Alluvial bottom geology inferred as a factor controlling channel flow along the Middle Vistula River, Poland

Within the channel zone of the Middle Vistula River, elevations of the Holocene alluvial basement occur, composed of deposits resistant to erosion. In this paper two kinds of such forms are discussed. Within the Małopolska gorge (an upstream part of the Middle Vistula, near Kępa Gostecka and Zakrzów), the elevations are composed mainly of Upper Cretaceous and Paleogene rocks covered with clayey debris and residual lags. Below this, the elevations are built of Paleogene and Neogene soils as well as of various Pleistocene glacial deposits, and are of ten covered by residual lags. This type of valley stretch occurs in the vicinity of Dęblin. The surface of the erosion-resistant Holocene alluvial basement averages at 5-7 m below the mean water level. It is exposed on the channel bottom during high-water stages, as seen from a residual layer and from the low density of the contemporary channel deposits above them. The morphology of these elevations influences the pattern of the main stream also during mean-water stages. The trend to flood flow concentrations is also seen in the relief of the floodplain.