Holocene fluvial history of Romanian Carpathian rivers

Abstract

The Holocene fluvial history of rivers draining the Romanian Carpathians has been assessed using a database resulting from the authors’ own field research at more than 50 sites comprising natural exposures (riverbanks), anthropogenic exposures (gravel pits) and manual boreholes carried out in floodplains, as well as on the analysis of over 30 published sources. Thus, 196 dated fluvial units were obtained, of which, subsequent to filtering, 142 units were retained for application of Cumulative Probability Functions (CPFs). To this database we added 274 radiocarbon-dated lacustrine sedimentary units originating in small lakes and bogs linked to the fluvial network of the study area. The resulting data bases were classified, processed and analysed under the methodological framework of meta-analysis in order to reconstruct the phases of geomorphic activity of rivers during major flood events and periods of low hydroclimatic intensity throughout the Holocene, and to understand the river response in terms of dominant fluvial processes. Based on the analysis of probability frequency, 17 major flooding episodes were identified, of which 12 (mostly from the last 4500 years) coincide with wet phases determined for lacustrine sedimentary units from the study area. At regional scale, of the 17 centennial intervals with flood events recorded in fluvial archives from Romania in the last 10 ka, 15 intervals are synchronous with at least one area from Central or Eastern Europe, leading to the conclusion that the climatic factor manifested dominantly in SE Europe, as well. At local scale, the orographic barrier of the Carpathians interfering with the dominant circulation of western air masses has modified the magnitude and timing of flood events, resulting in increased rates of hydroclimatic and geomorphic processes in western rivers compared to rivers east of the Carpathian range. Other controls which have local influence on the evolution of hydroclimatic processes (i.e. vegetation changes and human impact) determined an increase in the susceptibility of areas located between 700 and 1000 m asl throughout Romania and contributed to the acceleration of fluvial systems dynamics post ca. 4500 cal BP.