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

Abstract

Intensive hydro technical works were conducted in the middle course of the bra iver (oland) at the beginning of the nineteenth century. The ‘natural’ river course (functioning before the major construction works) was transformed into three artificial canals. Ground‐penetrating radar investigations, ground‐truthed with coring and remote sensing surveys, were conducted to reconstruct the course taken by the river prior to the hydro technical works. This work demonstrated that the bra formerly had an anastomosing planform. Radiocarbon dating indicated that the earliest of the retraced channels were active before 9000 bp. The retraced river system was formed in a proglacial stream valley perpendicularly cut by remains of subglacial tunnels, now partly filled with lakes. The planform consisted of one or two major channels and a number of secondary channels formed by avulsions. During the last 2000 years, some of the avulsions may have been caused by anthropogenic interventions. The sequences of channel fill deposits indicate that particular channels changed from major to secondary ones. Sand deposits filling the bottom and middle parts of the channels point to an important role of upstream sediment supply causing in‐channel aggradation triggering the avulsions. Moreover, spatial variability in river patterns was found within the anastomosing system. Valley sections with meandering anabranches, anastomosing patterns with traces of a lateral migration and traces of a transition from meandering to anastomosing planform were distinguished.