ABSTRACT1. Improved linkage between physical characteristics of rivers and biological performance or potential is a recurrent theme in contemporary river survey, management and design. This paper examines the degree to which flow biotopes and functional habitats may be differentiated with respect to physical habitat delimiters, i.e. flow depth, velocity and Froude number.2. Re-examination of published data demonstrates only very broad associations between biotopes, functional habitats and 'low', 'medium' and 'high' bands in the chosen physical habitat delimiter. The associations are also not consistent between different delimiters.3. Re-analysis is complicated by considerations of research design and methodology. Further studies are required with greater control on the circumstances of observation (particularly flow stage and seasonality), which explicitly control for variance within and between different river cases, and which adopt more consistent terminology.4. Field data for a single river reach at low and high flows indicate that use of the Froude number, in particular, requires careful interpretation. Very different velocity and depth combinations can exhibit similar Froude numbers. This may obscure important contrasts, such as those between channel margin and channel centreline environments.5. Field data also reveal that the flow characteristics of even basic (riffle or pool) units of channel morphology exhibit strong stage-dependence. There are also significant variations when channel margins are isolated from the definitions of the bedforms or analysis of the data. Current practice which seeks to simplify field survey to channel cross-section transects is therefore likely to be misleading.6. Given these difficulties, attempts to link biotopes with ecological response appear premature. Further research might, however, be directed to identifying possible associations between combinations of flow types and bedforms or functional habitats. In this sense, the biotope concept may be more profitably employed as one of several surrogate measures for potential biodiversity.

During the twentieth century, the planform and profile of the Lower Mississippi River from Cairo, Illinois, to New Orleans, Louisiana, have been transformed by a series of engineering modifications. These include steepening of the long profile by removal of the most sinuous bends, extensive bank stabilization, and regulating sediment movement by dyke field construction. Prior to these modifications, the Lower Mississippi River adjusted its morphology in the planform, long profile and crosssection. Planform adjustment has, however, effectively been negated and other adjustments are now constrained. Nevertheless, analysis of hydrographic surveys between 1949 and 1989 demonstrates that geomorphological response during the post-cutoff period remained complex. Morphological adjustments involved phased patterns of aggradation and degradation, together with changes in cross-sectional form, and in the number, size, location and shape of pools and crossings. Greatest changes occurred in the early post-cutoff period (1949-64) upstream from Vicksburg, Mississippi, but were accompanied by complementary changes elsewhere which propagated downstream. The combined set of responses may be interpreted with respect to a dynamic equilibrium in which the river responded to additional energy created by the cutoffs by increasing and adapting flow resistance over various scales and time periods. This study helps resolve paradoxes from previous analyses, and has significance for interpreting past engineering impacts and for suggesting future management strategies for the Lower Mississippi River.

This paper presents an analysis of the planform behaviour of the Lower Mississippi River (LMR) using a series of maps and hydrographic surveys covering the period 1765-1975. Data allow analysis at various time and space scales, using fixed and statistically defined reaches, both before and after extensive channel modification. Previous research has interpreted planform change in relation to geomorphological or engineering regime-type analyses of channel length and width for the LMR as a 'single system'. The analysis here is broadly consistent with these approaches, but highlights the importance of meander geometry, in the form of the radius of curvature:width ratio. This neglected factor helps resolve paradoxes relating to observed changes in sediment transport and channel stability. When viewed over smaller time and space scales, analysis of dynamics using fixed reach boundaries reveals a downstream trend in the pattern of planform behaviour, which is closely related to the distribution of valley floor deposits, and which also reflects neotectonic influences. Analysis of changes using statistically determined reach boundaries shows that, over shorter time scales, meander trains are continually formed and modified over a period of approximately 120 years. Zones of more-or-less dynamic behaviour thus move through the LMR. The research also provides a context for 20th century engineering interventions to the river. These have constrained the magnitude of planform adjustment, but also altered the kind of response that is now possible in relation to changes in discharge and sediment load, and as a consequence of internal feedbacks within the LMR system. A long-term planform record does exist, however, for the Lower Mississippi River. This provides a rare opportunity to analyse planform dynamics within their spatial and temporal context, and to offer explanations based upon the distribution of geological and neotectonic controls, changing discharge and sediment load regimes, and contrasting channel management strategies. In this paper, planform changes are studied between 1765 and 1975 using a sequence of planform maps, historic hydrographic surveys, long-term hydrological records and maps of abandoned channels in the Lower Mississippi alluvial valley. The period 1765-1975 is of particular interest because it combines both a phase of accelerated dynamics (in relation to average rates over the preceding several thousand years; Saucier, 1994), with a phase of constrained dynamics following channel shortening and bank stabilization in the 20th century.The purpose of this paper is twofold: to evaluate the complexity of planform dynamics by undertaking a multiscaled investigation using a unique large-scale and long-term planform record; and to improve understanding of the pre-modification planform dynamics of the Lower Mississippi River to inform shorter-term studies of geomorphological response to channel intervention. Analysis and discussion address the following research questions: (i) the extent to which planfo...

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Flood risk is influenced by the rate and speed of rural land runoff within the catchment and by flow attenuation in the floodplain. Most research to date has demonstrated that the overall flood risk benefits of multiple small scale land management changes at a large catchment scale are difficult to determine and, in some situations, may not provide significant benefits in flood events. Measures may, however, offer potential for attenuation, improving flood warning times and therefore reducing flood damages. Climate change and major legislative drivers such as the Water Framework Directive will also further drive the need to consider different approaches to how land is managed and the impact that this may have on flood and coastal risk management.