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The paper evaluates changes in the water temperature of the Vistula River – one of the longest rivers in Europe. Mean monthly and annual water temperatures from the period 1971–2017 for 11 stations along the entire length of the river revealed the increasing trends. The mean increase in water temperature in the analysed multi-annual period was 0.31 °C dec–1. In the majority of analysed stations, the key factor determining changes in the water temperature of the river was air temperature. The observed water warming in the Vistula River should be considered an exceptionally unfavourable situation in the context of importance of water temperature for a number of processes and phenomena occurring in river ecosystems. Given the scale of changes, fast measures should be undertaken to slow down the warming.

Main aim of the study was to determine the temporal and spatial patterns of relations between monthly and annual average river flow (RF) and water temperature (WT) for 53 rivers in Poland. The research made use of monthly and annual WT and RF for 88 water gauges for the period 1971–2015. Correlations were established using the Spearman’s rank correlation coefficient and the similarity of RF–WT relations was determined using the Ward’s hierarchical grouping. It was demonstrated that correlations between average annual RF and WT were negative (for >85% of water gauges) and statistically significant (p

Surface water temperature characteristics of the Vistula River mouth and the Gdańsk Bay were determined on the basis of available observations, remote sensing and hydrodynamic models. The water temperature in the Vistula River and the Gdańsk Bay was found to be much higher than the air temperature. In recent decades, the water temperature in the Vistula River mouth and the Gdańsk Bay has increased significantly. It has been determined that water temperature in the bay is affected not only by air temperature, but also by water discharge of the Vistula River and wind. [ABSTRACT FROM AUTHOR]