Climatic and morphological controls on diachronous postglacial lake and river valley evolution in the area of Last Glaciation, northern Poland

Abstract

The Wda River valley in northern Poland is a polygenetic valley located in a former subglacial meltwater channel that after ice sheet retreat hosted separate and then interconnected lake basins, subsequently replaced by a river. In one part of the river system an abandoned Lateglacial valley is found. This dry valley is a unique feature in the area located within the limit of the Scandinavian Ice Sheet during the Last Glaciation in the Central European Lowland. We investigated lacustrine and fluvial sediments and landforms in the valley and applied palynological analysis combined with radiocarbon dating to reconstruct the valley evolution from the early period of river inception in the Lateglacial through the formation of lakes during the Pleistocene/Holocene transition up to the establishment of a modern river system in the early Holocene. Three coeval processes were identified: (1) formation of lake basins in the subglacial channel connected by a river, (2) erosion of the river bed between the lakes, and (3) sediment deposition at the mouths of the channels in the lake basins first generating delta fans and eventually filling the lakes entirely. The valley formation was associated with diachronous melting of dead ice blocks buried in the subglacial channel controlled by the capacity of the hydrological system to evacuate the meltwater. Most of the lake basins in the study area were formed during the Bølling–Allerød period, but one section of the subglacial channel not affected by thermokarst processes survived protected by dead ice blocks throughout the entire Lateglacial. Accelerated dead-ice decay at the beginning of the Holocene triggered widespread lowering of the base level, which caused lake disappearance and the formation of the modern river system. The processes reconstructed in the central section of the Wda River indicate a highly dynamic and diachronous river valley development during the Lateglacial and early Holocene whereby the valley formation was interwoven with the formation of lake basins within the valley triggered by melting of dead ice blocks in the substratum.