Cobble-bed Rivers Research Articles

Hydrogeomorphic Scaling and Ecohydraulics for Designing Rescaled Channel and Floodplain Geometry in Regulated Gravel–Cobble Bed Rivers for Pacific Salmon Habitat

Societies are increasingly restoring and/or rehabilitating rivers below dams for keystone species such as salmon. A fundamental concept for rehabilitating river morphology below dams for salmon is that a rescaled version of the river corridor synchronized to the regulated flow regime can restore habitat quantity and quality. Downscaled and resized hydrographs have been shown to provide environmental benefits to fish communities including salmon as well as riparian vegetation communities. However, less research exists on how this can be achieved through the topographic rescaling of heavily modified and regulated river corridors. The goal of this paper is to review analytical methods to determine initial of size of rescaled channel and floodplain mesohabitat units in regulated gravel–cobble bed rivers for Pacific salmon (Oncorhynchus spp.) habitat using hydrogeomorphic scaling and ecohydraulics. Hydrogeomorphic flow scaling is the prediction of river morphology and geometry using empirical and analytical relationships. Ecohydraulic scaling refers to the use of ecohydrology, habitat suitability curves, and fish density relationships to determine the size of mesohabitat units for ecologically relevant flows. In practice, these are complimentary first order estimates of channel and floodplain configurations followed by iterative design in a hierarchical manner. This review advances the science of river design by synthesizing these complimentary ideologies for Pacific salmon habitat restoration in regulated rivers. Following the review, the layout of features is briefly discussed followed by a discussion of important considerations beyond the physical and topographic rescaling of river corridors for salmonid habitat restoration.

Fuzzy map comparisons enable objective hydro‐morphodynamic model validation

SummaryNumerical modeling represents a state‐of‐the‐art technique to simulate hydro‐morphodynamic processes in river ecosystems. Numerical models are often validated based on observed topographic change in the form of pixel information on net erosion or deposition over a simulation period. When model validation is performed by a pixel‐by‐pixel comparison of exactly superimposed simulated and observed pixels, zero or negative correlation coefficients are often calculated, suggesting poor model performance. Thus, a pixel‐by‐pixel approach penalizes quantitative simulation errors, even if a model conceptually works well. To distinguish between reasonably well‐performing and non‐representative models, this study introduces and tests fuzzy map comparison methods. First, we use a fuzzy numerical map comparison to compensate for spatial offset errors in correlation analyses. Second, we add a level of fuzziness with a fuzzy kappa map comparison to additionally address quantitative inaccuracy in modeled topographic change by categorizing data. Sample datasets from a physical lab model and datasets from a 6.9 km long gravel–cobble bed river reach enable the verification of the relevance of fuzzy map comparison methods. The results indicate that a fuzzy numerical map comparison is a viable technique to compensate for model errors stemming from spatial offset. In addition, fuzzy kappa map comparisons are suitable for objectively expressing subjectively perceived correlation between two maps, provided that a small number of categories is used. The methods tested and the resulting spatially explicit comparison maps represent a significant opportunity to improve the evaluation and potential calibration of numerical models of river ecosystems in the future.

Reach- and mat-scale differences in Microcoleus autumnalis (cyanobacterium) accrual along velocity and nitrate gradients in three New Zealand rivers

Proliferations of the toxic, mat-forming cyanobacterium Microcoleus autumnalis are an increasingly recognized problem in cobble-bed rivers worldwide. This study explored how flow and nutrient concentrations influence mat expansion. Microcoleus autumnalis was inoculated into cobbles placed in runs, riffles, and pools in three rivers with different nutrient conditions, and mat size was monitored over 21 days. The following hypotheses were tested: (i) mat expansion will reflect cover increases at the reach scale; (ii) biomass and cover will be highest in high-velocity habitats; and (iii) under similar velocities, nutrient concentrations will be more important than other abiotic and biotic variables in determining expansion rates. Mat expansion accurately reflected the increase in reach-scale cover, and expansion was most rapid at intermediate water velocities (0.25–0.45 m·s–1). Mats persisted the longest in riffles. Accrual cycles were terminated earlier in runs than in riffles, as high expansion rates resulted in patches reaching maximum mat size rapidly. Although M. autumnalis accrual differed among rivers, this was attributed to differences in shear stress and grazing pressure rather than nutrient concentrations.

Non-uniform flow over cobble bed with submerged vegetation strip

To better understand the influence of non-uniform flow in coarse-bed streams with a submerged vegetation strip on flow characteristics, data were collected from experiments in a laboratory flume and field investigations in a cobble-bed river reach in central Iran. The results of the experiments and field observations showed that the velocity profiles for non-uniform flows in cobble-bed streams with vegetation strips can be divided into either two or three sub-zones. The velocity profiles of non-uniform flows in the laboratory flume and cobble-bed stream deviated from the log law in the wake and mixing zones, but fitted approximately in the log-law layer zone. The distribution of Reynolds stress for non-uniform flows was influenced by the vegetation strip and showed a non-concave shape. Near the channel bed, sweep motion occurred more frequently than the ejection process. Close to the top of vegetation strip, the correlation coefficient (ru′w′) was negative, indicating downward momentum transport due to sweep and ejection processes. However, ru′w′was positive near the water surface, indicating upward momentum by the vegetation strip due to inward and outward interactions. The study shows that laboratory results cannot be easily applied to natural rivers without considerable assumptions.

Responses of macroinvertebrate assemblages to environmental variations in the river-oxbow lake system of the Zoige wetland (Bai River, Qinghai-Tibet Plateau)

The biodiversity value of river-oxbow lake systems in high plateau peatland has been little recognized, and there are many gaps in our understanding of their ecology. In this study, we investigated the river-oxbow lake system of the Bai River basin, the main tributary of the Yellow River Source in the Zoige wetland from 2015 to 2016, in attempt to show how the environmental variations, especially hydrological connectivity and macrophyte biomass in the river-oxbow lake system influenced macroinvertebrates. Habitat patches were investigated in 11 river cross-sections and 18 oxbow lakes in the Bai River basin. Through hierarchical clustering and non-metric multidimensional scaling, four main types of habitats were identified in the river-oxbow lake system in the plateau: sand-bed river, cobble-bed river, sparse-macrophyte oxbow lake, and luxuriant-macrophyte oxbow lake. The luxuriant-macrophyte oxbows were characterized by high dissolved oxygen concentrations, alkalinity, and higher macroinvertebrate richness, density, biomass, and the Improved Shannon-Wiener Index in comparison to the other habitat types. Additionally, influential patterns of environmental variables on macroinvertebrates were analyzed using redundancy analysis. Lasso regression models were established to describe how macroinvertebrate density responded to macrophyte biomass and other variables, and how macrophyte biomass responded to hydrological connectivity and oxbow size. It was revealed that reduced hydrological connectivity and reduced oxbow size played important roles in increasing the biomass of submerged macrophyte, and dense macrophyte was directly responsible for the high biodiversity of macroinvertebrates. Different from the commonly believed unimodal influential pattern that medium hydrological connectivity supports the highest biodiversity in oxbow lakes reported in previous studies, macroinvertebrates in the high plateau river-oxbow lake systems benefited from low connectivity and reduced size. Oxbow lakes, especially those covered with luxuriant macrophytes, diversified the macroinvertebrate assemblages and enhanced primary consumer biomass at the regional scale.

Scour from the interstitial spaces in cobble-bed rivers

The periodic removal of sand from the interstitial spaces between cobbles is extremely important for ecosystem functioning in cobble-bed rivers. In order to flush fine sediments from the interstitial spaces between cobbles in river reaches downstream of dams, specific dam releases known as flushing flows or sediment maintenance flows are utilised. This paper describes the development and calibration of a mathematical model to predict the equilibrium depth of scour of fine sands from between cobbles in terms of applied stream power principles. The model was developed with the aid of physical model experiments and is founded on a stream power model which defines the condition of dynamic equilibrium in a deformed sand-bed river. Calibration was done in the laboratory under clear water conditions and with uniform cobble sizes. The scouring of fine sands in cobble-bed rivers is associated with an increase in absolute bed roughness and an associated decrease in the unit stream power applied along the bed as the cobbles become exposed. When scour ceases, the sand particles on the bed are at the movement threshold and critical conditions exist. In order to establish the relationship between equilibrium scour depth and bed particle characteristics, the power which is required to suspend sand particles under laminar boundary conditions is equated with the turbulent power being applied along the bed.

Assessment of erosion and settling properties of fine sediments stored in cobble bed rivers: the Arc and Isère alpine rivers before and after reservoir flushing

AbstractCohesive sediment dynamics in mountainous rivers is poorly understood even though these rivers are the main providers of fine particles to the oceans. Complex interactions exist between the coarse matrix of cobble bed rivers and fine sediments. Given that fine sediment load in such environments can be very high due to intense natural rainfall or snowmelt events and to man‐induced reservoir or dam flushing, a better understanding of the deposition and sedimentation processes is needed in order to reduce ecohydrological downstream impacts. We tested a field‐based approach on the Arc and Isère alpine rivers combining measurements of erosion and settling properties of river bed deposits before and after a dam flushing, with the U‐GEMS (Gust Erosion Microcosm System) and SCAF (System Characterizing Aggregates and Flocs), respectively. These measurements highlight that critical shears, rates of erosion, settling velocities and propensity of particles to flocculate are highly variable in time and space. This is reflective of the heterogeneity of the hydrodynamic conditions during particle settling, local bed roughness, and nature and size of particles. Generally the deposits were found to be stable relative to what is measured in lowland rivers. It was, however, not possible to make a conclusive assessment of the extent to which the dynamics of deposits after reservoir flushing were different from those settled after natural events. The absence of any relationships between erosion and deposition variables, making it impossible to predict one from another, underlined the need to measure all of them to have a full assessment of the fine sediment dynamics and to obtain representative input variables for numerical models. While the SCAF was found to be effective, an alternative to the U‐GEMS device will have to be found for the erodibility assessment in cobble bed rivers, in order to make more rapid measurements at higher shears. Copyright © 2017 John Wiley & Sons, Ltd.

The Abundance of Toxic Genotypes Is a Key Contributor to Anatoxin Variability in Phormidium-Dominated Benthic Mats.

The prevalence of benthic proliferations of the anatoxin-producing cyanobacterium Phormidium are increasing in cobble-bed rivers worldwide. Studies to date have shown high spatial and temporal variability in anatoxin concentrations among mats. In this study we determined anatoxin quotas (toxins per cell) in field samples and compared these results to the conventionally-used concentrations (assessed per dry weight of mat). Three mats were selected at sites in two rivers and were sampled every 2–3 h for 24–26 h. The samples were lyophilized and ground to a fine homogenous powder. Two aliquots of known weights were analyzed for anatoxin congeners using liquid chromatography-mass spectrometry, or digital droplet PCR with Phormidium-specific anaC primers to measure absolute quantities of gene copies. Anatoxin concentrations in the mats varied 59- and 303-fold in the two rivers over the study periods. A similar pattern was observed among gene copies (53- and 2828-fold). When converted to anatoxin quotas there was markedly less variability (42- and 16-fold), but significantly higher anatoxin quotas were observed in mats from the second river (p < 0.001, Student’s t-test). There were no obvious temporal patterns with high and low anatoxin concentrations or quotas measured at each sampling time and across the study period. These results demonstrate that variability in anatoxin concentrations among mats is primarily due to the abundance of toxic genotypes. No consistent modulation in anatoxin production was observed during the study, although significant differences in anatoxin quotas among rivers suggest that site-specific physiochemical or biological factors may influence anatoxin production.

Spatiotemporal dynamics of Phormidium cover and anatoxin concentrations in eight New Zealand rivers with contrasting nutrient and flow regimes

Toxic benthic cyanobacterial proliferations, particularly of the genus Phormidium, are a major concern in many countries due to their increasing extent and severity. The aim of this study was to improve the current understanding of the dominant physicochemical variables associated with high Phormidium cover and toxin concentrations. Phormidium cover and anatoxin concentrations were assessed weekly for 30weeks in eight predominately cobble-bed rivers in the South Island of New Zealand. Phormidium cover was highly variable both spatially (among and within sites) and temporally. Generalized additive mixed models (GAMMs) identified site, month of the year, conductivity and nutrient concentrations over the accrual period as significant variables associated with Phormidium cover. Cover was greatest under low to intermediate accrual dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) concentrations. Accrual nutrients had a strong, negative effect on cover at concentrations>0.2mgL−1 DIN and 0.014mgL−1 DRP. The effect of flow was generally consistent across rivers, with cover accruing with time since the last flushing flow. Total anatoxins were detected at all eight study sites, at concentrations ranging from 0.008 to 662.5mgkg−1 dried weight. GAMMs predicted higher total anatoxin concentrations between November and February and during periods of accrual DRP<0.02mgL−1. This study suggests that multiple physicochemical variables may influence Phormidium proliferations and also evidenced large site-to-site variability. This result highlights a challenge from a management perspective, as it suggests that mitigation options are likely to be site-specific.

Effect of river flow, temperature, and water chemistry on proliferations of the benthic anatoxin-producing cyanobacterium Phormidium

AbstractProliferations of the benthic anatoxin-producing cyanobacterium Phormidium are increasing in prevalence in cobble-bed rivers worldwide. This proliferation is of particular concern when rivers are used as sources of drinking water or for recreation. Little is known about the physicochemical variables promoting proliferations, and our existing knowledge is based on data from only a few rivers. We assessed Phormidium cover, physicochemical variables, and anatoxin concentrations at 10 sites in 7 New Zealand rivers every week for 2 y. Generalized additive mixed models (GAMMs) identified dissolved inorganic N (DIN) over the accrual period 15°C, and conductivity as having positive and statistically significant effects on % Phormidium cover. Flow intensity, expressed relative to the long-term median, had a positive effect up to 0.4× the median flow and a negative effect when >0.5× the median flow. Quantile regression models showed ma...

Detection of anatoxin-producing Phormidium in a New Zealand farm pond and an associated dog death

ABSTRACTOver the last decade, proliferations of benthic mat-forming Phormidium have been reported with increasing frequency in New Zealand’s cobble-bed rivers. The dominant species within these mats (Phormidium autumnale) commonly produces a range of neurotoxins, collectively known as anatoxins. Ingestion of toxic Phormidium-dominated mats has resulted in the death of an estimated 100 dogs at New Zealand’s rivers since 2005. In September 2014, a 12-week-old border terrier died rapidly after contact with floating ‘algal mats’ at a small farm pond near Kaikoura (South Island, New Zealand). The aim of this study was to establish whether ingestion of the algal mats was the cause of the death and if the dominant species was similar to that found in rivers. Environmental samples from the pond and the dog’s stomach contents contained moderate levels of anatoxin-a (25 µg L−1 and 36 µg kg−1 wet weight, respectively) and high levels of dihydroanatoxin-a (2,118 µg L−1 and 4,437 µg kg−1, respectively). The dominant organism in the environmental sample was cultured and a polyphasic approach used for characterisation. Morphological analysis of environmental samples and six isolates indicated the dominant species was most similar to Ph. autumnale. Further confirmation was provided by 16S ribosomal RNA gene sequences which showed a high percentage identity (≥ 99%) to strains isolated from Phormidium-dominated mats in New Zealand rivers. Identification of anatoxin-a and dihydroanatoxin-a in the stomach contents, environmental samples and isolates, in concert with the rapid dog death provide strong evidence that anatoxin poisoning was the cause of death. To our knowledge this is the first report in New Zealand of a dog death related to ingestion of toxic Phormidium mats from a farm pond. This study highlights the wide diversity of habitats that toxin-producing Phormidium can inhabit, and the need for greater awareness of the risks Phormidium-dominated mats pose to human and animal health.

Assessing geomorphic sensitivity in relation to river capacity for adjustment

River sensitivity describes the nature and rate of channel adjustments. An approach to analysis of geomorphic river sensitivity outlined in this paper relates potential sensitivity based on the expected capacity of adjustment for a river type to the recent history of channel adjustment. This approach was trialled to assess low, moderate and high geomorphic sensitivity for four different types of river (10 reaches in total) along the Lower Tongariro River, North Island, New Zealand. Building upon the River Styles framework, river types were differentiated based upon valley setting (width and confinement), channel planform, geomorphic unit assemblages and bed material size. From this, the behavioural regime and potential for adjustment (type and extent) were determined. Historical maps and aerial photographs were geo-rectified and the channel planform digitised to assess channel adjustments for each reach from 1928 to 2007. Floodplain width controlled by terraces, exerted a strong influence upon reach scale sensitivity for the partly-confined, wandering, cobble-bed river. Although forced boundaries occur infrequently, the width of the active channel zone is constrained. An unconfined braided river reach directly downstream of the terrace-confined section was the most geomorphically sensitive reach. The channel in this reach adjusted recurrently to sediment inputs that were flushed through more confined, better connected upstream reaches. A meandering, sand-bed river in downstream reaches has exhibited negligible rates of channel migration. However, channel narrowing in this reach and the associated delta indicate that the system is approaching a threshold condition, beyond which channel avulsion is likely to occur. As this would trigger more rapid migration, this reach is considered to be more geomorphically sensitive than analysis of its low migration rate alone would indicate. This demonstrates how sensitivity is fashioned both by the behavioural regime of a reach and flow/sediment input from upstream. The approach to assess geomorphic river sensitivity outlined here could support ‘room to move’ or ‘freedom space’ approaches to river management by relating likely channel adjustments for the type of river under consideration to the area of land that is required to contain ‘natural’ patterns and rates of geomorphic functionality.

Hydrodynamic control of benthic mats of Didymosphenia geminata at the reach scale

Algal blooms caused by the stalk-forming diatom Didymosphenia geminata impact the habitat structure and functioning of stream ecosystems. We evaluated site-specific geomorphic controls on D. geminata in a cobble bed river in the Front Range of the Rocky Mountains. Entrainment potential, defined as the ratio of mean bed shear stress to the critical shear stress, was found to be the most important control on total abundance. A comparison with critical thresholds for sediment transport showed that the requirements for flushing flows to control D. geminata blooms are similar to that required for channel maintenance. Environmental parameters influencing the morphology of the algal mats and the potential for D. geminata to dominate the benthic algal community were also investigated. The results not only contribute to understanding of the controls on benthic algal growth in dynamic streams but also for management of water releases as mitigation for the control of future blooms of D. geminata.

Mixed Scaling for Open-Channel Flow over Gravel and Cobbles

AbstractMixed scaling that includes inner and outer variables is performed on open-channel flow over gravel and cobbles. A similarity analysis is performed following classification of the flow as an equilibrium turbulent boundary layer using the asymptotic invariance principle. A mixed scale that accounts for the variation of turbulent length in the vertical direction is used to parameterize the external boundary impact on the flow for the logarithmic and outer layers. The resultant scale uses free-stream velocity, hydraulic radius, and bed particle size in which 84% of the sediment is finer. The results are consistent with empirical gravel-bed river research and applications, and nicely include the variation of the turbulent flow structure into the scaling. The results are verified using data from gravel- and cobble-bed flumes and rivers and by comparison with earlier methods. A single coefficient, semitheoretical model collapses laboratory and field data well. An integral approach is applied to allow th...

Geospatial organization of fluvial landforms in a gravel–cobble river: Beyond the riffle–pool couplet

Morphological units (MU) are landforms with distinct local form–process associations at ~1–10 channel widths scale that may be the fundamental building blocks describing the geomorphic structure of a river. Past research has disproportionately focused on the two MUs of pool and riffle, conjecturing that they are the central linked couplet in the process–form association. The goal of this study was to delineate and map spatially explicit fluvial landforms in two-dimensional planview within a gravel–cobble bed river using two-dimensional hydrodynamic delineation and then to statistically examine MU geospatial patterns for indicators of deterministic geomorphic control. This procedure is not discharge-dependent like mesohabitat methods, but gets at the geometry of underlying landforms. Statistical testing confirmed that eight delineated in-channel MU types comprise a complex and diverse channel morphology in which pools and riffles are not directly coupled. Specifically, gravel–cobble river channels (1) exhibit nonrandom spatial organization of their longitudinally and laterally variable landform morphology; (2) consist of a variety of MU types, not just pools and riffles; and (3) show distinct MU collocations and avoidances, with riffles linked to chutes and runs, while pools are linked to slackwaters and glides. Planview MU delineation with two-dimensional hydrodynamic modeling provides a ‘bottom-up’ approach to understanding and linking channel morphology with ecosystem services and geomorphic processes and is being used to guide river management and rehabilitation strategies.

Field investigation on friction factor in mountainous cobble-bed and boulder-bed rivers

Abstract This paper presents results of the friction factor of cobble-bed and boulder-bed rivers based on 23 field measurements in three mountainous rivers in Iran. Results reveal that features of our measured velocity profiles in boulder-bed rivers in mountainous region do not present a specific shape but illustrate specific pattern in cobble-bed rivers. This difference is attributed to scale effect of relative submergence. It is found that the relative submergence in cobble and boulders-bed rivers is the only key parameter that influences friction factor estimation and other parameters such as particle Froude number and Froude number have few impacts on flow resistance of cobble-bed and boulder-bed rivers. The proposed relationship for estimation for the friction factor should be useful for large coarse-bed rivers.

In situ quantification of spatial and temporal variability of hyporheic exchange in static and mobile gravel‐bed rivers

AbstractSeepage meters modified for use in flowing water were used to directly measure rates of exchange between surface and subsurface water in a gravel‐ and cobble bed river in western Pennsylvania, USA (Allegheny River, Qmean = 190 m3/s) and a sand‐ and gravel‐bed river in Colorado, USA (South Platte River, Qmean = 9·7 m3/s). Study reaches at the Allegheny River were located downstream from a dam. The bed was stable with moss, algae, and river grass present in many locations. Median seepage was + 0·28 m/d and seepage was highly variable among measurement locations. Upward and downward seepage greatly exceeded the median seepage rate, ranging from + 2·26 (upward) to − 3·76 (downward) m/d. At the South Platte River site, substantial local‐scale bed topography as well as mobile bedforms resulted in spatial and temporal variability in seepage greatly in exceedence of the median groundwater discharge rate of 0·24 m/d. Both upward and downward seepage were recorded along every transect across the river with rates ranging from + 2·37 to − 3·40 m/d. Despite a stable bed, which commonly facilitates clogging by fine‐grained or organic sediments, seepage rates at the Allegheny River were not reduced relative to those at the South Platte River. Seepage rate and direction depended primarily on measurement position relative to local‐ and meso‐scale bed topography at both rivers. Hydraulic gradients were small at nearly all seepage‐measurement locations and commonly were not a good indicator of seepage rate or direction. Therefore, measuring hydraulic gradient and hydraulic conductivity at in‐stream piezometers may be misleading if used to determine seepage flux across the sediment‐water interface. Such a method assumes that flow between the well screen and sediment‐water interface is vertical, which appears to be a poor assumption in coarse‐grained hyporheic settings. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Correction to “Feedback between bed load and flow resistance in gravel and cobble bed rivers”

[1] In the paper “Feedback between bed load and flow resistance in gravel and cobble bed rivers” by A. Recking et al. (Water Resources Research, 44, W05412, doi:10.1029/2007WR006219, 2008), in section 5.1, paragraph 41, “increasing relative depth” should have been indicated. Corrected text appears here: The most interesting result that should be considered here is that the turbulence intensity of the roughness layer was observed to increase with increasing relative depth [Bayazit, 1976; Wang et al., 1993; Dittrich and Koll, 1997; Carollo et al., 2005; Tsujimoto, 1991]. This can be explained because the wake's frequency and size depend on the mean flow velocity [Nowell and Church, 1979], which is increased with increasing relative depth, and are controlled in the upper part by the turbulent mixing of the logarithmic zone [O'Loughlin and Annambhotla, 1969], whose importance is also assumed to decrease with decreasing relative depth [Nikora et al., 2001].

An experimental test of whether bar instability contributes to the formation, periodicity and maintenance of pool–riffle sequences

AbstractPool–riffle sequences (PRSs) are periodic river‐bed morphologies with wavelengths several times the channel width. Causes of PRS formation and maintenance are not clearly understood, which may limit the effectiveness of protection and rehabilitation measures. Some confusion has existed about whether the PRS morphology is the same as or distinct from alternate bars. In this paper we investigate whether the bar instability forming alternate bars also contributes to PRS formation, periodicity and maintenance. This was unclear because bar instability occurs only when the ratio of channel width/depth exceeds a critical value, generally understood to be approximately 10, which is larger than the width‐to‐depth ratio of many PRSs. A mobile‐bed physical model is used to test whether bar instability occurs in channels characteristic of PRS morphology, with low width‐to‐depth ratio, and high relative roughness. The physical model was scaled from a prototype PRS in a gravel and cobble bed river. Alternate bars formed in the model at channel width‐to‐depth ratios as low as 3·8. The wavelength of the alternate bars formed was generally 2·2–5 times channel width, which was similar to the prototype PRS. Therefore, bar instability can occur in virtually all PRSs, and it contributes to the widespread formation of periodic PRS morphology. The model showed that maintenance of the bar height in the prototype PRS also depends on variations in channel width. It is concluded that periodic PRSs are formed and maintained by the interaction between bar instability, and flow deflections associated with variations in channel geometry such as width variation. Resonance between bar instability and three‐dimensional bed forms such as alternate bars and variations in channel geometry. Variations in channel geometry are also important in determining the location and dimensions of individual pools and bars. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The impact of invasive fish and invasive riparian plants on the invertebrate fauna of the Rondegat River, Cape Floristic Region, South Africa

Invasive fish and plants are widespread in the rivers and riparian zones of the Cape Floristic Region of South Africa and represent potential threats to the highly endemic freshwater fauna. We investigated the impact of invasive smallmouth bass (Micropterus dolomieu) and black wattle (Acacia mearnsii) on assemblages of macroinvertebrates and fish in a cobble-bed foothill river, the Rondegat River, in the Cederberg mountains, by sampling from sites representing four invasion conditions: no invasion; invasive trees; invasive fish; invasive trees and fish. Invertebrate assemblages were sensitive to invasion from bass and/or wattle: in the presence of A. mearnsii the abundance of some cobble-dwelling taxa were reduced, while those of particle-feeding mayflies and chironomids increased, reflecting increased leaf litter and sediment produced by acacias. In the presence of bass the abundance of most invertebrate taxa, particularly Chironomidae and Baetidae, increased, whereas the algal-grazing Heptageniidae and Elmidae were moderately reduced. These changes in invertebrate assemblages, coupled with data on the biomass and diet of fishes, indicate that smallmouth bass, in contrast to indigenous fish, do not exert a strong influence on the abundance of their invertebrate prey. Bass remove indigenous insectivorous fish predators, thereby reducing predation on invertebrate prey, with consequent food-web effects that influence the invertebrate community assemblage.