Anthropogenic transformation of lowland river pattern and changes of spatial extent of lakes inferred from ground-penetrating radar and remote sensing surveys (the Obra River, Poland)

A significant environmental change took place in the middle course of the Obra River (western Poland). The natural river bed pattern was replaced here by three artificial canals in the beginning of the 19^th century. Before that time the Obra River bed had been characterized by various river patterns (multi-channel, meandering and disappearing in wetlands). GPR (ground-penetrating radar) surveys and the analysis of historical maps and satellite and aerial images were conducted to retrace the natural river course and changes of spatial extent of Obrzańskie Lake. Two types of the Obra river patterns, which had been functioning here before the period of the hydrotechnical works, were retraced: a large scale meandering pattern and a relatively smaller scale multi-channel pattern indicating features of anastomosing river. The transformation from the meandering to multi-channel pattern might have been caused by climatic changes and/or forming a bifurcation to the Odra River. Historical maps indicated that the multi-channel pattern was active in the second half of the 18^th century. Next, the multichannel pattern was artificially transformed into the North Obra Canal. Furthermore, the shorelines of Obrzańskie Lake from the period preceding the hydrotechnical works and from the 19^th century were delineated on the basis of the geophysical and remote sensing surveys. Before the anthropogenic intervention, the lake had been directly connected with a network of the Obra channels. Construction of the North Obra Canal resulted in significant decrease of the lake´s surface. The presented results may be used in a restoration project aiming to reconstruct the natural river course. Such intervention would increase natural values of the middle Obra valley and influence development of tourism in this area.