Frequency of fast geomorphological processes in high-gradient streams: case study from the Moravskoslezské Beskydy Mts (Czech Republic) using dendrogeomorphic methods

High-gradient streams are localities with the most dynamic activity of geomorphological processes in medium-high mountains. This study of the frequency of floods and debris flows in a se-lected high-gradient stream in the Moravskoslezské Beskydy Mts was based on a dendrogeomorphic approach. It makes use of the most accurate methods applied in the dating of historic geomorphologi-cal processes. Individual events were reconstructed on the basis of the dating of various growth dis-turbances displayed in 99 samples taken from 56 predominantly broad-leaved trees. As for the studied area, 26 years out of the last 113 years have been identified as years of rapid geo-morphological processes. The frequency of the processes has been high above average since the 1970s. A majority of the events can be considered as flash floods. Debris flows, which can only be observed sporadically, originate due to the reactivation of old accumulation material that subsequent-ly ends up re-accumulated on the alluvial fan at the mouth of a stream. A large number of events oc-cur in connection with extreme short-term precipitation in summer months. In addition, they are af-fected by fast snow melting in spring, which has also been proved by intra-seasonal dating of selected events.