The habitat-scale ecohydraulics of rivers

Abstract

This paper establishes the link between ‘functional habitats’ (biologically defined habitat units) and flow ‘biotopes’ (hydraulically defined habitat units) using Froude number. Froude number has been shown to be the hydraulic variable that best describes surface flow type (flow biotope). The approach used was to examine the relationship between functional habitat occurrence and Froude number, with the aim of developing ‘habitat preference curves’. Fifteen of the 16 functional habitats were found to be distributed with Froude number in a non-random fashion (‘woody-debris’ being the only exception). Eight functional habitats were most likely to occur in the lowest Froude number class. These were ‘silt’, ‘roots’, ‘trailing vegetation’, ‘marginal plants’, ‘leaf-litter’, ‘emergent macrophytes’, ‘floating-leaved macrophytes’, and ‘submerged broad-leaved macrophytes’. The remaining habitats occurred at higher Froude numbers, with a gradient of increasing Froude number going from ‘sand’, to ‘gravel’, to ‘moss’ to ‘macroalgae’ to ‘cobbles’ to ‘submerged, fine-leaved macrophytes’. This information can be applied to river rehabilitation projects. Hydraulic variables, such as Froude number, can be manipulated through changes to channel morphology, to maximise habitat heterogeneity and therefore biodiversity.