This paper examines the practical use of physical biotopes as the basic component of instream physical habitat. These are identified by dominant flow type as a particular combination of substrate and hydraulic parameters will have a characteristic surface flow type. Eleven rivers representing a range of geomorphological features and physical biotopes were selected, and depth, velocity, and substrate data were collected at metre intervals in subjectively-spaced transects. Flow type was recorded at each sample “cell” and transects classified on the basis of dominant flow type. Combined hydraulic indices were calculated in order to characterise more completely a particular biotope and to facilitate comparisons among different discharges and sites.

The subjective classification of flow types was tested statistically by discriminant analysis and units were shown to be hydraulically discrete in terms of Froude number and other combined hydraulic indices. Identification of physical biotopes by flow types has been incorporated into the Environment Agency’s (formerly National Rivers Authority) River Habitat Survey (RHS). The RHS is based on 500 m reaches selected randomly within a stratified selection from a national grid, to characterise the physical features of rivers in England and Wales, and to provide an objective method for assessing habitat quality.

Biotope mapping at different flows was undertaken to establish how the biotope sequence differs among sites and with increasing discharge. Threshold discharges at which biotopes change are related to flow duration data to predict annual patterns of biotope ‘patchiness’ and ‘diversity’. Flows may then be manipulated to maintain ‘natural’ or ‘desirable’ biotopes. Sustainable management of aquatic systems and their biota would require flow management of critical reaches or biotopes, at key life-stages of target organisms.

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