Gemma L. Harvey

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.

The impact of pre-restoration land-use and disturbance on sediment structure, hydrology and the sediment geochemical environment in restored saltmarshes.

Saltmarshes are being lost or degraded as a result of human activity resulting in loss of critical ecosystem services including the provision of wild species diversity, water quality regulation and flood regulation. To compensate, saltmarshes are being restored or re-created, usually driven by legislative requirements for increased habitat diversity, flood regulation and sustainable coastal defense. Yet, there is increasing evidence that restoration may not deliver anticipated ecosystem services; this is frequently attributed to poor drainage and sediment anoxia. However, physical sediment characteristics, hydrology and the sediment geochemical environment are rarely examined in restoration schemes, despite such factors being critical for plant succession. This study presents the novel integration of 3D-computed X-ray microtomography to quantify sediment structure and porosity, with water level and geochemical data to understand the impact of pre-restoration land use and disturbance on the structure and functioning of restored saltmarshes. The study combines a broad-scale investigation of physical sediment characteristics in nine de-embanked saltmarshes across SE England, with an intensive study at one site examining water levels, sediment structure and the sediment geochemical environment. De-embankment does not restore the hydrological regime, or the physical/chemical framework in the saltmarshes and evidence of disturbance includes a reduction in microporosity, pore connectivity and water storage capacity, a lack of connectivity between the sub-surface environment and overlying floodwaters, and impeded sub-surface water flow and drainage. This has significant consequences for the sediment geochemical environment. This disturbance is evident for at least two decades following restoration and is likely to be irreversible. It has important implications for plant establishment in particular, ecosystem services including flood regulation, nutrient cycling and wild species diversity and for future restoration design.

River bank burrowing by invasive crayfish: spatial distribution, biophysical controls and biogeomorphic significance

Invasive species generate significant global environmental and economic costs and represent a particularly potent threat to freshwater systems. The biogeomorphic impacts of invasive aquatic and riparian species on river processes and landforms remain largely unquantified, but have the potential to generate significant sediment management issues within invaded catchments. Several species of invasive (non-native) crayfish are known to burrow into river banks and visual evidence of river bank damage is generating public concern and media attention. Despite this, there is a paucity of understanding of burrow distribution, biophysical controls and the potential significance of this problem beyond a small number of local studies at heavily impacted sites. This paper presents the first multi-catchment analysis of this phenomenon, combining existing data on biophysical river properties and invasive crayfish observations with purpose-designed field surveys across 103 river reaches to derive key trends. Crayfish burrows were observed on the majority of reaches, but burrowing tended to be patchy in spatial distribution, concentrated in a small proportion (<10%) of the length of rivers surveyed. Burrow distribution was better explained by local bank biophysical properties than by reach-scale properties, and burrowed banks were more likely to be characterised by cohesive bank material, steeper bank profiles with large areas of bare bank face, often on outer bend locations. Burrow excavation alone has delivered a considerable amount of sediment to invaded river systems in the surveyed sites (3tkm(-1) impacted bank) and this represents a minimum contribution and certainly an underestimate of the absolute yield (submerged burrows were not recorded). Furthermore, burrowing was associated with bank profiles that were either actively eroding or exposed to fluvial action and/or mass failure processes, providing the first quantitative evidence that invasive crayfish may cause or accelerate river bank instability and erosion in invaded catchments beyond the scale of individual burrows.

Reintroduced large wood modifies fine sediment transport and storage in a lowland river channel

This paper explores changes in suspended sediment transport and fine sediment storage at the reach and patch scale associated with the reintroduction of partial LW jams in an artificially over-widened lowland river. The field site incorporates two adjacent reaches: a downstream section where LW jams were reintroduced in 2010 and a reach immediately upstream where no LW was introduced. LW pieces were organised into ‘partial’ jams incorporating several ‘key pieces’ which were later colonised by substantial stands of aquatic and wetland plants. Reach-scale suspended sediment transport was investigated using arrays of time-integrated suspended sediment samplers. Patch-scale suspended sediment transport was explored experimentally using turbidity sensors to track the magnitude and velocity of artificially generated sediment plumes. Fine sediment storage was quantified at both reach and patch scales by repeat surveys of fine sediment depth. The results show that partial LW jams influence fine sediment dynamics at both the patch and reach scale. At the patch-scale, introduction of LW led to a reduction in the concentration and increase in the time lag of released sediment plumes within the LW, indicating increased diffusion of plumes. This contrasted with higher concentrations and lower time lags in areas adjacent to the LW; indicating more effective advection processes. This led to increased fine sediment storage within the LW compared with areas adjacent to the LW. At the reach-scale there was a greater increase in fine sediment storage through time within the restored reach relative to the unrestored reach, although the changes in sediment transport responsible for this were not evident from time-integrated suspended sediment data. The results of the study have been used to develop a conceptual model which may inform restoration design.

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.

Large Wood Dynamics Along the Tagliamento River, Italy: Insights from Field and Remote Sensing Investigations

Large wood plays a vital role in many fluvial systems through its influence on a wide variety of hydromorphological and ecological processes. Growing recognition of the significance of large wood has led to its increasing use as a river restoration measure but this remains juxtaposed against its traditional perception as a flood risk hazard. Improved understanding of when, where and how large wood is recruited by rivers, and its behaviour and effects once entrained, is therefore of critical importance to river management. This paper integrates the findings of a number of recent studies to characterise and explain large wood dynamics along the Tagliamento River, Italy. A combination of remotely-sensed and field data are used together with historical hydrometeorological data to quantify and explain spatio-temporal variability in large wood recruitment and associated geomorphological adjustment. The results highlight strong longitudinal differences in large wood supply rates, dominant recruitment process and consequences as a result of the changing environmental conditions along the river. These insights serve to illustrate how universal rationales and approaches to the management of large wood in rivers can be misguided and should be informed by detailed knowledge of the system for which they are developed.