Influence of large woody debris on the morphology of six central European streams

The impact of large fallen trees on channel form is described for six short stream sections in central Europe influenced by large woody debris (LWD sections), five of which are compared to nearby reference sections free of LWD (reference sections). Three-dimensional models of streambed topography were generated by surveying cross-sections with a spacing of 1 per 1/15 channel width. Parameters derived from digital terrain models and cross-sections compared between LWD sections and reference sections include the extent of pools, bars, and cutbanks, streambed and bank complexity, cross-sectional area, width, depth, and cross-section complexity as described by Andrleʹs [Math. Geol. 26 (1994) 83] ‘angle-measurement-technique’ (AMT analysis), a measure of the deviation of a cross-section line from a straight line. Structural diversity is greater in LWD sections at almost all spatial scales, particularly in terms of pool volume (Mann–Whitney U-test, p<0.01) and cross-section complexity described by median angle of AMT analysis (Mann–Whitney U-test, p<0.05). Large pools are clearly associated with large fallen trees and attain volumes up to 36 m3. With the exception of the ratio of one LWD section where the fallen tree is oriented parallel to flow, the ratio of pool volume to bed planimetric area ranges from 424 to 693 m3/ha, which is in the upper range reported for small, high-gradient streams in Oregon, NW America (229–755 m3/ha) [Can. J. Fish. Aquat. Sci. 47 (1990) 1103]. Pool volume of LWD sections is strongly correlated to the blockage ratio (Spearman rank order correlation, rs=0.93, p<0.01). Differences in channel morphology between the LWD sections and reference sections indicate a strong morphologic control of large woody debris in these central European stream sections.