Step Pool Research Articles

Empirical and model‐based evaluation of a step‐pool stream restoration project: Consequences for a highly valued fish population

AbstractObjectiveWe sought to measure a step‐pool restoration project's effects on a steelhead Oncorhynchus mykiss population and explore the capability of process‐based modeling to enhance understanding of the results.MethodsWe used before‐after–control‐impact monitoring in combination with a process‐based, individual‐based, spatially explicit fish population model to evaluate a stream restoration project that reconfigured the channel and primarily added step pools to a reach of a second‐order stream in northwestern California.ResultFive years of monitoring both before (2012–2016) and after (2018–2022) restoration indicated that restoration caused substantial increases in the abundance and biomass of steelhead. Individual growth rates and retention of fish in the study reach did not exhibit consistent patterns, even in the first 2 years after restoration, when fish abundance and biomass exhibited extreme increases of about fivefold. Model simulations predicted about a twofold increase in the abundance and biomass of steelhead in the restoration reach, which corresponded with the empirical results 4–5 years after restoration. The model also predicted a similar increase in the production of steelhead out‐migrants, a response we did not attempt to measure in the field.ConclusionStep‐pool restoration benefitted steelhead. The model's correspondence with empirical observations indicates its potential applicability to more complex resource management questions in the study area, such as how restoration will combine with changes in climate to affect the sustainability of salmonid populations.

Types and morphologies of gully step pools of different process of formation in the Yuanmou dry-hot Valley

ABSTRACT Step pools are micro-geomorphology hosted in gullies which are closely related to the channel erosion rate and hydraulic characteristics of runoff. These features are widely developed in the Yuanmou dry-hot Valley area of the Central Yunnan Plateau, China. In this paper, the morphological characteristics of different types of step pools in the Yuanmou dry-hot Valley were analyzed using analysis of variance and cluster analyses. The results show that: (1) step pools can be divided into differential, bedrock, barrier, and sub-erosion step pools based on the environment of development; the bedrock and differential step pools develop on a large scale, while barrier and sub-erosion step pools develop on a small scale; (2) the width and depth of the step, the height of the step, the width of the pool, and the depth of the step of the differential type, bedrock type, and sub-erosion type step pools exhibited a significant positive correlation (P < 0.01); the height of the step and the depth of the pool of the barrier type step pools were significantly negatively correlated with the height of the step and the concave hole depth of the sub-erosion type (P < 0.01); (3) the differential type and undercurrent type step pools were seriously eroded. The study of the types and morphologies of gully step pools in the Yuanmou dry-hot Valley helps understand the formation and evolution mechanisms of erosion gullies and provides a scientific basis for monitoring and controlling soil erosion.

The Morphological Evolution of a Step–Pool Stream after an Exceptional Flood and Subsequent Ordinary Flow Conditions

Mountain streams are frequently characterized by step–pool morphology that provides stability and energy dissipation to the channel network. Large flooding events can overturn the equilibrium of the step–pool condition by altering the entire configuration. This work focuses on the impact of the “Vaia” storm (27–30 October 2018) on a step–pool mountain stream (Rio Cordon, Northeast Italy) and on its evolution after two years of ordinary flow conditions. To achieve the aims, this work uses both remote sensing data (LiDAR and UAV) and direct field measurements (i.e., longitudinal profiles and grain sizes distributions) performed pre-event, post-event, and 2 years later (current conditions). The results show a significant widening (width +81%, area +68%) and the creation of a new avulsion after the storm and a substantial change between the number of units (51 in the pre-event, 22 post-event, and 51 in the current conditions) and characteristics of step–pool sequences between pre- and post-conditions. Furthermore, it proves the ongoing processes of morphological stabilization since the current step–pool sequences parameters are heading back to the pre-event values. Such results suggest clear susceptibility of step–pool to exceptional events and fast recovery of such setting during barely two years of ordinary flow conditions.

Investigation on the effect of inclined crest step pool on scouring protection in erodible river beds

Investigation on the effect of inclined crest step pool on scouring protection in erodible river beds

Influence of stream channel characteristics on bed morphology and step–pool structure in a rhyolitic headstream watershed

The headstream area is an important channel segment for investigating the self-organization mechanism of a stream channel or the occurrence characteristics of stream bed morphology owing to its sediment source and low anthropogenic impact such as that from check dams or channel modifications. However, less work has been conducted on the transition of stream bed morphology in channels with very high gradients, unlike in step–pool channels. This study describes the field observation results of the stream bed morphological characteristics and the relationship between the step–pool structure and stream channel characteristics in the headstream areas of rhyolitic mountains. The transition of the stream bed morphology associated with different bed slopes was confirmed through field observations. The dominant stream bed morphology changed from a fall to a cascade to a step, from upstream to downstream. Furthermore, the relationship was different between the step formation processes (hydraulic control and keystone types) and the jamming state, which was caused either by the supply process of the step constituent boulder or the external force of the step. Mountainous streams are strongly affected by geology, and this tendency is likely to be more pronounced in headstream areas. Further, the factors influencing stream bed morphology or stream channel characteristics include the formation age of mountains and the climate, geology, and the absence of fault zones. Therefore, knowledge of the stream bed morphology of headstream areas under various geological and meteorological conditions is required.

Numerical Study on the Hydraulic Properties of Flow over Different Pooled Stepped Spillways

This work presents numerical simulations carried out to study the influence of geometric characteristics of pooled steps on the energy dissipation performance, flow patterns properties, velocity rates, and pressure distributions over a spillway. The localization of the inception point of air entrainment was also assessed, being a key design parameter of spillways. With this aim, different configurations of steps were taken in account, including flat, pooled, and notch pooled types. The computational procedure was first validated with experimental results from the literature and then used to test the hydraulic behavior derived from different geometric configurations. The flat step configuration showed the best energy dissipation performance as compared with other configurations. With the notched pooled step configuration, the efficiency performance of the pooled structure improved by about 5.8%. The interfacial velocities of the flat stepped spillway were smaller than those of the pooled structure. The pressure value at the beginning of the step in the pooled configuration was larger than the flat configuration, while for the notched pool the maximum pressure values decreased near the step pool. Pool configuration (simple or notched) did not have a significant influence on the location of air entrainment.

Analysis of rill step–pool morphology and its comparison with stream case

AbstractIn this paper, the morphology of step–pool features is analysed using rill measurements and literature data for streams. Close‐range photogrammetry was used to carry out ground measurements on rills with step–pool units, shaped on a plot having slope equal to 14, 15, 22, 24 and 26%. Data were used to compare the relationships between H/L, in which H is the step height and L is the step length, and the mean gradient of the step–pool sequence, Sm, for streams or the slope of the step–pool unit, S, for rills. The relationship of H/L against Sm is widely used to test the occurrence of the maximum flow resistance condition in streams, which is associated with the range 1 ≤ (H/L)/Sm ≤ 2. Further analyses were carried out to compare both the formation process and the profile of the pool in rills with those related to streams. Moreover, for a single rill channel, an analysis of flow characteristics expressed in terms of Darcy–Weisbach friction factor and Froude number was developed. The results allowed us to state: (i) the relationships of H/L versus Sm and S are quite similar and the steepness ratio for streams, (H/L)/Sm, and for rills, (H/L)/S, generally ranges from 1 to 2; (ii) the formation process and the profile of the pool in rills are not consistent with those occurring in streams; (iii) in the rills, the longitudinal size of the pool is dominant with respect to the maximum scour depth; (iv) the presence of a sequence of step–pool units within a rill segment noticeably increases flow resistance compared to segments with a flat bed; (v) the Froude number of the flow over the sequence of step–pool units in rills is slightly below the range of 0.8–1 corresponding to the maximum flow resistance in step–pool units.

Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charr in Hokkaido, Japan.

Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density-dependent growth and survival. Agonistic interactions (n=761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length. Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks. These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference. Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr. The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade-offs in ecological performance can contribute to a mechanistic understanding of species coexistence.

Morphology of Dune-Like Relief in Rivers

Linearized equations of two-dimensional hydraulics have been analyzed by the method of small perturbations within a wide range of alluvial features sizes at large values of Froude numbers and hydraulic resistance. The preservation of three-dimensional effects in depth-averaged equations of motion, continuity, and deformation enabled the authors to identify domains with combinations of flow hydraulic characteristics that correspond to alluvial features of different size and morphology. The study confirmed the results of the earlier analysis for subcritical flows with small hydraulic resistance and showed new types of relationships between alluvial features morphology and the hydraulic characteristics of flow for flows with large Froude numbers and high hydraulic resistance. In addition, a relationship between the length of two-dimensional ultramicroforms and hydraulic resistance has been established. A new class of macroforms—two-dimensional macroforms—have been identified. The verification of the results of theoretical analysis by measurements data on the morphology of channel formations and hydraulic characteristics of the flow has shown that the analysis of linearized equations of two-dimensional hydrodynamics by small-perturbation method can be used to determine the morphology and dimensions of channel forms in both subcritical and supercritical streams. The step–pool systems in torrential streams in mountain rivers are analogues of antidunes (in supercritical flows) and ripples (in subcritical flows), obtained in large flumes with sand alluvium. Three-dimensional macroforms are most common in rivers. If such macroforms are well developed in wide channels (at flow width greater than the half-width of the macroform), their length is determined by channel depth. Macroforms in narrower channels cannot develop completely, and their lengths are limited by channel width and can be calculated only through this width.

Fundamental study on the effects of the keystone height and structure on sediment deposition in a high‐gradient channel

AbstractStep–pool structures are among the most important structures in riverbed morphology that can contribute to the stabilization of riverbeds and riverbanks, energy dissipation during floods, and head reduction function. Two concepts have been suggested regarding step–pool formation: the development of antidunes (antidune concept) and keystone concept. The latter suggests that a large boulder (keystone) incidentally left in a river channel can trap pebbles and build a step–pool structure. The ultimate goal of this study is to reveal the step–pool formation process and establish a design method based on the keystone concept. This paper describes the experimental results of the effects of the keystone arrangement on sediment deposition. Step–pool formation based on the keystone concept has previously been focused on the process of sediment deposition a short distance from a keystone. However, this study reveals that the obstruction of sediment supply due to the decreased flow velocity in the main river course is an important factor. When the arrangement interval of keystones is small, the amount of sediment supply downstream decreases because the flow velocity at the main flow channel between the keystones does not increase. In contrast, the flow velocity at the main river course increases between the keystones, and supplied sediment is transported to the most downstream section if the keystone arrangement interval is large enough to join the flow from the bank side of the keystones.

In‐situ assessment of the spatial arrangement of step–pool units on eroded rills

AbstractRills are generated on homogeneous hillslopes by the action of different discharges and evolve morphologically over short timescales due to a strong interaction between the flow and bed morphology. Such an interaction generates a reconfiguration of the bed geometry. Previous works suggest that bed geometry is often characterized by alternation between pools and flat reaches (steps). Each step–pool unit may contribute to hydraulic resistance and affects flow behaviour. The objectives of this work are (i) to assess different (innovative) techniques for the in‐situ assessment of rill bed geometry, (ii) to use these techniques to assess the geometry of eroded rills in situ in order to determine the spatial arrangement in the bed macro‐scale roughness and (iii) finally to analyse the role of slope and discharge as driving factors associated with the development of these macroforms.Roughly rectilinear, long rills were formed in the field as a result of combining different slope and discharges. Photogrammetry provided detailed digital elevation models (DEMs) before and after the experiments. The rills were morphologically characterized from the DEMs. In each rill, the presence of step–pools was identified from long profiles according mainly to morphological criteria published elsewhere, but with ad hoc critical threshold values more appropriate for small eroded channels.The minimum slope required for the development of step–pool units seems to be somewhere between 5 and 15%. Discharge seems to affect pool size or roughness amplitude.There does not seem to be a clear step–pool periodicity. However, external factors could have affected the normal growth and alternation of these structures. Identification of steps and pools from longitudinal elevation profiles can be objectively accomplished using a series of geometric rules originally proposed for rivers and large channels, and adapted to rills. © 2019 John Wiley &amp; Sons, Ltd.

Particle transport in gravel‐bed rivers: Revisiting passive tracer data

AbstractData from tracer experiments were compiled and analysed in order to explore the role of geomorphological, hydrological and sedimentological constraints on fluvial gravel transport in gravel‐bed rivers. A large data set from 217 transport episodes of tagged stones were compiled from 33 scientific papers. Our analyses showed that while magnitude of peak discharge is a major control on gravel transport and mobility, tracer travel distances show some scale dependence on the morphological configuration of the channel. Our results also highlight differences in the way tracers are displaced between step–pool and riffle and pool channels. The riffle–pool sequence seems to be a more efficient trap for travelling gravels than the step–pool pair. In addition, in step–pool channels there are clear differences in tracer transport between observations of first displacements after tracer seeding (unconstrained‐stone conditions), and second and subsequent observations of tracer displacements (constrained‐stone conditions). The comparison between tracer experiments under constrained conditions and those under unconstrained conditions also highlights the importance of bed state and structures in gravel mobility. The results of this study confirm that sediment transport in gravel‐bed rivers is a complex process, whereby sedimentological and geomorphological controls are superimposed on the hydraulic forcing. © 2018 John Wiley &amp; Sons, Ltd.

Field Study of Morphological Parameters in Step-Pool Streams

Nowadays, step-pool formations have attracted a lot of attention, which are distinguished by the successive arrangements of the bed, suitable geometry, and the tumbling flow pattern, which can highly disperse water energy. Field study of a step–pool channel, along with one of the upper reaches of Kamandan River indicated a strong correlation between several morphological parameters of the river such as reach slope, step length, step height, pool depth, local slope, and the like. The length of the reach under the study is 145 meters and has an intermediate morphology based on Montgomery and Buffington’s classification. Therefore, twelve distinct step units were identified for 145 meters upstream while the rest was formed by steep morphology. In the present study, different definitions of wave length were applied to establish the relationships among the above parameters. For instance, the difference between apexes of every two successive step elevation was found to have a considerable relationship with the wavelength with a determination coefficient of 0.9. In addition, bankfull width and depth, along the profile for different cross-sections, were determined to establish a relationship between these parameters and pool spacing. Further, the parameters were applied to create a relationship with step heights.

Measurement of dimensionless Chezy coefficient in step-pool reach (Case Study of Dizin River in Iran)

Step–pool reaches are common bed forms in mountainous streams exceeding 3% gradient and provide an important means of flow resistance in steep rivers. Step-pool channels are characterized by longitudinal steps formed by large boulders which are constituted into discrete channel-spanning accumulations that separate pools comprising finer grain. Flow resistance, reflected by roughness materials, appears to be an important control on bed load transport rates and mean flow velocity. To estimate flow resistance some morphological features and velocity were measured in the step-pool channel of Dizin River, located in Karaj River watershed in Iran. Topographic surveys and bed sediment sampling were made in low flow condition while three-dimensional velocity measurements were made in low, medium and high flow conditions. As flow resistance is a function of geometric, bed material size, longitudinal slope and hydraulic radius, dimensional analysis was conducted to derive a non-dimensional relationship for Chezy coefficient in step-pool reaches. Thereafter, it was calibrated for the measured data set of Dizin, Ammameh and Rio Cordon. Comparable results of calibration with a river located in a different environment may suggest that flow resistance features in semi arid and humid streams may have similar effects on non-dimensional Chezy coefficient.

Bedload transport in a steep alpine stream: Assessment of sediment mobility and virtual velocity using the bedload tracking

The bedload transport is challenging to analyze in field, consequently, several assumptions about it were made basing on laboratory researches or on short-term field studies. During the last decades several monitoring methods were developed to assess the bedload transport in the fluvial systems. The aim of this work is to investigate the transport of the coarse sediment material in a steep alpine stream, using the bedload tracking. The Rio Cordon is a typical alpine channel, located in the northeast of Italy. It is characterized by a rough streambed with a prevalent boulder-cascade and step pool morphology. Since 2011, 250 clasts equipped with Passive Integrated Transponders (PIT) were installed in the main channel, to analyze their mobility along a reach 320 m long. From November 2012 to August 2015, the transport induced by a range of hydraulic forcing between 0.44 m3 s-1 and 2.10 m3 s-1 was assessed by 10 PIT-surveys. First, the mobility expressed by the tracers was analyzed, observing marked differences in terms of travel distance. Then, the average recovery rate achieved during the tracer inventories (Rr &gt; 70%) permitted to define the threshold discharge for each grain size class analyzed and, then, to assess the virtual velocity experienced by the tracers.

Flow Resistance in Step‐Pool Rills

Core Ideas A new theoretical flow resistance equation is proposed for step‐pool rills. In step‐pool rills, total resistance is higher than grain resistance in rill flows. Step pools in rills do not form as antidunes. Rills evolve morphologically, and the adjustment of rill channel geometry to flow affects the relationships among velocity, discharge, and slope. The resistance to flow in step‐pool rills is mainly due to form‐induced mechanisms and, in comparison, grain resistance is of minor significance. Previous studies on rill flow resistance have been performed exclusively for grain‐resistance conditions and use a stream flow equation. In this study, a new flow resistance equation, deduced by applying dimensional analysis and self‐similarity theory, was applied to rill flow in step‐pool channels. First, the incomplete self‐similarity hypothesis was used for establishing a power flow velocity profile whose integration gives the theoretical expression of the Darcy–Weisbach friction factor. Then the deduced theoretical resistance equation was tested by measurements of flow velocity, water depth, channel width, and bed slope performed in 49 reaches of some rills having a step‐pool bed. A relationship between the velocity profile, the channel slope, and the flow Froude number was established. The analysis showed that the Darcy–Weisbach friction factor can be accurately estimated by the proposed theoretical approach based on a power‐velocity profile. The Darcy–Weisbach friction factor values related to rills with step‐pool sequences, on average, are higher than those related to rills without step pools, signifying that rills where step‐pool sequences occur are characterized by a total resistance (grain and morphological) that is higher than grain resistance. Finally, this investigation showed that step pools that maximize flow resistance may develop at Froude numbers below the range at which step pools may form as antidunes.

Morphological dynamics of an englacial channel

Abstract. Despite an interest in the hydraulic functioning of supraglacial and englacial channels over the last 4 decades, the processes and forms of such ice-bounded streams have remained poorly documented. Recent glaciological research has demonstrated the potential significance of so-called "cut-and-closure" streams, where englacial or subglacial flow paths are created from the long-term incision of supraglacial channels. These flow paths are reported to exhibit step-pool morphology, comprising knickpoints and/or knickzones, exaggerated in dimensions in comparison to supraglacial channels. However, little is known of the development of such channels' morphology. Here, we examine the spatial organisation of step pools and the upstream migration of steps, many of which form knickzones, with repeated surveys over a 10-year period in an englacial conduit in cold-based Austre Brøggerbreen, Svalbard. The observations show upstream step recession to be the dominant process for channel evolution. This is paralleled by an increase in average step height and conduit gradient over time. Characteristic channel-reach types and step-riser forms are consistently observed in each of the morphological surveys reported. We suggest that the formation of steps has a hydrodynamic origin, where step-pool geometry is more efficient for energy dissipation than meanders. The englacial channel system is one in rapid transition towards a quasi-equilibrium form within a decadal timescale. The evolution and recession of knickzones reported here result in the formation of a 37 m deep moulin shaft, suggesting that over time an incising supraglacial channel may evolve towards an englacial channel form exhibiting a stable end-point characterised by a singular vertical descent, which potentially can reach the glacier bed. This challenges the prevailing notions that crevasses or hydrofractures are needed to form deep moulins. Our observations highlight the need to further examine the adjustment processes in cut-and-closure channels to better understand their coupling to supraglacial meltwater sources and potential significance in cold-based glacier hydrology and ice dynamics.

Do bio-physical attributes of steps and pools differ in high-gradient mountain streams?

The hydrologic and physical structure of streams strongly influences the biological composition of benthic macroinvertebrate communities. Research on step–pool systems in high-gradient streams has focused primarily on physical processes rather than on ecological characteristics. This study examined both the biological and physical attributes of 27 step–pool sequences in three steep mountain streams of the Smith River Basin in northern California, USA. Multivariate analysis using non-metric multidimensional scaling (NMS) found a biological separation of step and pool sites (based on benthic macroinvertebrate data) in all three study watersheds. Step habitats had greater taxa richness, diversity, %Plecoptera, %Heptageniidae, %Nemouridae, and %clingers compared to pools. Steps also differed from pools in physical characteristics such as grain size distribution, whereby steps were dominated by boulders compared to pools characterized by gravel and cobbles. Moreover, steps had higher dissolved oxygen, greater water velocity, and shallower water depths compared to pools. NMS ordinations showed a correlation between physical factors and biological communities. These results suggest the ecological importance of step–pools streams, in that the development of step sequences creates and maintains a repetitive pattern of high-quality ecological environments.

Hitting rock bottom: morphological responses of bedrock-confined streams to a catastrophic flood

Abstract. The role of extreme events in shaping the Earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how semi-alluvial channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 168 km2 show that the predicted discharge for the 2011 event ranged from 415 to 933 m3 s−1, with unit stream power estimates of up to 1077 W m−2. Estimated entrainment relationships predict the mobility of the entire grain-size population, and field data suggest the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat lidar data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of coarse-grained alluvium with a strong positive relationship between catchment area and normalised erosion (R2 = 0.72–0.74). The extensive scouring in the 2011 flood decreased thalweg variance significantly removing previous step pools and other coarse-grained in-channel units, forming lengths of plane-bed (cobble) reach morphology. This was also accompanied by the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps. Post-flood field data indicate a slight increase in thalweg variance as a result of the smaller 2013 flood rebuilding the alluvial overprint with pool-riffle formation. However, the current form and distribution of channel morphological units does not conform to previous classifications of bedrock or headwater river systems. This variation in post-flood form indicates that in semi-alluvial systems extreme events are significant for re-setting the morphology of in-channel units and for exposing the underlying lithology to ongoing erosion.

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

AbstractWe 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 D90s 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 &amp; Sons, Ltd.