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We compared habitat diversity and morphodynamics of 'actively' restored reaches (including removal of bank fixation, widening and large wood placement) with 'passively' restored reaches (abandonment of channel maintenance) and adjacent non-restored control reaches in medium-sized Central European mountain rivers. Habitat diversity and river morphology were mapped in two consecutive years and changes in habitat composition (channel features, substrates) and morphological changes (active depth and width) were quantified. In both years, habitat diversity was generally higher in the restored reaches compared to their non-restored counterparts, and significantly differed between restoration approaches, with average values in actively restored reaches being about 60% larger than in passively restored reaches. Channel feature composition differed significantly, both between restored and unrestored reaches, and between restoration approaches, whereas substrate composition was similar in all investigated reaches, indicating that restoration had a higher effect on mesohabitat than on microhabitat conditions. Interannual habitat dynamics in respect to channel feature composition were larger in the actively restored compared to the passively restored reaches, while substrate composition remained fairly constant in all reaches. Regarding morphodynamics, changes in depth and width of actively restored reaches differed significantly from changes in passively restored ones in three of the four elements compared. Our findings imply increased habitat richness, diversity and habitat dynamics in the restored reaches, especially in actively restored ones. Analysis of discharge data suggests that flood events exceeding critical shear stress of the bed material, and the time span since restoration determine the potential for morphological changes. [ABSTRACT FROM AUTHOR]

The objective of the current study was to identify hydromorphological variables that are suitable to define and describe hydromorphological degradation. Stream type-specific and spatial scale-dependent multivariate analysis (Non-metric Multidimensional Scaling, NMS) of 106 hydromorphological variables derived from 275 samples at 147 sites and indicator value analysis (IndVal) resulted in the identification of key factors describing hydromorphological differences in Central European lowland streams. Sample sites represented six European stream types from Sweden (1 stream type), The Netherlands (2 stream types), and Germany (3 stream types). The four large-scale hydro(geo)morphological variables: catchment size, geology (`% moraines', `% alluvial deposits'), and natural land use (`% natural forest') explained inter-stream type differences best. On the smaller site scale, riparian vegetation described inter-stream type differences best. On catchment scale, `% natural forest', and `agricultural land use' illustrated inter-stream type hydromorphological degradation of all six stream types very well. Four site related variables (`% wooded riparian vegetation', `% shading', `average stream width', and `% macrolithal (cobbles, 20 to 40 cm long) account for hydromorphological degradation on the smaller reach-scale. An analysis of indicator variables restricted to German stream types only resulted in four factors, namely `% xylal' (tree trunks, branches, roots, etc.), `no of debris dams >0.3 m3', `no of logs >10 cm ∅', and `% fixed banks' as important descriptors of hydromorphological degradation. Intra-stream type hydromorphological degradation is illustrated for `mid-sized sand bottom streams in the German lowlands'. For this stream type, a clear gradient of degradation was revealed, and 25 variables were identified to entirely characterize reference conditions and degradation. The variables that described the degradation gradient best were combined to the new German Structure Index (GSI), which can be implemented to continuously measure hydromorphological degradation. [ABSTRACT FROM AUTHOR]