Significance of extreme hydro-geomorphological events in the transformation of mountain valleys (Northern Slopes of the Western Tatra Range, Carpathian Mountains, Poland)

Extreme events disturb the development paths of geomorphological systems. The effect of such events on relief tends to persist for long periods of time and affects the future development of such systems. This paper focuses on changes in middle mountain valleys associated with an extreme hydro-geomorphological event that occurred in the Western Tatra Mountains (Polish Carpathian Mountains) in June of 2007. The geomorphological effects were mapped and an analysis of valley-bottom deposits and dendro-geomorphological data on tree roots and stems was used to identify patterns in the geomorphological transformation taking place during such events. In addition, the organic material obtained from fans was dated by means of carbon-14 dating. The effects of previous extreme events in the study area and their geomorphological outcomes were compared. Four main valley section types were identified in the valley longitudinal profile. Unevenly aged (from 14C 390 ± 30 years YBP (1441–1631 cal AD) to 14C 3760 ± 70 YBP (4318–3962 cal BP)) organic material was revealed at different depths of each studied fan. A complex structure with alternating layers of fine and coarse material from the analysed fans was identified. The colluvial-alluvial structure of fans suggests that extreme high-energy events did occur more than once per century during the Holocene. This type of fan structure can be referred to as a patchwork. In the study area, rainfall with high energy potential occurs once in every 6–9 years and extreme hydro-geomorphological events occur every 15 years. This type of extreme event may not be exceptional in the history of middle mountain areas.