River widening: An approach to restoring riparian habitats and plant species

Abstract

River widenings' are commonly used in river restoration to allow channel movement within a spatially limited area. Restoration seeks to restore fluvial processes and to re-establish a more natural riparian community. This study investigates the performance of five river widenings in Switzerland, focusing on the re-establishment of riparian (semi-)terrestrial habitats and species, and highlights some factors that seem to influence their performance. The restoration projects are compared with pre-restoration conditions and near-natural conditions, which are assumed to represent the worst- and best-case conditions along a gradient of naturalness. Fuzzy ordination of vegetation data and calculation of landscape metrics based on habitat maps revealed marked differences between the degree of naturalness achieved by each individual restoration project. However, in general river widenings were found to increase the in-stream habitat heterogeneity and enhanced the establishment of pioneer habitats and riparian plants. Analyses of species pools based on a hierarchic list of indicator species and correspondence analysis showed that the ability of river widenings to host typical riparian species and to increase local plant diversity strongly depends on the distance to near-natural stretches. Species dispersal and establishment might be hampered by decisions taken outside the scope of the restoration project. Therefore we conclude that action on the catchment scale is needed to maximize the benefits of local management. Copyright © 2005 John Wiley & Sons, Ltd.