Stephen P. Rice

Grain size along two gravel‐bed rivers: statistical variation, spatial pattern and sedimentary links

A new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel-bed rivers in northeastern British Columbia, Canada. A preliminary assessment of grain-size variability establishes a basis for examination of the spatial pattern of grain-size change. Standard ANOVA techniques are inappropriate because individual samples have unequal variances and are not normally distributed. Alternative tests for homoscedasticity and comparison of means are therefore utilized. Within-site, between-sample variability is not significant. The grain-size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled. In both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining. Textural variations along the two rivers studied here are complex and show negligible overall fining (in over 100 km). This is the consequence of a large number of tributary inputs and non-alluvial sediment sources which are the legacy of Late Pleistocene glaciation. The identification of lateral sources like these is fundamental for understanding textural changes within rivers. The sedimentary link (a channel reach between significant lateral sediment inputs) provides a means of isolating fluvial maturation processes (abrasion and sorting) from contingent lateral inputs. Strong fining trends are apparent in most links and classification of grain-size measurements according to their location within particular links greatly improves the statistical explanation of textural variation. Identification of sedimentary links provides a means of applying models of fluvial fining processes, so isolation of link networks will aid the development of basin-scale models of textural variation.

Which tributaries disrupt downstream fining along gravel-bed rivers?

Tributaries and other lateral sediment sources can have a significant impact on river bed sediment texture and, in turn, on channel form. Sufficiently voluminous or distinct sediment inputs redefine the mainstem grain-size distribution, punctuating downstream maturation and isolating a sequence of discrete sedimentary links. Within these links fining processes usually dominate, such that models of sorting and abrasion, when applied to individual links, provide reasonable predictions of grain-size change. Links represent the fundamental natural unit within which fining models can be tested, developed and applied. Identification of significant lateral sources is therefore important, yet, beyond vague references to relative tributary size, sediment load, and sediment calibre, no criteria exist for the a priori discrimination of such sources. In this paper a procedure for identifying significant lateral (tributary) sources, without the benefit of grain-size information, is outlined. A high-resolution characterisation of bed material texture along two Canadian gravel-bed rivers facilitated classification of all their perennial tributaries as either significant or insignificant. Three absolute tributary basin parameters and their relative counterparts, chosen to reflect the likely controls on tributary significance, are then used to develop a discriminant function which isolates a large proportion of significant tributaries while minimising incorrect classifications. Examination of consistently misclassified (anomalous) tributaries reveals the importance of lateral source spacing and of inconsistencies in the geomorphic history of the contributing basins. In turn, a general tributary categorisation procedure is suggested which includes a logistic regression model for attaching probability statements to individual classifications. The generality of the discriminant and logistic functions cannot be assessed because of the lack of other suitable data sets.

Bed material texture in low order streams on the Queen Charlotte Islands, British Columbia

Low order channels comprise a large proportion of the links of every drainage basin, and are often at the centre of land management concerns. They exhibit hydrological and geomorphological characteristics atypical of higher order links. This paper examines the nature and causes of variations in the bed material texture of two streams on the Queen Charlotte Islands, British Columbia. The extant, functional exponential model is found to be inadequate for explaining observed changes in grain size parameters with distance downstream. Recurrent disruption of sediment transport by large organic debris jams, and the sporadic contamination of the fluvial sediment population by colluvial inputs, preclude the development of longitudinal structure. Rather, grain size varies erratically over short distances. A stochastic model best describes the observed variations, and should be adopted as an alternative to the exponential model in low order links. Characteristic variances are controlled by the degree of hillslope-channel coupling, and the extent and characteristics of non-alluvial storage mechanisms.

Longitudinal profiles in simple alluvial systems

Theoretical considerations suggest that exponential or quadratic functions are apt for describing the longitudinal profiles of aggrading, alluvial systems that are unaffected by significant lateral inputs of water or sediment. A new set of field data confirms this for individual sedimentary links along a wandering gravel-bed river in British Columbia, Canada. Each link is viewed as a simple alluvial system, without major water or sediment inputs, within which grain size typically fines downstream in a relatively systematic manner. Such homogeneous reaches are suitable for the investigation of simple profile form. It is found that quadratic approximations are the most flexible descriptor for link longitudinal profiles. Overall, the river forms an irregularly cuspate long profile structured by these fundamental length-scale units. Both link gradients and link fining rate reflect size-selective aggradation associated with the bounding lateral inputs.

River confluences, tributaries and the fluvial network.

Preface. List of contributors. 1. Introduction: river confluences, tributaries and the fluvial network (Stephen P. Rice, Bruce L. Rhoads and AndrA(c) G. Roy). Introduction. Key aims of the book. Sections of the book. References. PART I: RIVER CHANNEL CONFLUENCES. 2. Introduction to Part I: river channel confluences (AndrA(c) G. Roy). Introduction. Individual chapters. Reference. 3. Modelling hydraulics and sediment transport at river confluences (Pascale M. Biron and Stuart N. Lane). Introduction. Hydraulics. Bedload, suspended and solute transport. Conclusion. Acknowledgments. References. 4. Sediment transport, bed morphology and the sedimentology of river channel confluences (James L. Best and Bruce L. Rhoads). Context. Bed morphology. Sediment transport. Sedimentology. Conclusions. Acknowledgements. References. 5. Large river channel confluences (Daniel R. Parsons, James L. Best, Stuart N. Lane, Ray A. Kostachuk, Richard J. Hardy, Oscar Orfeo, Mario L. Amsler and Ricardo N. Szupiany). Introduction. Bed morphology. Flow structure at large river channel confluences. Flow mixing at large river confluences. Conclusions. Acknowledgements. References. 6. Management of confluences (Robert Ettema). Introduction. Unruly confluences. Management approaches. Managing confluences for sediment transport. Managing confluences for ice passage. Summary. References. 7. Unconfined confluences in braided rivers (Peter Ashmore and J. Tobi Gardner). Introduction. General characteristics and significance of confluences in braided channels. Confluence scour depth. Confluence kinetics and bar formation. Confluence spacing and the length-scale of braided morphology. Sediment transport and sediment budgets. Sediment sorting and alluvial deposits. Prospect. Acknowledgements. References. PART II: TRIBUTARY-MAIN-STEM INTERACTIONS. 8. Introduction to Part II: tributary-main-stem interactions (Stephen P. Rice). Introduction. Individual chapters. References. 9. Spatial identification of tributary impacts in river networks (Christian E. Torgersen, Robert E. Gresswell, Douglas S. Bateman and Kelly M. Burnett). Introduction. Data measurement. Analytical tools. Future developments and challenges. Acknowledgements. References. 10. Effects of tributaries on main-channel geomorphology (Rob Ferguson and Trevor Hoey). Introduction. Conceptual considerations. Empirical evidence. Theoretical models: (1) Regime analysis of confluences. Theoretical models: (2) Numerical experiments with adjustable grain-size distributions. Discussion. Acknowledgments. References. 11. The ecological importance of tributaries and confluences (Stephen P. Rice, Peter Kiffney, Correigh Greene and George R. Pess). Introduction. Tributaries, confluences and river ecology. Tributaries, ecosystem functions and river management. Constraints on understanding and progress. A case study. Conclusion. Acknowledgments. References. 12. Tributaries and the management of main-stem geomorphology (FrA(c)dA(c)ric LiA(c)bault, HervA(c) Piegay, Philippe Frey and Norbert Landon). Introduction. Conceptual framework for assessing the geomorphological impact of tributaries. Managing the geomorphological impact of tributaries. Conclusion. Acknowledgments. References. 13. Confluence environments at the scale of river networks (Lee Benda). Introduction. River network structure and confluence environments. Symmetry ratios and confluence environments. Basin shape, network patterns and confluence environments. Local network geometry. Drainage and confluence density. River network scaling properties of confluence environments. The Law of stream sizes and the spatial scale of morphological diversity related to confluences. Longitudinal extent and size of confluence environments. Stochastic watershed processes. The role of hierarchical branching networks. Discussion. River networks, resource management and river restoration. Acknowledgements. References. PART III: CHANNEL NETWORKS. 14. Introduction to Part III: channel networks (Bruce L. Rhoads). Introduction. Individual chapters. References. 15. Hydrologic dispersion in fluvial networks (Patricia M. Saco and Praveen Kumar). Hydrologic dispersion effects on runoff response. Runoff response as travel-time distributions: the GIUH. Geomorphological dispersion in stream networks. Non-linear effects and the use of hydraulic geometry relations. Kinematic dispersion in stream networks. The effect of scale and rainfall intensity on the dispersive mechanisms. Hillslope dispersive effects. Kinematic dispersion effects using the meta-channel approach. Summary and future research directions. Acknowledgments. References. 16. Sediment delivery: new approaches to modelling an old problem (Hua Lu and Keith Richards). Introduction. The concept of sediment delivery. Difficulties in measuring and estimating sediment yield and SDR. Links between hydrology and sediment production and yield. Physical inferences of sediment delivery based on a simple lumped model. Practical large-scale application using a distributed model. Conclusions. Acknowledgements. References. 17. Numerical predictions of the sensitivity of grain size and channel slope to an increase in precipitation (Nicole M. Gasparini, Rafael L. Bras and Gregory E. Tucker). Introduction. Landscape-evolution models. Example simulation of network evolution. Discussion. Conclusions. Acknowledgements. References. 18. Solute transport along stream and river networks (Michael N. Gooseff, Kenneth E. Bencala and Steve M. Wondzell). Introduction. Review of current knowledge. Linking transport processes with the fluvial geomorphic template. Forward-looking perspective. Acknowledgements. References. 19. Fluvial valley networks on Mars (Rossman P. Irwin III, Alan D. Howard and Robert A. Craddock). Introduction. Early observations. Distribution, age, origin and morphology of valley networks. Morphometry. Alluvial deposits. Hydrology. Summary. Acknowledgements. References. Subject Index. Place Index.

Macroinvertebrate community changes at coarse sediment recruitment points along two gravel bed rivers

Field data from two gravel bed rivers are used to investigate the response of mainstream macroinvertebrate communities to changes in bed sediment character at sites of significant coarse sediment recruitment. There is clear evidence that these points, where downstream fining trends are punctuated, are associated with significant changes in community composition. Community differences are greater between pairs of riffles that straddle recruitment points than between similarly spaced riffles that do not. Taxa that prefer coarse substrate and energetic flows tend to be more abundant below recruitment points where there is also a tendency for taxa diversity to increase. Classification procedures discriminate faunal assemblages at those sites which occur immediately downstream of recruitment points from assemblages at other sites. Although most of the lateral sediment sources examined are tributaries, a number of arguments suggest that, at least in this case, faunal changes are primarily due to abrupt changes in bed sediment character and are not due to concomitant changes in water quality or quantity. Our results suggest that at moderate spatial scales, patterns of coarse sediment recruitment are partly responsible for the spatial organization of macroinvertebrate communities in gravel bed rivers.

Evaluating the role of invasive aquatic species as drivers of fine sediment-related river management problems: The case of the signal crayfish (Pacifastacus leniusculus)

Sediment quantity and quality are key considerations in the sustainable management of fluvial systems. Increasing attention is being paid to the role of aquatic biota as geomorphic agents, capable of altering the composition, mobilization and transport of fluvial sediments at various spatiotemporal scales. In this paper invasive species are presented as a special case since: (1) populations may not be constrained by factors characteristic of their native habitats; and (2) they represent a disturbance to which the system may not be resilient. Discussion is centred on the signal crayfish which has rapidly colonized catchments in Europe and Japan, but the hypotheses and models presented provide a framework applicable to other invasive species. This paper explores the mechanisms by which signal crayfish may influence sediment dynamics from the patch scale to the catchment scale. There is potential for signal crayfish to impact significantly on river sediments and morphology as a function of their interactions with river bed and bank material, and with other aquatic organisms, combined with their large body size and aggressive nature, their presence in very high densities, and the lack of effective mitigation strategies. Potential catchment-scale management issues arising from these factors include habitat degradation, mobilization of sediment-associated nutrients and contaminants, and sediment-related flood risks. Further interdisciplinary research is required at the interface between freshwater ecology, fluvial geomorphology and hydraulics, in order to quantify the significance and extent of these impacts. The paper points to the key research agendas that may now emerge.

Physical modelling of water, fauna and flora: knowledge gaps, avenues for future research and infrastructural needs

Physical modelling is a key tool for generating understanding of the complex interactions between aquatic organisms and hydraulics, which is important for management of aquatic environments under environmental change and our ability to exploit ecosystem services. Many aspects of this field remain poorly understood and the use of physical models within eco-hydraulics requires advancement in methodological application and substantive understanding. This paper presents a review of the emergent themes from a workshop tasked with identifying the future infrastructure requirements of the next generation of eco-hydraulics researchers. The identified themes are: abiotic factors, adaptation, complexity and feedback, variation, and scale and scaling. The paper examines these themes and identifies how progress on each of them is key to existing and future efforts to progress our knowledge of eco-hydraulic interactions. Examples are drawn from studies on biofilms, plants, and sessile and mobile fauna in shallow water fluvial and marine environments. Examples of research gaps and directions for educational, infrastructural and technological advance are also presented.

24 Movements of a macroinvertebrate (Potamophylax latipennis) across a gravel-bed substrate: effects of local hydraulics and micro-topography under increasing discharge

Flow refugia provide a mechanism that can explain the persistence of macroinvertebrate communities in flood-prone, gravel-bed rivers. The movement behaviour of macroinvertebrates is a key element of the flow refugia hypothesis, but surprisingly little is known about it. In particular, little is known about how local near-bed hydraulics and bed micro-topography affect macroinvertebrate movements. We used a novel casting technique to reproduce a natural gravel-bed substrate in a large flume where we were able to observe the movement behaviour of the cased caddisfly, Potamophylax latipennis at different discharges. The crawling paths and drift events of animals were analysed from video recordings and used to classify sites on the substrate according to the type of insect movement. We used acoustic Doppler velocimeter (ADV) measurements close to the boundary to characterise hydraulic conditions at different sites and a detailed Digital Elevation Model (DEM) to characterise sites topographically. Animals made shorter more disjointed crawling journeys as discharge increased, although they tended to follow consistent paths across the substrate. As hypothesised, crawling behaviour was locally associated with low elevations, low flow velocities and low turbulent kinetic energies, while sites that insects avoided were characterised by higher elevations, velocities and turbulence. Discrimination was greater at higher discharges, indicating that movement behaviour is contingent upon flow conditions. We suppose that these relations reflect the need of animals to reduce the risk of entrainment and minimise energy expenditure by avoiding areas of high fluid drag. As discharge increased, there was a general upward shift in the frequency distributions of local velocities and turbulent kinetic energies. The animals responded to these shifts and it is clear that their different activities were not limited to fixed ranges of velocity and turbulence. We assume that the absolute hydraulic forces would become a limiting factor at some higher discharge. At the discharges examined here, which are below those required to instigate framework particle entrainment, patterns of animal movement appear to be associated with the animals’ experiences of relative rather than absolute hydraulic forces.

The activity of signal crayfish (Pacifastacus leniusculus) in relation to thermal and hydraulic dynamics of an alluvial stream, UK

Signal crayfish (Pacifastacus leniusculus) are an invasive species of global significance because of their detrimental impacts on freshwater environments and native organisms. The movement of signal crayfish was continuously monitored for 150-days through a 20-m reach of an alluvial stream in the UK. Passive integrated transponder-tags were attached to crayfish, allowing their location to be monitored relative to 16 antennae which were buried beneath the river bed. The activity of crayfish was related to water depth and temperature, which were continuously monitored within the instrumented reach. Crayfish were highly nocturnal, with less than 6% of movements recorded during daylight hours. Activity declined from September and was minimal in November when water temperature was low and flow depth was high. However, relations between environmental parameters and crayfish activity had poor explanatory power which may partly reflect biological processes not accounted for in this study. Water depth and temperature had a limiting relationship with crayfish activity, quantified using quantile regression. The results extend existing data on signal crayfish nocturnalism and demonstrate that, although signal crayfish can tolerate a range of flows, activity becomes limited as water temperature declines seasonally and when water depth remains high in autumn and winter months.