The links between rapid channel degradation and related large wood (LW) recruitment and storage are still not precisely understood, especially in aquatic systems with high human and natural pressures such as intermittent rivers in the peri‐Mediterranean region. In this study, by using multidisciplinary research (analysis of satellite images time series, detailed field inventory, and simulated hydrological data), we focused on rapid upstream propagation of channel degradation (i.e., incision and consequent channel widening) in an intermittent Mediterranean river (Evrotas River, southern Greece) as a response to short‐term intensive gravel extraction from the active channel and removal of riparian vegetation in the 2013–2016 period. Subsequently, our objective was to link this channel transformation with the spatiotemporal changes of LW storage in the active channel. We found that short‐term and spatially limited human impact, coupled with high flows, allowed rapid upstream progression (~2 km) of erosion processes (approximately twofold widening of the active channel) during the relatively short 2‐year period (2017–2019). This morphological response accelerated LW recruitment through bank erosion and its subsequent storage in the active channel. Incision and channel widening processes were interrupted in the 2019–2021 period due to the prevalence of relatively low, geomorphologically non‐effective flows. However, the storage of LW in the channel continued to increase as a result of the higher trapping efficiency of the widened channel, a delay between tree death and its final uprooting, or by LW recruitment processes not directly related to flood events. We suggest that freshly recruited LW has the potential to accelerate the establishment of a new channel equilibrium and increase channel heterogeneity through its interaction with flows and sediments.

The geological environment is undoubtedly one of the basic factors that influence the formation of surface runoff. The extent to which this factor can also affect the hydrodynamic characteristics of flash floods, which is also indirectly associated with flood risk, is the main topic of this study. In two geologically different areas of the Bohemian Massif (crystalline rocks predominate) and the western Carpathians (flysch rocks predominate), a total of 40 watersheds characterised by sharing a certain hydrological analogy were selected (20 watersheds from the Massif and 20 from the Flysch zone). In each of these watersheds, 1-year, 10-year and 100-year flash flood return periods were constructed using the two-dimensional hydrodynamic model Iber. The outputs from this model included raster datasets of areas, depths, and flow velocities during inundations. Subsequently, these rasters were analysed and compared with an emphasis on differences within the individual geological study areas. The outputs showed clear differences in the individual hydrodynamic characteristics (e.g. the average inundation area during Q100 was 29.07% larger in the Flysch than in the Massif). Overall, the Flysch zone appeared to be far riskier in terms of flash floods than in the case of the Bohemian Massif.

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