Do the coarsest bed fractions and stream power record contemporary trends in steep headwater channels?

• The coarsest bed fraction and the unit stream power were evaluated in headwaters.
• Calculated flow competence reflected local bedload transport observations.
• Sediment coarsening and increase in the unit stream power occurred in incised reaches.
• Debris-flow influenced stream showed different behaviours than others.
• Bankfull discharges were not effective discharges in steep headwaters.

Three stream channels that were devoid of evidence of past debris flows and one headwater channel that contained debris flow deposits in the flysch western Carpathians, Czech Republic were selected to test relationships between in-channel processes, bed sediments, and unit stream power calculated for bankfull and Q20 flows. Contemporary depositional or erosional trends in the examined headwaters were linked with bed sediments that were represented by the coarsest cobble and boulder fraction with a mean calculated from the five largest particles. The downstream trends of the unit stream power were derived for a bankfull discharge and a well-documented 20-year flood event. In addition, the flow competences during the discharges were calculated using indirect bedload transport measurements. Downstream fining of the cobble and boulder fraction was observed in all of the studied headwaters, and unique downstream variations of the unit stream power were calculated for the longitudinal profiles. The single-thread streams that were devoid of evidence of debris flow events exhibited direct relations between the coarsest sediment size and the unit stream power, especially as calculated for the 20-year flood event and for erosional/depositional trends of the channel. The downstream coarsening of the bed material that was accompanied by an increase in the unit stream power was usually observed in the case of deeply incised (> 0.5 m above the assumed bankfull depth) channel reaches. The calculated competence of the 20-year flow was up to twofold higher than that required to entrain the largest bed particle diameters in those channel reaches, and even the bankfull flow was potentially capable of transporting the coarsest bed particles in certain of the reaches. On the other hand, some depositional channel reaches evidently led to the disconnectivity of transport of the coarsest bed material even in the case of the 20-year flood event. The longitudinal profile of the channel that had evidence of past debris flows did not clearly show a direct relationship between boulder size and calculated unit stream power and contemporary channel trends. This finding reflected the presence of significantly coarser bed material, which points to a higher stability of the debris flow-influenced channel during ordinary floods when compared to fluvially dominated headwaters that are devoid of debris flows.