GPR and aerial imageries to identify the recent historical course of the Obra River and spatial extent of Obrzańskie Lake, altered by hydro-technical works

Marcin Słowik

doi:10.1007/s12665-012-2215-9

Abstract

Ground penetrating radar (GPR) measurements have been conducted to retrace the natural course of the Obra River and changes of spatial extent of Obrzanskie Lake (western Poland). The Obra River valley was subjected to intensive anthropogenic transformation. Three artificial canals were constructed here in the beginning of the nineteenth century. The GPR surveys were done using a georadar MALA ProEx equipped with 250 MHz antenna. The measurement was verified by coring and analyses of historical maps, aerial photographs, and satellite images. The research has shown that there are at least three elongated depressions preserved in the modern land surface near the village of Solec, situated in the middle course of the valley. Several structures suggesting functioning of a multi-channel pattern in the past have been detected inside and in the vicinity of the depressions. Filling the channels with silts and peats might have been the effect of flow deceleration caused by the construction of the Obra Canals. Moreover, situation of two of the retraced depressions suggests that the landforms might be the remains of a river bed disappearing in wetlands. Further studies including groundwater flux measurements in the area between the depressions are needed to confirm such hypothesis. Furthermore, the study allowed retracing the spatial extent of Obrzanskie Lake. The sediment structures detected by the GPR measurements and calibrated with coring were interpreted to be the traces of the lake shoreline. The interpretation was supported by the findings inferred from aerial and satellite images. It was shown that Obrzanskie Lake had had at least three islands and had been directly connected with a network of braided channels before the anthropogenic intervention. The spatial extent of the lake was significantly altered by the construction of the North Obra Canal. Disconnecting the lake from the Canal resulted in more than 50 % decrease of the lake surface.

Highlights

Many hydrological and geomorphological studies regard retracing changes of river bed patterns
Ground penetrating radar (GPR) measurements are a very useful tool in such studies as they allow imaging the fragments of river beds and alluvial structures preserved in the shallow subsurface
The faint or lack of the GPR reflectors can be found above the concave-up pattern

Summary

Introduction

Many hydrological and geomorphological studies regard retracing changes of river bed patterns. Such reconstruction may be especially difficult in case of a river course subjected to hydro technical works. The GPR method was applied in many studies concerning reconstruction of meandering (Leclerc and Hickin 1997; Vandenbenghe and van Overmeeren 1999; Huisink 2000; Gourry et al 2003; Bersezio et al 2007; Bowling et al 2007; Kostic and Aigner 2007), braided (Bristow et al 1999; Skelly et al 2003; Lunt et al 2004; Froese et al 2005; Best et al 2006; Bowling et al 2007), wandering (Rice et al 2009; Hickin et al 2009) and transitional patterns (Vandenbenghe and van Overmeeren 1999; Bowling et al 2007) in case of both sand and gravel bed river courses. A number of research works referred to the application of the GPR method to study the sediments’ structures in wetlands, mires, and bogs. Jol and Smith (1995) reported that GPR surveys provide information concerning gyttja layers and other sediments under peats. Holden (2004) used

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