channel complexity, check dams, mountain stream, Moravskoslezské Beskydy Mts.

The application of transversal structures as sequences of check dams represents widely used approach in mountain basins to stabilise stream longitudinal profiles and decrease bedload transport rates. Our talk will demonstrate consequences of check dams on selected dimensions of channel complexity (bed sediment distribution, longitudinal heterogeneity, cross-sectional heterogeneity and large wood distribution) by comparisons of two pairs of check dam managed and untreated streams of similar dimensions draining flysch Carpathians (Moravskoslezské Beskydy Mts.). We focused on the intermediate reaches between two successive check dams, which were not directly affected by erosional (scoured pool) or depositional processes (sedimentary wedge) of individual check dam bodies. We observed a decrease primarily in the heterogeneity of bed sediments in the intermediate reaches of the steep managed channel and degraded complexity metrics of longitudinal and cross-sectional profiles in the unconfined foothill stream managed by check dams when compared with the streams without check dams. The different adjustment of channel complexity to the presence of check dams in the intermediate reaches corresponds to the position of a particular reach on the fluvial continuum. Despite a decrease in sediment heterogeneity in steep reaches managed by check dams, the presence of coarse bed sediments helped to preserve their stepped-bed morphology but with lower steps and shallower pools. In contrast, the longitudinal profile of the treated foothill stream completely lost its vertical bed oscillations because of the complete transformation of original pool-riffles to a uniform plane bed channel. Decreased cross-sectional heterogeneity in the managed foothill stream likely resulted from the artificial straightening of the channel and the occasional presence of bank stabilisations. We did not observe differences in large wood abundance between treated and untreated streams.