Evaluating effects of dam operation on flow regimes and riverbed adaptation to those changes

The operation of dam and reservoirs is one of the main reasons for the transformation of river runoff. The change in the hydrological regime affects a number of other processes taking place in the river channel, including the transformation of its geomorphological features. The article presents the impact of the Włocławek Reservoir on the hydrological regime of the lower Vistula. The alterations of the river flow downstream of the dam in the conditions of the hydropeaking regime of hydroelectric plant operation and after its change to the run-of-river and interventional operation were characterised. It was assessed how the change in flow conditions in connection with the deficit of sediments affected the transformation of the Vistula river channel downstream of the dam. The nature and magnitude of changes in the Vistula river flow regime caused by the functioning of the "Włocławek" dam were determined using IHA (Indicators of Hydrologic Alteration) and the RVA (The Range of Variability Approach) method. It was demonstrated that the operation of the hydroelectric power plant in the hydropeaking system is the cause of a large flow alteration in respect of the frequency and duration of low- and high-flow pulses and the rate and frequency of change in the flow. The change in the manner of operation of the hydroelectric power plant affected the reduction in the degree of transformation of most features of the flow. The increase in water erosion force downstream of the dam led to the deepening and narrowing of the river channel. The presence of more resistant formations at the bottom of the river channel inhibited riverbed erosion, at the same time intensifying lateral erosion, which increased the width of the river channel. During the operation of the reservoir, the river channel downstream of the dam evolved towards the anabranching type system.