequent and smaller than the bankfull. Only in the IMG streams does the effective discharge approximate a channel-forming discharge. In FMG and SG streams, the effective discharge bears little relation to the size or dimensions of the channel and is at best a channel-maintaining flow; at worst it is geomorphically meaningless. Effective discharge should not therefore be used in isolation as a proxy for channel-forming discharge for mountain stream channel design or management. [ABSTRACT FROM AUTHOR]

Abstract: This study describes the structure of gravel bars in Nahal Zin, an ephemeral stream in the Negev desert. The internal structure of the bars was examined along trenches and in shallow pits. Gravel sheets and unit bars form during transporting flow events in the main channel, on intra-bar channels and near bar heads. Unit bars are dominated by the Go facies. Compound bars develop from accretion around, and modification of, unit bars. Compound bars are active under the current flow regime and the average depth of the fill layer is about 35cm. The structure of compound bars is dominated by Gm (massive), containing large amounts of sand. The second most common facies is clast-supported, openwork, and well sorted sediments of the Go (pebbles) facies. Bar formation, and the development of the range of facies evident in the bars is controlled by sediment supply, particularly the high volumes of sand-sized sediment, the passage of gravel sheets and bedforms during floods, and the lateral and vertical instability of the channel. Repeated scour and fill events have produced a diverse arrangement of facies, with numerous erosional contacts between depositional units. Lateral and downstream shifts in the pattern of scour and fill due to flow and antecedent conditions shape the channel morphology and bar internal structure. Ephemeral river bars differ from those of humid and proglacial rivers in terms of the dominant facies present, the arrangement of the facies within the bars, and the sedimentary structures developed within the depositional units and on the bar surface. [Copyright &y& Elsevier]

Abstract: Colluvial sediment dynamics are examined using a 70-year landslide inventory in formerly glaciated mountain drainage basins of coastal British Columbia, Canada. Landslide sediment transfer is documented by identifying landslide types, and by characterizing preferential sites of landslide initiation, delivery, and storage across spatial scales. Data analysis reveals that open-slope landslides delivering material to seasonal or perennial channels and fluvial terraces are the dominant mechanisms of sediment transfer. This finding suggests high instability of the low-order channel network and its ongoing re-organization (degradation) after generalized sediment recharge occurred in the last glacial maximum. In the study period, landslide activity across the landscape has generated net degradation on planar slopes and first-order colluvial channels, whereas unchannelled valleys, higher-order colluvial channels and alluvial channels have accumulated material. The scaling relation of the landslide sediment yield appears to be controlled by the spatial arrangement of the relict glacial macro-forms. Landslide yield is highest in unchannelled topography, decreases at the scale of channel initiation (A d ~0.002 km2), and remains constant for drainage areas where length scales of cirque/valley walls and hanging valley floors overlap (0.002< A d <0.06). Injections of landslide material start declining consistently beyond areas larger than 0.6 km2 (the scale of relict glacial trough initiation), where fluvial environments become increasingly disconnected from landslide inputs. Cumulative yield indicates that colluvial sediment redistribution is limited to small basins; specifically, 90% of the colluvial load is released at scales smaller than about 0.6 km2. [Copyright &y& Elsevier]

It has been observed that the relation between sediment storage and sediment transport rate in gravel-bed rivers exhibits counter-clockwise hysteresis, or aggradation-degradation cycles. Previous work has attributed these cycles to external changes to the sediment supply rate. We present a set of nine flume experiments that test the hypothesis that aggradation-degradation cycles can occur even under steady sediment supply conditions when the sediment transport rate temporarily nears the sediment supply rate. The experiments are designed to examine the transport-storage relations under a range of conditions with different initial bed condition, sediment supply rate, discharge, and supply material texture. The sediment transport efficiency of the experimental channel was strongly influenced by the existing bed morphology inherited from historical flows. The discharge and grain size distribution of the supply material played a secondary role, with higher discharge conditions and finer sediment feed material producing increased transport efficiencies. The transport-storage relations revealed that the transport rate of the channel changed with the volume of sediment stored and was, therefore, mediated by the channel morphology. Aggradation-degradation cycles were observed even under steady sediment supply and discharge conditions and were mediated by morphologic adjustments within the channel that temporarily led the transport rate to be near equal to the sediment supply rate. In other words, the probability of hysteresis in the transport-storage relation increases when the channel is near a transport equilibrium. The findings demonstrate the complexity associated with developing a unique relation between sediment transport and storage in gravel-bed rivers, and highlights the importance of considering the morphologic evolution and historical bed conditions. • Flume experiments conducted to investigate cycles of aggradation-degradation. • Sediment transport rate variability linked to bed morphology and texture changes. • Transport rate adjusted according to volume of sediment stored. • Aggradation-degradation cycles tend to occur when channel is near equilibrium. [ABSTRACT FROM AUTHOR]

The article presents a study which explores the concept of gravel transport measurements through a video-based technology with a flume mounted light table. The researchers have incorporated a high-resolution video camera with a light table to continuously capture images of 2-181 mm material exiting a flume. Video images were recorded at a rate of 15 to 20 frames per second and were post processed by using LabView™ software. It asserts that the technology is inexpensive and provides high-resolution data on a coarse sediment transport out of a flume. The Grain-size distribution and Solid Discharge system are also explored.

Desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below, while gravel bed streams in humid and snowmelt areas typically have well-armored surfaces. The degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size. A set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0.5 to 2.4 and with an average value of 1.2. The armor ratio of snowmelt-fed gravel bed streams in the same set ranges from 2 to 7, with an average value of 3.4. The reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes. Thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions. The experiments have two limiting cases: a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods. All constant hydrograph experiments developed a well-armored structured surface, while short asymmetrical hydrographs did not result in substantial vertical sorting. All symmetrical hydrographs show some degree of sorting, and the sorting tended to become more pronounced with longer duration. Sediment supply appears to be a first-order control on bed surface armoring, while the shape of the hydrograph plays a secondary role. [ABSTRACT FROM AUTHOR]

This paper reviews sediment transport and channel morphology in small, forested streams in the Pacific Northwest region of North America to assess current knowledge of channel stability and morphology relevant to riparian management practices around small streams. Small channels are defined as ones in which morphology and hydraulics may be significantly influenced by individual clasts or wood materials in the channel. Such channels are headwater channels in close proximity to sediment sources, so they reflect a mix of hillslope and channel processes. Sediment inputs are derived directly from adjacent hillslopes and from the channel banks. Morphologically significant sediments move mainly as bed load, mainly at low intensity, and there is no standard method for measurement. The larger clastic and woody elements in the channel form persistent structures that trap significant volumes of sediment, reducing sediment transport in the short term and substantially increasing channel stability. The presence of such structures makes modeling of sediment flux in these channels – a potential substitute for measurement – difficult. Channel morphology is discussed, with some emphasis on wood related features. The problem of classifying small channels is reviewed, and it is recognized that useful classifications are purpose oriented. Reach scale and channel unit scale morphologies are categorized. A "disturbance cascade" is introduced to focus attention on sediment transfers through the slope channel system and to identify management practices that affect sediment dynamics and consequent channel morphology. Gaps in knowledge, errors, and uncertainties have been identified for future research. [ABSTRACT FROM AUTHOR]

This paper reviews suspended sediment sources and transport in small forest streams in the Pacific Northwest region of North America, particularly in relation to riparian management. Mass movements, roading and yarding practices, and burning can increase the supply of suspended sediment. Sediment yields recovered to pre-harvest levels within one to six years in several paired catchment studies. However, delayed mass movements related to roads and harvesting may produce elevated suspended sediment yield one or more decades after logging. There is mixed evidence for the role of streamside tree throw in riparian buffers in supplying sediment to streams. Harvesting within the rip arian zone may not increase suspended sediment yield if near stream soils are not disturbed. Key knowledge gaps relate to the relative roles of increased transport capacity versus sediment supply, the dynamics of fine sediment penetration into bed sediments, and the effects of forest harvesting on suspended sediment at different scales. Future research should involve nested catchments to examine suspended sediment response to forest practices at multiple spatial scales, in combination with process-based field studies. [ABSTRACT FROM AUTHOR]

The mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable. The bed surface remained intact throughout the freshet, and scour was limited to small areas of the bed. Observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction. Fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature. At low flows, no size fraction was fully mobile. At intermediate flows (shear stress >27 Pa) sand was fully mobile while larger material remained partially mobile. At high flows (>43 Pa) the division between the full and partial mobility regimes occurred around 16 mm, which remains finer than the median size of the subsurface material. The largest material scarcely moves at mean annual flood. Magnetic tracers confirmed results obtained from the pit traps. Three distinct zones of mobility can be defined in fractional transport plots: partial mobility, full mobility, and overpassing/suspended. Critical shear stress for incipient motion varied over an order of magnitude; a qualitatively similar result for bimodal sediment was reported by Wilcock and McArdell [1993]. Incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface, and a lower envelope that corresponds with our (nonsimilar) entrainment criterion. [ABSTRACT FROM AUTHOR]

Sediment dynamics in river catchments are controlled by tectonics, climate, and biota effecting material production on hillslopes and transport of sediment from their source to the catchment outlet. Tectonics create topography and control erodibility of the bedrock material, whereas climate controls the efficiency of weathering and transporting processes. In contrast, the effects of biota (i.e. vegetation) are more ambiguous. Vegetation accelerates bio-chemical weathering and effects mass wasting through uprooting, trapping and stabilizing hillslope material. Furthermore, vegetation exerts a strong control on the grain size distribution of hillslope sediments and thus on sediment rooting through catchments. In this study we compare grain size distributions of hillslope and channel sediments collected in four headwater catchments located in the Chilean Coastal Cordillera covering a strong bio-climatic gradient. 76 volumetric bulk samples were taken with grain sizes ranging from clay (d = 0.3 μm) to boulders (d > 300 mm). Results show that the production of fine material is strongly dependent on bio-chemical weathering intensity and hence humidity. The coarse fraction in hillslope material and channel sediments increases from arid to sub-humid conditions, presumably reflecting a higher intensity of mass wasting processes. Channels show varying degrees of armouring reflecting nonselective and thus transport-limited conditions under arid climate that are contrasted by relatively well sorted and armoured channel sediments in the humid and vegetated catchments, indicating size selective and supply-limited transport conditions. Channel grain sizes generally show high similarity to hillslope grain sizes of the same catchment, revealing the strong dependency of channel sediments on hillslope supply, at least in headwater catchments, transferring a bio-climatic control from the hillslopes to the channel system. • Bulk sampling (covering clay to boulder) reveals differences in transport conditions along the sediment flow path • Grain size sorting between hillslope and river sediment shows complex response along the Chilean bio-climatic gradient • Hillslope vegetation affects channel dynamics via grain size in headwaters of the Chilean Coastal Cordillera [ABSTRACT FROM AUTHOR]

Abstract Massive bed degradation (20 m in 7 years) has been observed in the Shi‑ting River, Sichuan Province, China, since the 2008 Wenchuan Ms. 8.0 earthquake. The reason for the massive bed degradation has not been well understood. A hypothesis has been proposed that relates bed degradation to the augmentation of sand supply after the earthquake. The effect of sand on gravel mobility (magic sand effect) has long been observed in laboratory experiments. In this paper, we study whether the augmentation of sand supply and its magic sand effect can lead to the observed massive degradation at decadal scales. A one-dimensional river morphodynamic model is implemented to study the problem in general at field scale and in context of the gravel-bed Shi‑ting River. Sediment transport is calculated with the Wilcock and Crowe (2003) relation, in which the magic sand effect is explicitly embedded in terms of a function relating reference Shields number to the surface sand fraction. We find that the augmentation of gravel supply leads to bed aggradation, whereas the augmentation of sand supply can indeed lead to bed degradation and surface fining. The magnitude and timescale of bed degradation are not sensitive to the sand supply rate but are sensitive to the flood intermittency factor , i.e., the fraction of time the river is in flood. However, an unrealistic flood intermittency factor (≥0.3) would be required in order to match the observed timescale of bed degradation, thus indicating that the magic sand effect might not be the governing reason for massive bed degradation in the Shi‑ting River, and by implication in gravel-bed rivers in general. Our simulation results also indicate that despite the fact that magic sand effects are not explicitly included in most sediment transport relations, they are at least partly built in via the hiding function that is contained in most sediment transport relations for gravel-sand mixtures. While here we use the Shi‑ting River as an example, our results have applicability to gravel-bed rivers subjected to augmentation in sand supply in general. Highlights • Augmentation of gravel supply leads to bed aggradation • Augmentation of sand supply leads to enhanced gravel transport and bed degradation • Magic sand effects alone cannot induce the observed river incision at decadal scale • Magic sand effects can be partly incorporated with a dimensionless hiding function [ABSTRACT FROM AUTHOR]

• The density and spatial distribution of keystones exert a minor control on the bed surface grain-size distribution (GSD). • Keystones tend towards a random spatial distribution regardless of their initial configuration. • Sediment transport has a positive correlation to the proportion of dislodged keystones. Large particles strongly influence flow resistance, energy dissipation, bed stability, and channel morphology in mountain streams. We conducted flume experiments to evaluate effects of the density and spatial distribution of coarse particles (i.e., keystones) on bed stability, surface texture and sediment transport in steep channels. Keystones were placed on the channel bed surface at the beginning of each experiment, both in clustered and in uniformly spaced (i.e., anti-clustered) configurations and in different numbers (i.e., for different values of keystone density). The flow rate was increased by 20 % every hour and each experiment continued until the bed was completely scoured. Sediment transport was measured with a sediment trap located at the flume outlet. Topographic data from high-resolution surface imagery and second-order structure functions (SSF) of bed elevations, were used to characterize bed surface structuring while the Ripley's K function was used to assess the degree of clustering of keystones relative to random spatial distributions. Our results indicate that (1) the density and spatial distribution of keystones exert a minor control on the bed surface grain-size distribution (GSD), (2) for increasing flow rates, the spatial distribution of keystones naturally evolves towards a random distribution, regardless of the initial spatial arrangment, and (3) sediment transport has a higher correlation with the proportion of dislodged keystones than with flow discharge. [ABSTRACT FROM AUTHOR]

The estimation of bedload transport, a type of coarse fluvial sediment motion, is challenging because of higher-order, complex, and time-variable environmental processes. Still, models for calculating bedload transport mostly rely on semi-empirical formulae derived from time-averaged laboratory flume or field measurements and involve cross-section averaging of hydro-environmental parameters. Additionally, many models assume infinite, constant availability of sediment, which is unrealistic in natural environments. While most published models yield excellent results on the dataset they were trained on, they may be significantly inaccurate when applied to other cases. Also, more recent formulae relying on considerable amounts of data still lead to high calculation uncertainty. While the importance of short-term dynamics, for example, driven by turbulence, for bedload transport is well known, this study investigates the relevance of time-variable processes at monthly to seasonal scales. Our analysis of a large dataset and remote sensing data shows evidence for seasonal variability in measurements of bedload transport rates and challenges the validity of Gaussian statistics for interpolating bedload transport models. Measured bedload transport rates were smaller when snowmelt was taking place and when no glacier was present in the catchment, and were best represented by extreme value distributions. By controlling for biases from measurement devices, geomorphic patterns, and the presence of an upstream dam, we show that steady, constant at-a-station hydraulics only explain a limited share of the variance in the measurements. Ultimately, a model for estimating bedload transport rates should consider the temporal variability of processes across all scales, from turbulence to climate. [Display omitted] • Bedload transport estimation is challenging in complex natural hydro-environments. • Models rely on semi-empirical, time and cross-section averaging formulae and data. • Short-term dynamics are recognized to be critical factors for estimating bedload. • This study finds additional importance of monthly to seasonal time scales. • Normal Gaussian statistics are unsuitable for interpolating a bedload transport model. [ABSTRACT FROM AUTHOR]