LAST CENTURY OVERBANK DEPOSITION IN THE GORGE SECTIONS OF THE VISTULA AND WARTA RIVERS (POLAND): A MULTIDISCIPLINARY STUDY

Overbank deposits accumulated in the gorge sections of river valleys are valuable archives of natural and anthropogenic environmental changes, including the climate change. Such river reaches are undiked what provides conditions for undisturbed sediment accumulation across the whole floodplain, from the river banks to the hillslope base. This study was performed in the gorges of the Vistula and Warta rivers in southern Poland. Both gorges are cut through the Jurassic limestones of the Krakow Upland downstream large industrial centers which have significantly altered the geochemistry of both river valleys during the last millennium. Vertical profiles of overbank sediment properties were used to reconstruct spatial patterns of recent deposition on the floodplains. The applied multidisciplinary approach included analyses of lithofacies and of the contents of the selected radionuclides (¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra) and metals.

Despite the narrowness of the valleys the lithology of their overbank deposits shows a typical spatial pattern with the diversified coarse-grained material near river channels and more homogenous, fine-grained material in distal parts of the floodplains. Concentrations of all metals are distinctly higher, even by two orders of magnitude, than the geochemical background. Periods of extreme metal pollution are reflected even in the heterogeneous near-channel deposits. Vertical changes in metal concentrations were correlated with the history of industrial activities in river catchments providing estimates of overbank accretion rates which decrease with the distance from river channels and agree with the estimates based on radionuclide profiles. The maximum thickness of overbank sediments accumulated in the 20th c. is 40 and 100 cm for the Warta and the Vistula, respectively. Accumulation rates during this time were not constant reflecting land-use changes or variations in flooding intensity.

The study was financed by the Polish Ministry of Science and Higher Education from the budgetary funds in years 2005 – 2006 (2 P04G 012 28) and 2008-2009 (N N306 424834).

.