Variations in bar material grain-size and hydraulic conditions of managed and re-naturalized reaches of the gravel-bed Bečva River (Czech Republic).

European multi-thread rivers have undergone rapid morphological changes during past centuries due to the extensive direct and indirect human impacts on fluvial systems. As a consequence, we can identify altered patterns of bed sediment calibre reflecting disturbed sediment connectivity and modified flow hydraulics. Changes in the grain-sizes of samples collected on 68 gravel bars in August 2015 were studied along 14.0-km river reach of the Bečva River (Outer Western Carpathian Mts., Czech Republic). The grain-size characteristics obtained were confronted with modeled flow hydraulics and the present stage of the channel. The studied channel reach is presently characterized by several distinctive sections: for a long time (ca. 100 years) regulated single channel sections with artificial bank stabilizations incised several meters in the floodplain and by contrast, multi-thread channel patterns of two sections, which have witnessed retrograde development after large floods in 1997 and 2010 with 100- and 50-year recurrence intervals, respectively. The present channel behaviour of managed (regulated) and re-naturalized (multi-thread) river sections corresponded well with the modeled hydraulic variables for one-year discharge recurrence interval. Especially, re-naturalized river sections showed lower values of flow competence which facilitated the deposition of sediment material in the form of gravel bars. The high occurrence of lateral sediment sources (e.g., tributaries, bank failures) together with sediment disconnectivities (e.g., boulder ramps) in the longitudinal river reach were observed, and grain-size parameters did not particularly reflect the hydraulic conditions. Especially tributaries as sediment inputs had significant effect on bar grain size and increase of channel diversity, although, in general results indicate a gradual downstream fining.