Holocene fluvial terraces reveal landscape changes in the headwater streams of the Moravskoslezské Beskydy Mountains, Czechia

Abstract

Fluvial terraces of headwater streams in mountainous forested areas provide important insight into the development of sensitive fluvial system in response to natural and anthropogenic changes. Usually they are poorly investigated because of incomplete preservation and difficulties with mapping caused by dense canopy cover. We utilized new high-resolution LiDAR data, field mapping, sedimentological analysis, pollen analysis and radiocarbon dating to reconstruct the evolution of fluvial terraces in seven headwater catchments located in the highest parts of Moravskoslezské Beskydy mountains (Western Carpathians, Czechia). In these catchments, we identified three Holocene river terraces set within older alluvial deposits or flysch bedrock. The oldest (TH) terrace has a minimum age of the onset of accumulation of ~9 ka, the minimum age of the middle (TM) terrace is ~4.4 ka, and that of the lowest (TL) terrace is ~0.7 ka. The formation of the TH and TM terraces was controlled by climatic deterioration during the Subboreal (~4.3 ka) and Subboreal/Subatlantic transitions (~2.9–2.6 ka), respectively. The lowest TL terrace is dominated by <500-year-old fine-grained overbank deposits, which we interpret as legacy sediments due to deforestation and shepherd activity during the so-called Wallachian colonization of the study area. Our study reveals peculiarities in the development of the river terraces of small mountainous forested sub-catchments: (i) the limited preservation of pre-Holocene river terraces, (ii) sensitivity to extreme climatic events, and (iii) the decisive role of combined effects of climate change and grazing and forest management in sediment delivery during the last several centuries.