Chronology and dynamics of fluvial style transformation in theYounger Dryas – Holocene transition (lower San River, SEPoland)

Reconstruction of fluvial style changes in the San River in the Subcarpathian Basins is based on
geomorphological and sedimentological analyses. The time control of alluvial fills and temporal
changes in the river channel are derived from radiocarbon and optically stimulated luminescence
dating combined with independent pollen-based biochronostratigraphy. The results showed that the
alluvial plain of the braided (BR) or braided-meandering (BR-M?) river was abandoned before
12,800 cal BP. Large meanders (LM) were cut off in the older part of the Younger Dryas (YD; ca.
12,600 cal BP), and in the younger part of this period (ca. 12,450 cal BP). The small meanders (SM)
developed at the end of the YD and were abandoned at the onset of the Preboreal (PB; ca.11,550
cal BP). The erosion phase at the YD-PB transition, reported from many valleys in Central Europe,
was not confirmed in the study area. The full cycle of San River channel transformation (BR (BRM?)->
LM->SM); was estimated to be approximately 1200 years. According to the palynological data,
open pine forests with birch that survived from the end of the Allerød dominated the landscape of
the river valley during the YD cooling and did not undergo major changes during the warming in the
early PB. Therefore, we assume that the influence of vegetation changes in the San River channel
pattern transformation was nonsignificant. The location of the studied palaeochannels in the
floodbasin filled with silty clayey deposits may have influenced the formation of relatively narrow
and deep channels, than that of much the wider and shallower meanders from the YD, situated
several kilometres downstream of the surveyed sites.