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)

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 smallerscale 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.

Reconstruction of anastomosing river course by means of geophysical and remote sensing surveys (the middle obra valley, western poland)

“…c). Figure f shows that the change of in‐channel deposition in the anastomosing channels from section 14 (see Słowik , for the internal structure of the channels) was similar to that at site I (Fig. b), with medium sands in the bottom part of the channel fill overlain by fine sands, silty sands and silts (Fig.…”

“…In the site situated near Kopanica (described by Słowik ), the anastomosing pattern was formed after a transition from a meandering planform that took place before 9190 bp (Fig. a, b).…”

“…d). This is inferred from the historical maps presented by Słowik (). All the channels had become inactive by the end of the nineteenth century (Fig.…”

“…The location of detailed research sites are marked with red rectangles. Yellow rectangle marks the site studied by Słowik ().…”

“…The ground‐penetrating radar ( GPR ) surveys in the present study provide a more detailed evaluation of the internal structure of former lake islands than that provided by Słowik (). Moreover, a network of channels situated north of the lake, previously classified by Słowik () as braided, are reinterpreted as anastomosing with traces of a lateral migration on the basis of geophysical and geological surveys (site III). Additional radiocarbon dates are presented here, enabling the temporal and spatial evolution of the anastomosing system near Kopanica (described in Słowik ) to be established.…”