David A. Sear

Comparative biodiversity of rivers, streams, ditches and ponds in an agricultural landscape in Southern England

Information about the relative biodiversity value of different waterbody types is a vital pre-requisite for many strategic conservation goals. In practice, however, exceptionally few inter-waterbody comparisons have been made. The current study compared river, stream, ditch and pond biodiversity within an 80 km2 area of lowland British countryside. The results showed that although all waterbody types contributed to the diversity of macrophytes and macroinvertebrates in the region, they differed in relative value. Individual river sites were rich but relatively uniform in their species composition. Individual ponds varied considerably in species richness, with the richest sites supporting similar numbers of taxa to the best river sections, but the poorest sites amongst the most impoverished for all waterbody types. At a regional level, however, ponds contributed most to biodiversity, supporting considerably more species, more unique species and more scarce species than other waterbody types. Streams typically supported fewer species and fewer unique species at local and regional level than either ponds or rivers. Ditches (most of which were seasonal) were the least species-rich habitat, but supported uncommon species, including temporary water invertebrates not recorded in other waterbody types. Multivariate analysis indicated that permanence, depth, flow and altitude were the main environmental variables explaining invertebrate and plant assemblage composition. The findings, as a whole, suggest that ponds and other small waterbodies can contribute significantly to regional biodiversity. This contrasts markedly with their relative status in national monitoring and protection strategies, where small waterbodies are largely ignored.

Process-based Principles for Restoring River Ecosystems

Process-based restoration aims to reestablish normative rates and magnitudes of physical, chemical, and biological processes that sustain river and floodplain ecosystems. Ecosystem conditions at any site are governed by hierarchical regional, watershed, and reach-scale processes controlling hydrologic and sediment regimes; floodplain and aquatic habitat dynamics; and riparian and aquatic biota. We outline and illustrate four process-based principles that ensure river restoration will be guided toward sustainable actions: (1) restoration actions should address the root causes of degradation, (2) actions must be consistent with the physical and biological potential of the site, (3) actions should be at a scale commensurate with environmental problems, and (4) actions should have clearly articulated expected outcomes for ecosystem dynamics. Applying these principles will help avoid common pitfalls in river restoration, such as creating habitat types that are outside of a sites natural potential, attempting to build static habitats in dynamic environments, or constructing habitat features that are ultimately overwhelmed by unconsidered system drivers.

SEDIMENT TRANSPORT PROCESSES IN POOL–RIFFLE SEQUENCES

This paper reviews the published data for sediment transport in pool–riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport. This conclusion is explored in more detail using observations of sediment transport from the regulated River North Tyne. Sediment transport rates at discharges of <30 per cent bankfull are initially highest on riffles. As discharge increases, sediment transport rates in pools rise sharply, and values locally exceed those on riffles. Tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool–riffle sequence. Coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods. Analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds. An appraisal of the possible mechanisms controlling sediment entrainment, transport and deposition in pool–riffle sequences is made which reveals evidence for the operation of a combination of hydraulic, sedimentological and interactive processes. A descriptive model of sediment transport processes in pool–riffle sequences is presented.

Groundwater dominated rivers

This paper explores the significance of groundwater dominance in the surface water system through a combination of review and an exposition of the general hydrology, ecology and geomorphology of rivers draining the main UK aquifers. Groundwater dominance is shown to vary according to the nature of the aquifer lithology, the mechanism of groundwater:surface water interaction and the scale at which one examines this interaction. Using data derived from a range of studies including the UK Environment Agency River Habitat Survey and the UK Institute of Freshwater Ecology RIVPACS invertebrate database it is shown that the nature of the aquifer and mode of influent discharge strongly control the hydrological and ecological characteristics of the environment but that a specific groundwater ecology or hydrogeomorphology is masked by the overriding controls exerted by aquifer geology and catchment topography. Despite this, it is clear that river systems dominated by groundwater flows have specific hydrological characteristics and management issues that require holistic, multidisciplinary approaches that recognise the significance of groundwater and the nature of the interaction with the surface water environment.

Sediment transport and siltation of brown trout (Salmo trutta L.) spawning gravels in chalk streams

Deposition rates of fine sediment into brown trout spawning gravels were measured at monthly intervals for a period of one year in a small channel of the River Test, Hampshire. Data were also collected on stream discharge, water depth, flow velocity and suspended sediment concentrations. Deposition rates followed a seasonal pattern and were maximal during periods of high discharge in the late winter/early spring when suspended sediment concentrations were high. The material deposited in the spawning gravels included silts and fine sands (<250 μm) that were transported in suspension and coarser fragments of low density tufalike material that were transported as bed load. The ecological implications of fine sediment deposition for salmonid egg survival in chalk streams are considered.

The influence of vegetation and organic debris on flood-plain sediment dynamics: case study of a low-order stream in the New Forest, England

The presence of large woody debris (LWD) has important implications for the physical and ecological behaviour of rivers, and these aspects have been researched extensively in recent years. However, this research has so far focused primarily on interactions between LWD and in-channel processes, and the role of LWD in flood-plain genesis is still poorly understood. Established conceptual models of flood-plain evolution are, therefore, lacking because they neglect the complex interaction between water, sediment, and vegetation in systems with accumulations of LWD. This study examines the effect of LWD on patterns of sediment deposition within a small area of forest flood plain along the Highland Water, S. England. In-channel debris dams locally increase the frequency and extent of overbank flows, and the impact of such dam on flood-plain sedimentation was observed. Nine separate flood events were monitored through the exceptionally wet winter of 2000–2001. During each of these, water and sediment fluxes were quantified and correlated with general rates of overbank sedimentation. Flood-plain topography, vegetation, and LWD were surveyed and related to micro- and mesoscale patterns of sediment accretion. The amount of overbank sediment deposition was correlated most closely with flood hydrology and sediment input. The amounts (0–28 kg m?2) and patterns of sediment deposition were both greater and more variable than have been observed on nonforest flood plains. The highly variable pattern of accretion can be explained by the combined effects of topography and organic material present on the surface of the flood plain.

The geomorphological basis for classifying rivers

1. This paper sets out the background to river channel classification using geomorphological features; these are highly relevant to the description and understanding of physical habitat. 2. The River Habitat Survey (RHS) database contains much geomorphological information which has been statistically analysed for those semi-natural sites surveyed in 1994. Additional geomorphological data have been gathered in an attempt to make the resulting typology ‘dynamic’, i.e. capable of predicting channel stability, an important component of the information needed for sustainable river management. 3. The outcome of TWINSPAN and redundancy analyses is disappointing in statistical terms and does not, therefore, constitute an objective taxonomy. However, the river types selected by the analyses are intuitively realistic, and show promise that, with a broader database (both spatially and in terms of independent variables) the development of a predictive, dynamic typology will be possible.

Exploring the Relations Between Riverbank Erosion and Geomorphological Controls Using Geographically Weighted Logistic Regression

The relations between riverbank erosion and geomorphological variables that are thought to control or influence erosion are commonly modelled using regression. For a given river, a single regression model might be fitted to data on erosion and its geomorphological controls obtained along the rivers length. However, it is likely that the influence of some variables may vary with geographical location (i.e., distance upstream). For this reason, the spatially stationary regression model should be replaced with a non-stationary equivalent. Geographically weighted regression (GWR) is a suitable choice. In this paper, GWR is extended to predict the binary presence or absence of erosion via the logistic model. This extended model was applied to data obtained from historical archives and a spatially intensive field survey of a length of 42 km of the Afon Dyfi in West Wales. The model parameters and the residual deviance of the model varied greatly with distance upstream. The practical implication of the result is that different management practices should be implemented at different locations along the river. Thus, the approach presented allowed inference of spatially varying management practice as a consequence of spatially varying geomorphological process.

Restoration of the rivers Brede, Cole and Skerne: a joint Danish and British EU‐LIFE demonstration project, III—channel morphology, hydrodynamics and transport of sediment and nutrients

1. A comprehensive monitoring programme was initiated for the Brede, Cole and Skerne river restoration projects in order to elucidate the impact of re-meandering on flood levels, floodplain inundation, adjustment of river morphology, sediment transport and overbank sediment deposition. 2. Reducing the bankfull capacity, raising the bed level and lowering the bank level allowed an increase in flooding frequency and in the amount of water passing onto the floodplain in all three rivers. In the river Brede, restoration of the natural hydrological contact between the river and its floodplain resulted in high deposition of sediment (189 t year−1) and sediment-associated phosphorus (770 kg P year−1). 3. Construction work caused excessive downstream loss of sediment and phosphorus as documented from sediment mass balances for the River Brede and River Cole. Short-term adjustments in river morphology were recorded in the River Cole based on the fluvial auditing procedure. Post-restoration morphology changed compared with that before restoration in terms of both total diversity and the type of features recorded.

Environmental change in river channels: a neglected element. Towards geomorphological typologies, standards and monitoring.

Rivers integrate the impacts of change in atmospheric and terrestrial systems; they then deliver these to the coast. En route geomorphological processes create dynamic and diverse habitats, both in-stream and in riparian/floodplain ecotones. The dynamics of channel change conflict with human resource development, the outcome is that many river and riparian environments have been significantly modified, complicating the interpretation of change. Collection of geomorphological data on both form and process has to date been overwhelmingly an academic pursuit; standard measurement networks and long-term monitoring have, as a result been largely absent-as in the Environmental Change Network (ECN), despite the emerging requirements of legislation such as the EU Water Framework Directive. In this paper, we utilise a unique set of repeat channel surveys and long-term bed-load sediment yields to provide guidance on both definitions of change and those variables and survey techniques which might form the basis, in future, of improved national-scale monitoring. The Environment Agencys River Habitat Surveys suggest the basis for channel typologies that could structure a sampling framework and rationalise the variables to be monitored. We also point to the value of more detailed geomorphological procedures in use at the catchment/project scale-Catchment Baseline Surveys and Fluvial Audits-as a standardised basis for monitoring the detail of change in the fluvial sediment system. A perfect opportunity to lay foundations for such monitoring activity has been provided in England and Wales by the winter floods of 2000/2001.