Identification and measure of hydromorphological degradation in Central European lowland streams

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 m(3,), 'no of logs >10 cm 0', 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.