Identifying the pre‐Odra river system with hydroacoustic and seismic reflection imagery offshore Rügen Island, southern Baltic Sea

Understanding the course and dynamics of ancient river systems, such as the pre‐Odra, provides valuable insights into the post‐glacial evolution of landscapes and riverine processes. The northwest‐trending pre‐Odra was an important drainage system of the European mainland into the Baltic Basin during and after the Scandinavian Ice Sheet retreat. However, there are still uncertainties about the course and extent of the pre‐Odra river and its evolution. In this study, we combine parametric sediment echo‐sounder and high‐resolution multi‐channel reflection seismic data to identify and map the actual course of the former river and analyse its morphology and stratigraphic infill since the Baltic Ice Lake formed around 15 ka BP. Our integrated analysis reveals that biogenic gas anomalies within the Late Pleistocene and Early Holocene valley infill closely align with the seismically identified pre‐Odra valley. In areas where channel morphology is poorly resolved, gas‐related seismic signatures provide an additional method to trace the buried river system. In the southern Pomeranian Bay, a dense forking network of narrow channels records the braided phase of the river, with possible connection to the rivers Ryck and Peenestrom near Mönchgut (southeast Rügen Island). The narrow channels merge northwards to a single, wider channel, which can be traced as far as the Tromper Wiek. These results indicate that under suitable conditions, multi‐channel marine reflection seismic‐based gas mapping can support the reconstruction of buried fluvial systems even where high‐resolution parametric sediment echosounder data are limited. These findings reaffirm that the pre‐Odra exploited ancient meltwater drainage systems after the last glacial and improve our understanding of the post‐glacial landscape evolution in the southern Baltic Sea.