1. 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.