Hydromorphological parameters indicating differences between single- and multiple-channel mountain rivers in Germany, in relation to their modification and recovery

1. Despite a growing number of river restoration projects in Central Europe targeting hydromorphological improvements, it is still uncommon to evaluate the effects of restoration. Hydromorphological diversity in straightened, single-channel sections was compared with restored or naturally developed multiple-channel sections in German mountain rivers to identify parameters suitable for judging conservation value or restoration success. 2. Seven multiple-channel sections were compared with nearby straightened single-channel sections. Six hydromorphological parameters at macro-, meso- and micro-scales were recorded with transect point protocols, including aquatic, terrestrial and transient areas and width of channel features (main and secondary channels, sidearms, standing water bodies, bars, banks, floodplains and embankments). Depth, velocity and substrate type were measured at 400 points per river section. With these data 12 metrics were calculated, to provide comparison between the sections. 3. All macro- and meso-scale parameters were well differentiated between single-channel and multiple-channel sections: mean channel width and shoreline length increased by factors of 2.1 and 2.4, respectively; the average number of channel feature types increased from two to 10 per section. 4. Micro-scale parameters, such as velocity and depth variance, were significantly different between single-channel sections and most multiple-channel sections. Substrate composition was more diverse in the restored sections; the share of the major substrate was on average reduced from 75% to 62%. The Spatial Diversity Index described differences in substrate patterns best. The individual parameters correlated only within scales, but rarely between them. 5. Rapid assessment of restoration success or judgment of conservation value is made possible by combining simple macro- and meso-scale parameters, e.g. increase of overall width or of transient areas, and presence of bars and islands. Increase in aquatic area and diversity of habitats are the most appropriate micro-scale parameters required to evaluate habitat suitability for aquatic organisms. For depth and velocity data, variance should be analysed. Copyright (c) 2008 John Wiley & Sons, Ltd.