Changes of river bed pattern and traces of anthropogenic intervention: The example of using GPR method (the Obra River, western Poland)

The Obra River valley is an example of the river bed transformations which were the result of anthropogenic intervention. The analysis of archival cartographic materials has shown that the Obra River bed was altered in several places by hydro technical works. Three artificial canals were constructed in the middle course of the river in the 19th century. The Obra River bed was straightened by cutting off the meander necks in its lower course. Two detailed research areas have been chosen on the basis of the analysis. One was situated in the place where the river bed had disappeared in wetlands and then was altered by the construction of the canals. The other one was situated in the lower course of the river with oxbow lakes and traces of palaeochannels on the floodplain’s surface.The main goal of the study was to retrace the natural course of the Obra River using the GPR method. Measurement was done using a MALA ProEx georadar equipped with shielded 100 MHz and 250 MHZ antennae. The lithologic variability of sediments was recognized by coring to correlate the GPR reflectors with the deposits’ interfaces. The study has shown that traces of the natural Obra River bed pattern were preserved in the floodplain deposits. At least three channel fills which may represent migration phases of one river bed or simultaneous functioning of a multi-channel pattern have been detected in the lower course of the river. The spatial extent of the water reservoir filled with organic deposits was determined in the middle course of the river. Moreover, remains of an artificial dike which had been constructed to dam up the water were discovered. Further research is needed to determine the spatial extent of the reservoir north and south of the research area and to analyze changes of the river bed pattern in a more detailed way.

Research highlights
► The Obra River valley is an example of the river bed transformations which were the result of anthropogenic intervention. Three artificial canals were constructed in the middle course of the river in the 19th century. The Obra River bed was straightened by cutting off the meander necks in its lower course.
► The main research problem was to retrace the natural course of the Obra River using the GPR method. The objectives of the study included discerning types of the GPR reflections, structures of alluvial deposits and remains of hydro technical constructions based on the interpretation of the GPR images and core data.
► The presented research allowed to study the influence of hydro technical works on transformation of the Obra River bed pattern and to retrace the natural river course. Remains of a dike made of sand deposits were detected in the middle course of the Obra River. Its construction caused damming up of water and forming a reservoir. Its spatial extent was determined on the basis of deposits’ structures inferred from the GPR surveys supported by sedimentary information and land surface configuration.
► The natural river bed pattern has been retraced in the lower course of the Obra River. Detected alluvial structures represent both lateral migration of a meandering channel and a braided river pattern. The influence of hydro technical works is not obvious here. Detected alluvial structures are situated beneath the layer of sand deposits which might have been formed by artificial covering up of wetlands.
► Using the antennae of 100MHz and 250MHz allowed to analyze alluvial deposits’ structures and remains of hydro technical constructions in a view of different resolutions and depth ranges. The two antennae frequencies can be treated as complimentary: the 250MHz for detailed analysis of floodplain shallow subsurface and the 100MHz for obtaining more general but providing information beyond the range of the higher frequency antenna images.