Phases of fluvial activity in loess landscapes: Findings from the Sió valley (Transdanubia, central Europe)

•Three fluvial phases identified in the Sió valley floor (Transdanubia, Hungary).•First evidence for large-scale meanders in Transdanubia in the Late Pleniglacial.•Isolation by loess hills favored formation of anabranching channels ~18000 cal. BP.•Small meanders formed by incision, evolved in the Holocene in aggradation conditions.•The fluvial phases differ in timing and type of planform in other rivers of Europe. Knowledge about the evolution of rivers planforms in loess landscapes of Europe is incomplete. While there are studies on river evolution conducted in loess areas of central and western Europe, the area of Transdanubia (Hungary) remains poorly recognized. Our research work extends previous findings by studying a unique example of alluvial fill maintaining a sedimentary record of river planforms from the Late Pleistocene to the modern times (the Sió Valley, Hungary). The main goal was to determine the main fluvial phases shaping the valley floor. Our study was based on geological, geophysical and remote sensing methods supported by AMS radiocarbon and OSL dating. Three fluvial phases were identified: the formation of large-scale meanders in the Late Peniglacial, transition to an anabranching pattern ~18000 cal. BP, and formation of small-scale meanders ~12700 cal. BP. Their traces are present in the sedimentary record of other rivers evolving in loess landscapes, however, their appearance differs in the timing and type of river planform changes. Phase I was marked by large-scale meanders in the Great Hungarian Plain and Transdanubia, and braided rivers in western Europe. Phase II, marked by transitions to anabranching planforms, or periods of meander cutoffs, appeared first in the Late Pleniglacial in isolated valleys of Transdanubia and northern part of the Pannonian basin. The latest appearance of this phase took place in western Europe owing to a cooler climate and sparse vegetation compared to Transdanubia and Great Hungarian Plain. Phase III, marked by transitions from anabranching rivers to small-scale meanders, started in the Sió valley in conditions of sediment deficit causing river bed incision. Aggradation appeared owing to anthropogenic influence starting ~7000 cal. BP. Its maximum occurred 3000–4000 cal. BP. This triggered phase III in western Europe, and increased deposition of fines, resulting in flow discontinuation in some valleys of Transdanubia.