The evolution and disappearance of “false delta” multi-channel systems in postglacial areas (Central Europe)

Abstract River deltas are complex aggradational systems preserving the history of their evolution in the depositional record. While coastal deltas are extensively studied, little is known about “false delta”, multi-channel systems situated at a certain distance from the sea. The term “false delta” is used here in reference to a system of bifurcating alluvial channels influenced by sea level changes, and variations in discharge and sediment delivery. These interactions resulted in the formation of a multitude of channel planforms, and a “delta-like” fluvial landscape. We present the first study that aims to determine the influence of downstream controls (sea level changes) and upstream controls (discharge and sediment delivery) on the evolution of a “false delta” multi-channel system in the postglacial areas of central Europe. Our study is based on geophysical, geological and remote sensing methods, alongside radiocarbon dating (AMS–accelerator mass spectrometry). We use the example of a “delta-like” system of palaeochannels that connected the Obra and Odra Rivers (central Europe, Poland). The “false delta” formed within an ice-marginal valley by the activity of large-scale meandering channels in the Late Glacial period. Between 13,300 and 11,200 cal. BP, a transition to an anabranching and sinuous planform was caused by the coupled effects of a lowering Baltic Sea level, an increase in flood frequency and in sediment delivery. During this period, bifurcation began conveying 30% of the flow through a former subglacial tunnel to the Warta River. The Holocene evolution of this multi-channel system was driven by the interplay of successive sea level rise and sediment delivery from the middle Obra valley, and former subglacial tunnels. The upstream controls dominated during the last 4400 cal. BP, and were enhanced by mill constructions and deforestation in the last millennium. The false delta “disappeared” in the 18th century owing to a 63% decrease in flows. Based on these findings, we propose a model describing the evolution of “false delta” river systems in postglacial areas of Europe. Three stages of evolution are identified: i) formation by high flows, ii) sustaining the flow through particular branches in conditions of dominating deposition, and iii) discontinuation of the false deltas by hydro technical works. The model can be extrapolated to other areas in postglacial Europe and North America after refinement by further field studies.