Nicholas J. Clifford

Seed bank, seed dispersal and vegetation cover: Colonization along a newly-created river channel

Abstract Question: What is the relative importance of the initial seed bank and subsequent seed dispersal for floristic composition of bank vegetation two years after creation of a newly-cut reach of a river channel? Location: River Cole, West Midlands, United Kingdom. Methods: We took bank and bed sediment samples from a 0.5-km reach of a new river channel cut into intact flood-plain. After river diversion, seed samples deposited on artificial turf mats placed on the river banks and flood-plain edge were taken in summer and winter 2002 and 2003. Seed rain samples from funnel traps were taken during summer 2002 and 2003. We undertook greenhouse germination trials to assess viable seed species within these samples. In summer 2004, we surveyed river bank vegetation. Agglomerative cluster analysis was used to investigate floristic similarity between seed bank, seed rain, seed deposition samples and final bank vegetation cover. DCA was used to explore contrasts between the samples and to assess whether these reflected interpretable environmental gradients. Results: Seed rain samples contained a small subset of species in the summer depositional samples. 38 species were found within the final vegetation, the seed bank, and at least one of the four sets of depositional samples; a further 30 species not present in the seed-bank samples were present in at least one of the four sets of depositional samples and the final vegetation. Floristic composition of the vegetation was most similar to the depositional samples from winter 2002 and 2003 and summer 2003. DCA axis 1 reflected a time sequence from seed-bank samples through depositional samples to the final vegetation. Conclusions: Newly cut river banks were colonized rapidly. Seed remobilization and hydrochorous transport from the upstream catchment are important for colonization. Species richness was highest in samples deposited during winter when high river flows can remobilize and transport viable seeds from upstream. This process would also have enhanced the species richness of seed production along the banks during the second summer (2003). Nomenclature: Stace (1999).

The future of Geography: when the whole is less than the sum of its parts

Abstract In a recent article, Thrift has presented an optimistic account of the future of Geography. While this reply is broadly supportive of his claims that Geography is more diverse, and has more to offer than ever before, it is less optimistic with respect to the prospects for the survival of Geography as a unitary academic discipline. Experiences over the last 20 years in the UK higher education, in particular, the 2001 RAE exercise, point to an unfavourable institutional climate for the discipline. Within Geography, the ever-increasing diversity of its subject matter and research philosophy poses problems for disciplinary identity. This is reflected in the more restricted perspective of the subject outside the universities, and is compounded by a weakening of the link between Geography in the universities and the schools. In these circumstances, serious attention must be given to the changing nature of the discipline, to its positioning with respect to other subjects, and to its relations with the wider world. At a time of academic, cultural, technological and social dynamism, there are, nevertheless, opportunities as well as dangers for the subject. Although this reply is an explicitly bleak one regarding the future, the implicit message is that Geography can (and should) still prosper. The more positive outcome, however, rests on an appreciation and nurturing of a more traditional geographical heritage than Thrift identifies, as well as a more creative view of the relationship between fundamental and applied research.

Formation of riffle-pool sequences: field evidence for an autogenetic process

Abstract Riffle—pool sequences are the characteristic reach-scale bedforms of mixed- and gravel-bedded rivers. While most research has been directed to the quantitative description of these sequences and to their maintenance, comparatively little attention has been given to the processes responsible for their formation. In this paper, field results from quasi-continuous velocity records obtained with an array of electro-magnetic current meters are analysed with respect to a model of macro-scale structure in the turbulent velocity field of natural channels. This model has been suggested as the most plausible basis for riffle—pool formation. The results demonstrate the occurrence of a variety of coherent flow structures, some of which scale on channel dimensions as predicted by the model, but which also suggest that some revision of the association between flow structure and riffle—pool formation is necessary. It is suggested that riffle—pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers. The obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form, but is ultimately removed as part of the process of bed modification via the extension of scour. Three distinct stages in the process are involved: local scour of a single pool creates deposition downstream, which then generates the next-downstream flow irregularity. A riffle—pool sequence is thus created autogenetically as the summation of a sequence of irregularities, each unit of which is formed and maintained only by flow dynamics operating at the local scale. The process is essentially both deterministic and statistical in character. It is further suggested that spatial differences in the near-bed turbulence field arising from incipient riffle—pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism. Similar reasoning might possibly be extended to the entire range of bedform sizes.

Differential bed sedimentology and the maintenance of riffle-pool sequences

Abstract Riffle-pools are the characteristic reach-scale bedforms of gravel- and mixed-bedded channels of low to moderate slope. The purpose of this paper is to clarify several aspects of the sedimentology of riffle-pool sequences, and to suggest how these both reflect and control riffle-pool stability in the presence of spatial differences in the turbulent near-bed flow field. High-frequency velocity measurements obtained using an array of electromagnetic current meters, are combined with results from grid and transect sampling of bed sediments from 9 sites on 3 small upland rivers in N. Exmoor, U.K. Riffle sediments are shown to possess greater development of microtopographic structuring than adjacent pools, which enhances riffle stability, and which may reflect greater turbulent kinetic energy there over most of the flow range. Data on surface sediment arrangement and turbulent flow characteristics can be combined with different transport threshold equations to explain the maintenance of riffle-pool sequences with respect to a particle queuing (‘kinematic wave’) model, without recourse to a competence reversal, as conventionally assumed.

Fluvial geomorphology: structured beds in gravelly rivers

Last year’s progress report (Richards, 1990a) reviewed research into the process of initial entrainment of bed material in gravel-bed rivers. This highlighted the debate about ’selective transport’ or ’equal mobility’ of the widely ranging grain sizes in gravelly rivers, and a key issue in this debate is the structured nature of gravel river bed sediments. Bedload transport in alluvial gravel-bed rivers is generally a close-to-threshold process, and sedimentological structures influence transport capacity as a result of their control of the threshold condition; however, they are also often considered to control the supply of material as the structuring of the bed delays the release of particles (Laronne and Carson, 1976). Since transport rates may be defined as capacityor supply-limited, this introduces terminological and analytical confusion. An additional ambiguity arises because bed structures influence the flow hydraulics both through their fluid drag (Davies and Sutherland, 1980) and through the process of vortex shedding which determines the structure of near-bed turbulence (Clifford

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.

Numerical modeling of river flow for ecohydraulic applications: some experiences with velocity characterization in field and simulated data.

Information regarding the spatial and temporal organization of river flow is required for many applications in river management, and is a fundamental requirement in ecohydraulics. As an alternative to detailed field surveys and to mesohabitat reconnaissance schemes, potential exists to deploy numerical flow simulation as an assessment and design tool. A key question is the extent to which complex hydrodynamic models are really practical in river management applications. This paper presents experiences using sediment simulation in intakes with multiblock, a three-dimensional modeling code, in conjunction with a statistical approach for classifying the spatiotemporal dynamics of flow behavior. Even in a simple configuration, the model is able to replicate well flow structures which associate with the mesohabitat concepts used in field reconnaissance techniques. The model also captures spatiotemporal dynamics in flow and depth behavior at these scales. However, because the model shows differential performance between flow stages and between differing channel (bed form) units, the smaller-scale and discharge-dependent dynamics of some zones within the channel may be less-well represented, and the implications of this for future research are noted.

Globalization: a Physical Geography perspective

Although globalization is a term usually restricted to economics and the social sciences, there are aspects of the phenomenon that are intimately linked to the practice and purpose of the physical and environmental sciences and exemplified through Physical Geography. At a fundamental level, Physical Geography has always sought to describe and understand the multiple subsystems of the environment and their connections with human activity: it is global and globalizing at its very roots. Globalization may be seen historically in the global export of western science, including Physical Geography, that underpinned colonial resource exploitation, and which subsequently laid the foundations for the worldwide conservation movement, and for critiques of environment-development relations, such as Political Ecology. Globalization is evident today in the burgeoning productivity and increasing organization of science as well as in the growing accessibility of scientific information. It is also at work in setting contemporary scientific agendas that are focused on larger-scale issues of environment and development and environmental change, particularly in an emergent Earth System Science, and also in Sustainability Science. These global agendas are not simply shared with but also co-produced by the public, politicians and commercial interests, providing both opportunities and challenges for traditional disciplines and traditional disciplinary practices such as Physical Geography.

Three seedling emergence methods in soil seed bank studies: implications for interpretation of propagule deposition in riparian zones

Samples of soil and recently deposited sediments were collected from the river bed, bank face and bank tops of two lengths (reaches) of the River Frome, Dorset, UK and one reach of the River Tern, Shropshire, UK. Soil propagule bank samples were collected in May 2003, and depositional samples were collected subsequently over four consecutive 4-month periods between June 2003 and October 2004. The samples were subjected to three emergence trials under drained, waterlogged and submerged conditions. Significantly more seedlings germinated in the drained than waterlogged trial, and waterlogged than submerged trials. Drained, waterlogged and submerged trials identified 186, 76 and 37 species, respectively. Six species identified in the waterlogged trials were not identified in the drained trials, and five species in the submerged trials were not found in the drained trials. Submerged trials added two species to the drained and waterlogged results. Application of detrended correspondence analysis (DCA) to average species abundance data, for the sampled hydrological habitats (bed, bank face, bank top) within the soil propagule bank and four depositional data sets, generated different results for the three trials. The drained treatment revealed significant differences between sites, seasons and hydrological habitats, whereas the waterlogged and submerged treatments presented an increasingly homogeneous view of the samples. Our results confirm other propagule bank emergence comparisons and extend them to depositional samples, demonstrating that the strong environmental sieves imposed by waterlogging and submergence restrict emergence of numerous terrestrial, wetland and even some aquatic species that were successfully identified using the drained conditions.

Distribution of biologically functional habitats within a lowland river, United Kingdom

The assessment and appraisal of river habitat quality for purposes of legislative compliance requires a sound understanding of the interactions between the hydrogeomorphological and ecological components of the instream environment. Reconciliation of complementary habitat concepts developed from geomorphology (‘physical biotopes’) and from ecology (biologically ‘functional’ habitats) offers improved integrity of both concepts, but may be limited by factors such as the distributional characteristics of functional habitats. Field surveys conducted on a lowland river in Shropshire, UK, reveal differences in both the spatial organisation and seasonal variations in abundance among functional habitat categories. Furthermore, the level of human modification to the channel and banks appears to influence the spatial organisation of certain functional habitats within the river channel. These factors have implications for rapid field survey methods and for the potential for linking functional habitats with the complementary geomorphological concept of physical biotopes.