Use of ‘functional habitats’ to link ecology with morphology and hydrology in river rehabilitation

Abstract

1. This paper examines the influence of channel morphology and hydraulics on the occurrence and diversity of ‘functional habitats’ (also called ‘meso-habitats’) in semi-natural and physically degraded rivers. 2. The depths and velocities at which each functional habitat is most likely to be found are described using ‘occurrence matrices’. In most cases, each habitat was associated with distinct depth-velocity conditions. 3. The relationship between habitat occurrence, diversity and river degradation was examined. The deviation of a site from its predicted natural summer wetted width was used as an index of physical degradation. Habitat diversity (measured as the Shannon index) was low for sites that were wider than expected for natural conditions. This was independent of the type of physical perturbation causing the width deviation. 4. Two main types of physical perturbation were identified in the degraded river: over-widening and ponding. Over-widened sites were shallow, slow and dominated by silt and emergent macrophyte habitats. Ponded sites were deep, slow and dominated by floating-leaved macrophyte and macroalgal habitats. 5. River reaches of uniform depth had low habitat diversity compared with reaches containing both shallow and deep areas. 6. It is recommended that objective assessment of the type and extent of physical degradation, prior to restoration, should be made by measurement of depth, velocity, width and functional habitat distribution. Changes in channel morphology should then be designed to create the necessary range of depths and widths, and therefore velocities and habitat types, deemed desirable for that river site. Habitats that are over-dominant or rare can be identified and the ‘occurrence matrices’ used to inform decisions about the change in physical conditions needed to enhance or suppress that habitat. 7. The approach is a cost-effective way to link ecology with morphology and hydrology in river channel rehabilitation. Copyright © 1999 John Wiley & Sons, Ltd.