Przemysław Tomalski

The aim of the study is to identify the flow seasonality of the Vistula river in 14 water-gauge cross-sections located along the river course (from Skoczów to Tczew). For this purpose, the following characteristics were used: seasonality index (IS), period of seasonal concentration (PK) half-flow date (TPO), and concentration index (GMO).  Hydrometric data from the 1951–2016 period were shared by IMGW-PIB. The average time of seasonal concentrations and half-flow dates in the analysed cross-sections occurred in April or May. Along the Vistula river the mean PK and TPO regress towards winter (are earlier and earlier). The average seasonality index is comprised in the range 22–32%, while  the values of the concentration index fall into the range 10.0–14.5. The highest IS values characterize the upper part of the river down to the Goczałkowice water-gauge (here it reaches the maximum). Next the IS rapidly decreases, due to the summing of the flows from the Vistula river tributaries varying conditions of flow formation (from Carpathian Mountains and Uplands). A significant increase in the IS is visible below the mouths of the Dunajec and then the San rivers. A local maximum of the seasonality index occurs in the Zawichost cross-section. Next, the IS slowly decreases along the river down to the Vistula river mouth. One exception is the Włocławek water-gauge where a small increase of the IS can be noticed. It is due to the functioning of a dam and a big reservoir in the town. The changes of the concentration index along the Vistula river are similar to the seasonality index changes but the differences between the values of extremes are less visible. In the five analysed flood years, both the seasonality indexes and the concentration indexes were quite high in the upper part of the Vistula river and average in the central and lower parts of the river. The half-flow dates and the periods of seasonal concentration were stable along the whole river’s course but very diversified in particular years. The changes of the examined seasonality characteristics in dry years were very similar to analogical changes in normal years. Only in 1959 was the situation different when the IS was very low and the PK and TPO occured later and later down the river’s course.