Geomorphology, ecology and river channel habitat: mesoscale approaches to basin-scale challenges

Abstract

The physical component of river channel (instream) habitat is of acknowledged significance, particularly in headwater streams; furthermore, physical habitats have been heavily impacted by human needs for river services: principally flood defence (channel modification) and water resources (flow regulation). Despite the control exercised on physical habitats by fluvial geomorphology (channel shape, bed material size, bedforms and bars) and flow regime (including the varying hydraulics of flow around these forms), their interaction has hitherto lacked a distinctive spatial formulation and biological validation. This article describes the challenges faced by a need (in both theoretical and practical areas) to understand the detail of physical habitat conditions in stream channels. A number of mesoscale approaches are emerging, both from ecology and geomorphology. We outline the field validation of a ‘habitat hydraulics’ approach to the interaction between river discharge and channel form. Qualitative ‘flow types’ are shown to be representative of discrete hydraulic conditions within mesoscale units of the channel bed described as ‘physical biotopes’. The approach is compared with parallel ecological research on ‘mesohabitats’ and ‘functional habitats’. The extent, pattern and discharge-variability of physical biotopes can be surveyed in the field and used as a spatial guide to biological sampling (in this case for benthic invertebrates). Biological patterns (at the scale sampled here) appear to respond first to the river continuum concept’s longitudinal zonation of the channel, but there is a marked secondary signal in statistical analyses from the pattern of biotopes. Given the promise of the physical biotope approach, its logical extension in predictive mode is via the hierarchical concepts shared by freshwater ecologists and geomorphologists; however, there is little agreement on scale terminology, hierarchical principles and, above all, a truly geo-morphological channel classification, based on reaches, into which mesoscale habitat typologies could be fed. Assuming a relationship between biological diversity and that of physical biotopes it should be the aim of river managers, whether intervening in the channel form or its flow regime, to optimize certain parameters of the pattern of biotopes; these parameters should be a focus for future research. Both the rapid survey and proper description of these patterns will benefit from the application of remote-sensing technology, improved instruments for microscale hydraulic studies and a GIS approach based upon the spatial principles of landscape ecology.

Indicators of movement and space use for two co-occurring invasive crayfish species

“…Mesohabitats were classified as pools, riffles and runs according to current velocity and turbulence, which was visually assessed. Correspondence of the observed mesohabitat types to Newson and Newson (2000) surface flow types is as follows. Pools: no perceptible flow, smooth surface, reflections with no or very minor distortion; Runs: smooth boundary turbulent flow (very little surface turbulence, very small turbulent flow cells are visible, reflections are distorted); Riffles: rippled flow (water surface has regular disturbances, which form low transverse ripples across the direction of flow), broken standing waves (standing waves present which break at the crest originating white waters), chute flow (fast flow over boulders and bedrock).…”

Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications

“…In the early phase of heating molecular diffusion is sufficient for transporting the heat upwards, dissipating it and depleting the temperature gradient in the kettle. When the stove temperature and consequently the temperature gradient increase further, convection cells begin to form (Nicolis and Prigogine, 1977;Prigogine and Stengers, 1984), as these allow for a more efficient energy dissipation at the higher temperature gradient than does molecular diffusion. When the vertical temperature gradient increases even further, above a second threshold, turbulent eddies begin to form (Nicolis and Prigogine, 1977;Haken, 1983) and water begins to "boil".…”

“…Identification of cause-effect relations in the context of threshold behaviour is made difficult as a result , especially when these are accompanied by structural or morphological changes as well (Newson, 1980;Newson and Newson, 2000;Phillips, 2004Phillips, , 2006Faulkner, 2008). Hence, threshold behaviour in hydrological and geoecosystems becomes much more difficult to detect, understand and predict in comparison to elementary threshold phenomena, which can be predicted on the basis of well observable dimensionless variables such as the Reynolds number.…”

Influence of morphohydraulic habitat structure on invertebrate communities (Ephemeroptera, Plecoptera and Trichoptera)

“…These tasks lead to the cooperation of several branches of science such as fluvial geomorphology, ecology, zoology, botany, etc. Contribution of fluvial geomorphology represents understanding and analysis of processes which form the river channel and methods of assessment of physical habitats (Newson & Newson 2000;Newson 2002;Gordon et al 2004;Clifford et al 2006). It was recognised in stream ecology that the diversity of stream communities is directly influenced by the heterogeneity of abiotic environment (Hynes 1970).…”