Detailed characteristics of 73 log steps (i.e., steps formed by a single key wood piece) in steep headwaters draining managed Carpathian forests were assessed together with channel- and wood-based thresholds for log step occurrence. The upstream threshold of log step occurrence can be assigned to those locations in stream longitudinal profiles, where transport mode during high flow events switches from suspended to bedload. Downstream thresholds were controlled by the ratio between the stream transport capacity and wood dimensions, which reflected the limited recruitment of large wood pieces (in the terms of their size and frequency) in managed forests. The majority of observed log steps (93.1%) occurred in channels less than 2 m wide. The possible occurrence of a log step at a particular channel width was controlled by the ratio between the wood length and channel width, but the resultant step parameters (step height and step crest width, volume of stored sediments) better correlated with wood diameter. The wood orientation (both horizontal and vertical) had less importance on the possible occurrence of a log step. Wood pieces shorter than 4 m accounted for 72.2% of the step-forming pieces (avg. length=3.18 m), and the maximal diameter was less than 0.1 m for 20.5% of the pieces (avg. diameter=0.17 m). It implies the significance of relatively small wood in the log step development and sediment routing in the studied streams; the average sediment storage generated by a log step was 0.51 m3. Based on previous dendrochronological cross-dating of instream wood in neighbouring basins, we suggest longer residence time for steps created by Norway spruces than steps originated from European beech. The calculated mean residence time by accounting medians for observed decay classes is equal to 18.7 years for log steps formed by beeches and 43.9 years for steps formed by spruces.

Carpathians, headwater stream, log step, sediment storage