Effect of slope failures on river-network pattern: A river piracy case study from the flysch belt of the Outer Western Carpathians

•Landslides caused river piracy in Flysch Belt of the Outer Western Carpathians.•The landslides forced lateral erosion and about 400 m long abandoned valley developed.•Retrograde erosion was intensified in the elbow of capture and in the captured stream.•Estimated minimum erosion rates are 5.22 times higher in the captured catchment.

Landslides are important geomorphic agents in various mountainous settings. We document here a case of river piracy from the upper part of the Malá Brodská Valley in the Vsetínské Mts., Czech Republic (Rača Unit of the flysch Magura Group of Nappes, flysch belt of the Outer Western Carpathians) controlled by mass movement processes. Based on the field geological, geomorphological and geophysical data, we found out that the landslide accumulations pushed the more active river of out of two subparallel river channels with different erosion activity westwards and forced intensive lateral erosion towards the recently abandoned valley. Apart from the landslide processes, the presence of the N-striking fault, accentuated by higher flow rates of the eastern channel as a result of its larger catchment area, were the most critical factors of the river piracy. As a consequence of the river piracy, intensive retrograde erosion in the elbow of capture and also within the upper portion of the western catchment occurred. Deposits of two landslide dams document recent minimum erosion rates to be 18.8 mm.ky− 1 in the western (captured) catchment, and 3.6 mm.ky− 1 in the eastern catchment respectively. The maximum age of the river piracy is estimated to be of the late Glacial and/or the early Holocene.