Step-pool Channels Research Articles

Flow resistance in a subtropical canyon river

We conducted several field surveys to measure velocity and channel geometry in 15 cross-sections distributed along three reaches in the Boi River (Southern Brazil), a canyon river that belongs to the most extensive canyon chain in South America. The three reaches present, respectively, canyon, transition, and floodplain landscapes. The resulting data were used to compute the flow resistance. A downstream hydraulic geometry analysis indicates that the exponent describing the velocity increases with the discharge and that these values were between 0.3043 and 0.3507. A downstream hydraulic geometry analysis indicates that changes in velocity with discharge are greater than the rate of change of width or depth. It is typically found in steep mountain streams laterally confined, and it was verified in the canyon reach. However, despite the transition presents a considerable lateral constriction and the floodplain reach presents mountain river characteristics, both reaches did not present such behavior. The findings are coherent with other mountain river datasets, although the Boi River presents distinct geomorphic units such as glide-runs. In contrast, other available datasets are usually characterized by step-pool channels. Flow resistance in the canyon and transition reaches differ in many ways from floodplain reach, indicating larger resistance inside the canyon than outside, probably related to the landform and grain size in the canyon. Regression analysis of the combined field dataset indicated that dimensionless unit discharge is an important independent variable to explain variations in velocity and flow resistance. We also verified that the use of Rickenmann's Equation can be helpful to estimate velocity in canyon rivers. Our findings include: i) canyon landscape presents higher friction factors than floodplain landscape; and ii) canyon and transition landscapes are similar in terms of flow resistance during high-flow conditions, while floodplain present remarkably differences.

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.

Wood controls on pool spacing, step characteristics and sediment storage in headwater streams of the northwestern Cascade Mountains

Wood influences channel morphology in headwater streams by creating steps and pools in the longitudinal profile and trapping sediment. We used field data from 32 headwater stream reaches in the northwestern Cascade Mountains to test the hypotheses that 1) larger diameter classes of large wood (LW, defined as pieces with diameter>10cm and length>0.5m) are critical step-keying materials in headwater channels despite narrow channel widths (1–4m), 2) wood-keyed steps trap more sediment than clast- and root-keyed steps, and 3) the negative relationship between LW frequency and the distance between pools observed elsewhere in large streams extends to headwater streams. We found that the frequency of step key pieces peaked in the 20–40cm diameter class. Similarly, 40–100cm diameter pieces were disproportionately associated with key-piece function compared to their overall association with step formation (wood with diameter<10cm was an important step-keying material in channels of width<2m). Steps keyed by wood were significantly more likely to store sediment than clast- or root-keyed steps. In contrast to previous work that did not detect a relationship between wood loading and pool habitat in step-pool channels, pool spacing ranged between ∼17 and 1.5 channel widths with an apparent relationship described by a negative exponential function of LW frequency (RMSE=2.85 channel widths/pool), although the range of LW frequency and the functional relationship were different than those of lower gradient channels. Additionally, linear and nonparametric models demonstrate that sediment storage and step and pool characteristics are related to wood loading and stand history with important distinctions based on channel width. These results confirm the importance of maintaining riparian buffers sufficient to provide functional wood to headwater streams.

Interacting geomorphic and ecological response of step-pool streams after wildfire

AbstractDetailed study of the response of step-pool mountain channels to wildfire is rare despite increasing vulnerability of these systems. This paper reports the initial biogeomorphic response of step-pool streams to storms following the 2012 Waldo Canyon Fire, Colorado, USA. Field surveys, benthic macroinvertebrate sampling, and light detection and ranging (LiDAR) terrestrial laser scanning through 2014 generated a substantial data set comparing changes within seven channels burned by a range of severity with three unburned reference reaches. Results showed that wildfire increased the susceptibility of step-pool channels to destabilize according to the severity of burn. Whereas the step-pool morphology (step height H, step length L, and H/L) remained relatively unchanged in unburned and low-severity burned channels—even through an extreme rainfall event with recurrence intervals up to 500–1000 years—study sites affected by high burn severity altered significantly following the first comparatively minor storms. Study channels burned by moderate severity exhibited variable responses ranging from minor to substantial morphological changes, suggesting that additional factors, including slope, local topography, and proximity to other highly disturbed sites may play explanatory roles. The ecological response corroborated the geomorphic findings, with data for benthic macroinvertebrates varying according to burn severity along with the changing post-fire step-pool morphology. Ordination analysis enabled data synthesis and showed that precipitation intensity (r = 0.59) and the severity of burn (r = 0.60) together influenced the interacting morphologic and ecological responses after the Waldo Canyon Fire. Variations in H (r = 0.72), L (r = 0.56), and thalweg elevation (r = 0.75) correlated the most with the types of benthic macroinvertebrates present. Following the Waldo Canyon Fire, the highest rainfall intensities coincidentally occurred over study sites in high-severity burn areas. Although disentangling the effects of rainfall intensity and burn severity proved difficult given this scenario, the interacting and complementary roles of burn severity and rainfall intensity likely led to the greatest morphologic and ecological impacts at those sites. Findings offer insights for the post-fire management of step-pool streams that characterize frequently burned mountain areas. As wildfires grow in frequency and magnitude under warming regimes, management becomes increasingly important for anticipating future changes in burned river landscapes, mitigating potential floods and hazards, and promoting sustainable river ecosystems.

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.

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.

Effective discharge in Rocky Mountain headwater streams

Summary Whereas effective discharge ( Q eff ) in mountain streams is commonly associated with a moderate flow such as bankfull discharge ( Q bf ), this study found that the maximum discharge ( Q max ), and not bankfull discharge, is the channel forming or effective flow for gravel transport in plane-bed streams where partial bed mobility causes steep gravel transport rating curves. Q eff may approach bankfull flow in some step-pool channels where gravel moves over a static cobble/boulder bed. Our conclusions are based on magnitude–frequency analyses conducted at 41 gauged Rocky Mountain headwater streams. Because these gauged streams lacked gravel transport data, as is typical, comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics. Those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites. The measured flow frequency distributions were characterized by two fitted power functions. The steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude–frequency analyses to compute Q eff .

Comparison of different methods to predict the mean flow velocity in step-pool channels

Steep mountain streams have irregular bed topography, where the mean flow velocity is heavily affected by the coarsest bed components and by their arrangement to form step pools, cascades, and rapids. According to literature findings the mean flow velocity is often related with water discharge, channel slope, and grain-size related variables through power relationships. Several approaches consider dimensionless hydraulic geometry terms to develop the analysis over a wide range of channel sizes and hydraulic conditions. The aim of this research is to test the performance of some literature formulas to directly compute the mean flow velocity (V) in step-pool sequences. The study area deals with two fish ladders located in the Vanoi torrent (Trento Province, Italy), which were built by mimicking the steppool morphology. Three reaches were selected to cover different channel slopes (2.6-10%). Data collection entailed three main phases: (1) topographical surveys, (2) granulometric analysis, and (3) flow discharge measurements (salt dilution method). Geometric and hydraulic variables were measured for the following step-pool cross sections: step head, pool center, and intermediate position between pool end next step. Particular attention has been reserved to determine the effective mean flow velocity over the whole path of each step pool sequence. The performance of different literature equations to predict V has been verified. The relations have been shared in three groups: dimensional (V), dimensionless with respect to the grain size (V*) or to a combination of grain size and slope (V**). In general, the V group of equations has produced the highest errors between computed and measured values. The dimensionless V*, V** groups have shown the best performance. In particular the V* equations, which use unit discharge and channel slope, have provided the better fitting, and the lowest root mean square error. The results highlight the difficult to estimate flow velocity in step-pool sequences, and the attitude of this channel-bed morphology to be highly dissipative. The good performance of some dimensionless equations to predict V could also support the hydraulic designer in case the ‘morphological rebuilding’ of mountain creeks is opportune. Further analyses are required to better understand the flow behavior in streams where very rough bed forms and hydraulic drops are the primary sources of flow energy dissipation.

Comparison of different methods to predict the mean flow velocity in step-pool channels

Steep mountain streams have irregular bed topography, where the mean flow velocity is heavily affected by the coarsest bed components and by their arrangement to form step pools, cascades, and rapids. According to literature findings the mean flow velocity is often related with water discharge, channel slope, and grain-size related variables through power relationships. Several approaches consider dimensionless hydraulic geometry terms to develop the analysis over a wide range of channel sizes and hydraulic conditions. The aim of this research is to test the performance of some literature formulas to directly compute the mean flow velocity (V) in step-pool sequences. The study area deals with two fish ladders located in the Vanoi torrent (Trento Province, Italy), which were built by mimicking the steppool morphology. Three reaches were selected to cover different channel slopes (2.6-10%). Data collection entailed three main phases: (1) topographical surveys, (2) granulometric analysis, and (3) flow discharge measurements (salt dilution method). Geometric and hydraulic variables were measured for the following step-pool cross sections: step head, pool center, and intermediate position between pool end next step. Particular attention has been reserved to determine the effective mean flow velocity over the whole path of each step pool sequence. The performance of different literature equations to predict V has been verified. The relations have been shared in three groups: dimensional (V), dimensionless with respect to the grain size (V*) or to a combination of grain size and slope (V**). In general, the V group of equations has produced the highest errors between computed and measured values. The dimensionless V*, V** groups have shown the best performance. In particular the V* equations, which use unit discharge and channel slope, have provided the better fitting, and the lowest root mean square error. The results highlight the difficult to estimate flow velocity in step-pool sequences, and the attitude of this channel-bed morphology to be highly dissipative. The good performance of some dimensionless equations to predict V could also support the hydraulic designer in case the ‘morphological rebuilding’ of mountain creeks is opportune. Further analyses are required to better understand the flow behavior in streams where very rough bed forms and hydraulic drops are the primary sources of flow energy dissipation.

Geometric scaling of step-pools in mountain streams: Observations and implications

Understanding the geometric structuring of alluvial step-pools is an enduring problem in mountain stream geomorphology. Many previous studies propose that the key control parameter governing the step-pool form is mean bed slope. As explored through several examples, however, we find that slope control formulations are inconsistently demonstrated for broad data sets. On this basis we reexamine the bed form architecture of step-pools with a new descriptive parameter termed the aspect ratio ( α): active stream width divided by step drop height. We test α with data collected from three different mountain settings for step-pools observed over mean bed slopes ranging from 2% to 22%. We find that step wavelength and height are well described by α when these dimensions are normalized by the step drop height; furthermore, we observe that α varies inversely with mean bed slope. Context for our findings is provided by recent field and laboratory work which highlights that the cross-stream component to the overall flow structure in step-pools and scour pools is significant. This is important because it expands upon, and relates our work to formative models which are based on jet scour, and the associated three-dimensional flow structure of step-pools. Our work has practical implications for the design of step-pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry. We observe, however, that under certain conditions this approach may yield constructed step-pools that are unstable. As an alternative approach, we propose that the geometric relationships developed here can be applied to identify artificial step-pool geometries that will be more fully reflective of natural analogs, and thus possibly more stable.

Reply to comment by Keith Richardson on “Flow regimes, bed morphology, and flow resistance in self‐formed step‐pool channels”

Reply to comment by Keith Richardson on “Flow regimes, bed morphology, and flow resistance in self‐formed step‐pool channels”

Geomorphic implications of hydroclimatic differences among step-pool channels

We used nine step-pool channel segments from six regions to examine the magnitude, frequency, and duration of flows hypothetically capable of mobilizing the D50 and D84 of streambed sediment. The six regions examined represent different hydroclimatic regimes: Switzerland (snowmelt runoff and moderate-intensity summer rainfall), Puerto Rico (runoff from high-intensity, short-duration rainfall throughout the year), Colorado (snowmelt runoff), Oregon and northern California (low-intensity winter rainfall runoff), and Idaho (snowmelt and rain-on-snow). We hypothesize that differences in hydroclimatology will correlate with differences in the magnitude and frequency of sediment mobilization such that the tropical site with high-intensity rainfall and abundant total precipitation will have the greatest magnitude and frequency of sediment mobilization, whereas the snowmelt site will have the lowest magnitude and frequency of sediment mobilization but relatively high values of cumulative sediment mobility as a result of longer duration of flows above the threshold of sediment mobilization. The results partially support this hypothesis. The tropical site does have significantly greater magnitude and frequency of exceedence for flows hypothetically capable of mobilizing coarse sediment and the snowmelt site has substantially lower values, but the tropical site also has the highest value of cumulative sediment mobility. ANOVA models indicate that total precipitation and precipitation intensity are the best predictors of magnitude and frequency of exceedence of threshold discharge and cumulative mobility of coarse sediment.

Bed morphology and generation of step–pool channels

AbstractFlume experiments have been carried out to study the formation processes and the bed morphology of step–pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step–pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sand‐bed rivers. This observation points out that step–pool field‐data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand‐ and gravel‐bed rivers or step–pool units in boulder‐bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self‐stabilization by modifying the step–pool structure towards a geometry that provides maximum flow resistance and maximum bed stability. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Identification of steps and pools from stream longitudinal profile data

Field research on step–pool channels has largely focused on the dimensions and sequence of steps and pools and how these features vary with slope, grain sizes and other governing variables. Measurements by different investigators are frequently compared, yet no means to identify steps and pools objectively have been used. Automated surveying instruments record the morphology of streams in unprecedented detail making it possible to objectively identify steps and pools, provided an appropriate classification procedure can be developed. To achieve objective identification of steps and pools from long profile survey data, we applied a series of scale-free geometric rules that include minimum step length (2.25% of bankfull width ( W b)), minimum pool length (10% of W b), minimum residual depth (0.23% of W b), minimum drop height (3.3% of W b), and minimum step slope (10° greater than the mean slope). The rules perform as well as the mean response of 11 step–pool researchers who were asked to classify four long profiles, and the results correspond well with the channel morphologies identified during the field surveys from which the long profiles were generated. The method outperforms four other techniques that have been proposed. Sensitivity analysis shows that the method is most sensitive to the choice of minimum pool length and minimum drop height. Detailed bed scans of a step–pool channel created in a flume reveal that a single long profile with a fixed sampling interval poorly characterizes the steps and pools; five or more long profiles spread across the channel are required if a fixed sampling interval is used and the data suggest that survey points should be located more frequently than the diameter of the step-forming material. A single long profile collected by a surveyor who chooses breaks in slope and representative survey points was found to adequately characterize the mean bed profile.

The role of morphology on the displacement of particles in a step–pool river system

Although studies of sediment transport in steep and coarse-grained channels have been more numerous in recent years, the dynamics of sediment transport in step–pool river systems remain poorly understood. This paper investigates displacements of individual clasts through Spruce Creek (Québec, Canada), a classic step–pool channel, and the effects of the channel morphology on the path length of the clasts. Passive integrated transponder tags (PIT) were used to track the displacement of 196 individual particles over a range of discharges including the bankfull stage. Clasts were tracked after five sequences of flood events. The results showed that the distance distributions match a two-parameter Gamma model. Equal mobility transport occurs for the particle size investigated during each sequence of flood events. Mean travel distance of the clasts can be estimated from excess stream power, and the mobility of the clasts is more than an order of magnitude less than the model reported in riffle–pool channels. The dominant morphological length scale of the bed also controls the path length of the clasts. These results confirm some preliminary observations on sediment transport in step–pool channels.

Reach-scale channel geometry of mountain streams

The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength ( H/ L). Pool-riffle channels have greater width/depth ratios and relative grain submergence ( R/ D 84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence ( R/ H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.

Field measurements of three-dimensional hydraulics in a step-pool channel

We investigated the effects of morphologic position and discharge on flow structure in a steep (0.10 m/m) mountain channel by collecting three-dimensional measurements of time-averaged and turbulent velocity components with a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter) on a 30-m reach of a step-pool channel in the Colorado Rockies. Velocity profiles were measured at morphologic positions characteristic of steep channels (above steps, step lips, base of steps, pools, cascades, runs), and at five different discharges. A marked three-dimensionality of flow structure was documented in East St. Louis Creek. Velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes; cross-stream and vertical components contributed averages of 20% and 15%, respectively, to overall vector magnitudes. Turbulence intensities were especially multi-dimensional, however, with large contributions to turbulent kinetic energy from the vertical component of velocity. Analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities, with substantially higher velocities upstream from steps than in pools. Discharge and morphology effects on cross-stream and vertical velocity components, however, were not significant. Discharge and morphologic position also significantly affected turbulence intensities for all flow components, with the greatest turbulence intensities occurring in pools and at high discharges. These results illustrate the strong discharge-dependence of hydraulics in step-pool channels, where relative submergence of bedforms changes rapidly with discharge, and the substantial spatial variation in hydraulics created by step-pool sequences.

Local scouring and morphological adjustments in steep channels with check-dam sequences

This paper describes bed profile and grain size distribution adjustments in a mountain river (Maso di Spinelle River, Italian Alps) stabilized by a sequence of boulder check-dams. The control works were originally designed to simulate the geometry of natural step-pool channels, where tumbling flow is the dominant hydraulic regime. Local scouring downstream of 29 drop structures is analysed through the use of nondimensional parameters where maximum scour depth and scour length are normalised to the drop height. Prior laboratory data reveal a pattern similar to field scours, where complex interactions occur between drop height, critical flow depth, and step spacing. The linkage between scour length and depth is also discussed. There seems to be a maximum step height for impinging jets that is approximately twice the drop height; this maximum may explain the upper limit of the steepness factor found in high-gradient step-pool streams. If such a maximum upper limit is confirmed by further studies, this may aid designs of foundation heights for transverse control works in steep channels.

Large woody debris and flow resistance in step-pool channels, Cascade Range, Washington

Total flow resistance, measured as Darcy–Weisbach f, in 20 step-pool channels with large woody debris (LWD) in Washington, ranged from 5 to 380 during summer low flows. Step risers in the study streams consist of either (1) large and relatively immobile woody debris, bedrock, or roots that form fixed, or “forced,” steps, or (2) smaller and relatively mobile wood or clasts, or a mixture of both, arranged across the channel by the stream. Flow resistance in step-pool channels may be partitioned into grain, form, and spill resistance. Grain resistance is calculated as a function of particle size, and form resistance is calculated as large woody debris drag. Combined, grain and form resistance account for less than 10% of the total flow resistance. We initially assumed that the substantial remaining portion is spill resistance attributable to steps. However, measured step characteristics could not explain between-reach variations in flow resistance. This suggests that other factors may be significant; the coefficient of variation of the hydraulic radius explained 43% of the variation in friction factors between streams, for example. Large woody debris generates form resistance on step treads and spill resistance at step risers. Because the form resistance of step-pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height, we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel. Hence, the distribution and function, not just abundance, of large woody debris is critical in steep, step-pool channels.

The periodic nature of step-pool mountain streams

Rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers, but whether periodicities exist in the occurrence of step-pools has not been investigated in a comprehensive way, and the implications for such periodicities have not been explored. Spectral analysis was applied to bed elevation data from streams in the Santa Monica Mountains of southern California to investigate periodicity in the occurrence of step-pools. Results indicate significant step-pool periodicities, confirmed by independent field measurements, that vary from 0.43 channel widths to 2.40 channel widths. Spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step-pool spacing. Underlying periodicities are nevertheless de- tected, suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms. Periodicity in the occurrence of step-pools suggests that step-pools are bedforms that represent a fundamental mutual adjustment between flow, channel morphology, and energy expenditure. A rhythmic channel thalweg supports the notion that step- pools are analogous to meandering in the vertical dimension, and implies that, because step-pool channels are not able to adjust energy expenditure in the plane dimension, they instead adjust boundary roughness in the vertical dimension. A continuum of rhythmic gravel bedforms that displays the results of such adjustments, and that shows how step-pools merge into pools and riffles in the stream channel system, is discussed and illustrated with empirical data from the Santa Monica Mountains. Although many differences exist between steep mountain streams and broad alluvial rivers, the findings of this study suggest that generalized principles of uniform energy expendi- ture may also apply to rhythmic step-pool streams. These findings are potentially valuable in the management and design of steep channels.