Evolution of the hydrographic network in the middle Wieprz River Basin (E Poland)

The Wieprz River Valley (Polesie Plain, east Poland) has evolved with regard to its dimensions, position and course since the end of the Neogene, its geological record showing how the Wieprz channel migrated and modified its character. Changes have also been observed in the water balance of the entire middle river basin, related to climate changes in the Pleistocene and Holocene. We provide new maps of the sub-Quaternary surface and trace topographical changes during the Pleistocene. Two separate rivers, the Pre-Bystrzyca and Pre-Wieprz, existed in Preglacial time. During the Narevian Glaciation, a network of subglacial troughs evolved in the area, largely disturbing and overprinting the earlier hydrographic system. Some of the troughs developed in older river valleys. The trough in which the present-day Wieprz gorge near Łęczna is located was most probably formed at that time. During subsequent glaciations and cold periods, sedimentary changes took place in the troughs and valleys – from erosion and removal of the accumulated material to their complete burial by sediments. During the Mazovian Interglacial (MIS 11c), several lakes formed in the study area, recorded by their infills of organic and mineral-organic deposits. During subsequent advances of the Scandinavian ice-sheet, water flow was dammed in towards the north, resulting in the formation of backwaters. Ice-sheet retreat resulted in the flow of proglacial waters towards the south-east, fluvioglacial sediment transport, and then unblocking of flow and subsequent reversal of flow directions to the north and west. During the Odranian Glaciation (MIS 6), catastrophic flow unblocked and shaped the present-day Wieprz gorge.