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EN
This study presents the diversity of sediments forming sand bars located in the Vistula channel between 695 km and 775 km of its course. The analysed section of the Vistula channel is located in the Toruń Basin. During the fieldwork carried out on 20-21 September 2011, 45 channel bars were identified and sampled. The collected samples were analysed in a laboratory. The obtained results were presented in the form of figures and graphics. The research related to the Vistula bed load performed to date was based on various research methods. The authors applied the sampling method used by Z. Babiński (1992). It was employed also by other authors (Habel, 2013). The obtained results were referred to other studies and the findings clearly indicate that such detailed research must be conducted along the whole middle section of the Lower Vistula valley.
EN
Because of large amount of deposited material and the varied discharge of water, the Middle Vistula possesses the morphologic and lithological characteristics of braided river. During a geological survey of the valley which holds the bed of the Vistula River, culminations of the alluvia basement was detected in channel zone. There were outcrops of Mesozoic rocks — gaizes, marls and limestones, Paleogene and Neogene sediments as clays and sands and Pleistocene sediments as boulder clays, ice-dam deposits and compressed coarse grains fluvioglacial sediments are occasionally found. On the surface of these structures, the presence of residual lags was detected. The shallow location of these types of structures in the river channel proves its erosion-resistant nature. In long section of river bottom they are marked by a considerable increase of their gradient. Resistant steps of this kind are crucial for the natural stability of the river valley in longi-tudinal profile. They can also cause culmination of flood flows. The presence of the culminations of the alluvia basement in the river channel may be a factor increasing the probability of ice jam formation and ice jam floods. Those phenomena are causes of characteristic morphology of terrace surface — presence of erosional troughs of flood event flows. They also affect depositional processes. In the paper the morphologic characteristics of the Middle Vistula (braided lowland river) in the zones of various lithological types of alluvial basement culminations are presented, as well as characteristic structures of channel and flood deposits.
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