The majority of research on instream large wood (LW) focuses on perennial streams in temperate zones, and much less is known about wood distribution and its driving factors in other environments, such as semiarid bioclimatic regions. In this study, we sought to predict LW retention by assessing reach‐scale variables to infer the influence of external factors on the presence of LW in 37 independent reaches of ephemeral and intermittent streams draining a mountain range in a Mediterranean semiarid environment (Lefka Ori Mountains, Crete, Greece). In addition, we focused on LW pieces that had been mobilized by fluvial transport to find links between wood mobility and the external reach‐scale characteristics of this environment. Markedly low average LW frequency (6.5 LW/100 m) and volume (9.06 m3/ha) were observed in the studied reaches in comparison with streams draining other bioclimatic regions. Only one complex general linear model was found to predict LW volume (including valley confinement and basal area of coniferous trees), and five explanatory variables (elevation as a proxy of annual precipitation, valley type, valley confinement index, basal area of coniferous trees, and basal area of all trees) were significant in separated models predicting LW volume. Only two variables (valley floor width and the basal area of broadleaf trees) were significant in separated models for prediction of LW frequency. The mobilized LW in the studied streams tends to be stored as individual, dispersed pieces rather than in jams, likely due to overall low LW abundance which effectively prevents clustering of wood.
High-gradient streams are integral parts of the fluvial systems, which connect the mountainous landscapes with lowland rivers in the sense of water and sediment transport. We analysed downstream grain-size characteristics of bed sediments and sphericity of grains with respect to the local flysch lithology and channel geometry in the 5.4 km long reach of the high-gradient Kobylská Stream (the Vsetín highlands, Outer Western Carpathians, Czech Republic). Results showed a high variability of bed-sediment sizes in the longitudinal course of the Kobylská stream. This condition is given by the occurrence of anthropogenic impact in the studied channel (local bank stabilisations, grade-control structures) and lateral sediment input from the adjacent landslides and bank failures. The lithology of the flysch nappe structure of the basin significantly influenced variations of bed sediment grain-sizes. Different bedrock resistance resulted into inverse correlation between the local river gradient and related bed grain-sizes. Especially the mudstone bedrock layers had significant impact on the fining of bed sediment in the upper steep part of the basin (D50 ≤ 40 mm) and by contrast, sandstone bedrock caused the coarsening of the bed sediment in the middle and lower parts (D50 up to 50 mm) characterised by lower bed gradients.
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