Transformation of a lowland river from a meandering and multi-channel pattern into an artificial canal: retracing a path of river channel changes (the Middle Obra River, W Poland)

Marcin Słowik

doi:10.1007/s10113-013-0453-3

Abstract

At the beginning of the nineteenth century, a significant change in sedimentation took place along the middle course of the Obra River (W Poland): a “natural” channel bed was replaced by three artificial canals. Before that time the Obra River bed had been characterized by various river patterns (braided, meandering and disappearing in wetlands). GPR (ground-penetrating radar) surveys and the analysis of satellite and aerial images were carried out to retrace the natural river course near the village of Kopanica. The GPR, ground-truthed with core data, revealed that at least two types of the river pattern had been functioning here before the period of the hydrotechnical works: a large-scale meandering pattern (width-to-depth (w/d) ratio ranging from 15 to 40) and a relatively smaller-scale multi-channel pattern (w/d = 11–22). Possibly, the transformation was caused by forming a series of large floods followed by in-channel aggradation and avulsions. Next, the multi-channel pattern was artificially transformed into the North Obra Canal. This paper discusses possibilities of a creation of network of channels that were active after the construction of the North Obra Canal in the beginning of the nineteenth century. Such intervention would increase natural values of the presented section of the Obra valley. The presented study also shows that a detailed analysis of history and evolution of a given river course is needed before undertaking any restoration works.

Highlights

An analysis of the state of human-modified rivers strongly depends on the ability to reconstruct the natural form and functioning of a given river basin
River courses were modified by an anthropogenic intervention in such a way that it is impossible to discern humaninduced changes from the natural state
The following sequence of the Obra River pattern changes may be proposed in the light of the conducted research: 1. A large-scale meandering pattern featured with w/d ratios from 15 to 40

Summary

Introduction

An analysis of the state of human-modified rivers strongly depends on the ability to reconstruct the natural form and functioning of a given river basin. Such information may be a useful guide for river managers in case of planning a restoration project (Hoffmann et al 2010). There are many examples of transformation of natural river bed patterns into artificial canals (e.g., the Trent River: Large and Petts 1996; the Morava River: Babek et al 2008). Conventional field studies (coring and trenching, collecting sediment samples or cores to recognize lithology of an alluvial fill) are used to study both natural and anthropogenic changes of river patterns

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