Zmiany układu koryt Wisły spowodowane funkcjonowaniem stopnia wodnego „Włocławek” na podstawie analizy zdjęć lotniczych

Abstract

In line with the classification after Brice (1975), the system of the contemporary Vistula channel may be identified as a complex anabranched (anastomosing) river system, consisting of the main channel of the braided river type with bars and islands, as well as meandering side channels separated by holms. In turn, following the views of Smith and Smith (1980), this system is in a transition phase from a braided to an anastomosing river system. The phase of transformation to an anastomosing one in the central and lower Vistula braided system could have started at the beginning of the Holocene and continued uninterrupted through to the present day (Starkel, 2001). Currently, the Vistula fluvial system changes in the vicinity of the dam on the Wloclawek reservoir and is under the strong influence of the anthropogenic impact the functioning of the dam is able to exert. To analyze the effects of this transformation, we have studied changes in channel structure using a method proposed by Brice and Blodgett (1978), as modified by Szmanda and Luc (2010). Analyzing aerial photographs, we have measured the widths of channels (w), as well as the lengths (L) and widths of bars (w’) and inter-channel forms in the bottom of the valley (islands fixed by vegetation and inter-channel areas called holms). Dimensionless parameters of forms were also calculated: L/w – for length, w’/w – for width, and L/w’ for shape. Aerial photographs from the area of the two holms located in the Vistula valley – i.e. the Bogpomoz Holm (below the Reservoir) and the Wykowska Holm (above it) were our research material. These photos come in both cases from two periods, before and after the construction of the dam in Wloclawek. The survey allowed the following conclusions to be drawn: (1) In the research period, the average width of the river channels below the dam has increased slightly, while above the barrier there has been a slight decrease. (2) By numbers of given forms, islands fixed by vegetation are found to be in the majority, whereas holms were more frequent in the floodplain inter-channel area. Irrespective of the type of form, most were elongated. (3) Both above and below the Reservoir, there has been a decrease in the number of (channel and valley) forms. The number of bars in the Vistula channel has decreased. The area of the inter-channel islands colonised by vegetation has increased through evolution in the direction of inter-channel and floodplain areas. As a result of the disappearance of the side arms, inter-channel areas have been absorbed by floodplains. (4) These processes have resulted in the slow preservation of the fixed multiple-channel fluvial system. We suggest calling this system an “anabranching river” as opposed to the classical anastomosing river (Smith and Smith, 1980).