Connectivity of the coarsest fraction in headwater channels: imprints of fluvial processes and debris‐flow activity

Abstract

Some steep headwater streams of the mid‐mountains landscape of the flysch belt of the estern arpathians are affected by debris flows. Connectivity of the largest boulder and cobble fractions has been evaluated in steep streams found in the transitional zones between the hillslopes and alluvial cones of similar lithology and watershed morphometry. Two longitudinal profiles affected by past debris‐flow activity and two longitudinal profiles void of such processes were selected. In the first case, active channels were characterised by the presence of the coarsest fraction, while a downstream trend of sediment coarsening or fining lacked any influence of contemporary fluvial processes and sediment supply. In addition, debris‐flow‐affected streams showed greater differences between the middle axis of the coarsest particle and the particle‐size index including the lengths of all three axes, which is most likely the result of limited active fluvial transport. All the studied streams demonstrated general downstream fining of the largest particles, which resulted from local flysch lithology containing more resistant sandstones in the upper parts of the watershed. The trend of sediment fining was more rapid in the longitudinal profiles unaffected by past debris flows. These types of streams also showed adjustment of the coarsest bed fraction to contemporary processes in channels by sediment coarsening in incised reaches and sediment fining in depositional reaches as well as by downstream response to some lateral sediment inputs. The index of the unit stream power showed no correlation with the considered coarsest fraction in both fluvial‐dominated channels and debris‐flow‐affected channels.

Key words:

• headwater stream
• bed sediments
• connectivity

Acknowledgements

We would like to thank three anonymous reviewers and the associate editor Stephan Gruber whose careful and comprehensive comments significantly led to the improvement of the manuscript. The authors are also sincerely thankful for the revision of the English style to Monika Hradecká, Joseph G. Spark and Elsevier Language Editing Services. The study was funded by the Postdoc‐2013 project of the University of Ostrava and the student grant competition SGS19/PřF/2014.

Additional information

Notes on contributors

Jan Hradecký

Tomáš Galia, Václav Škarpich, Jan Hradecký, Department of Physical Geography and Geoecology, Faculty of Science, University of Ostrava, Chittussiho 10, 710 00 Ostrava, Czech Republic

E‐mail: [email protected], [email protected], [email protected]