Long-Term Trends and Characteristics of Water Temperature Extremes of a Large Lowland River During the Summer Season from 1920 to 2023—Case Study of the Bug River in Poland, Eastern Europe

Sustainable water resource management must address the current challenges posed by changes in the thermal regime of water bodies. The ongoing increase in air temperature leads to higher water temperatures, resulting in various unfavorable changes in the aquatic environment and limiting its capacity to meet human subsistence and economic needs. The aim of this study was to evaluate the impact of climate change on the thermal regime of the Bug River, a large lowland river in Eastern Europe (Poland) that remains thermally unpolluted. Using archival sources, we collected, reconstructed, and standardized water temperature data for the Bug River at the Wyszków profile during the summer months (April to October) from 1920 to 2023, despite several gaps in the data. Our analysis revealed a significant increase in the average summer water temperature, along with maximum and minimum temperatures, although there was considerable month-to-month variability throughout the study period. The observed upward trend in water temperature corresponds to the rise in air temperature. Between the subsequent 30-year climate norms of 1971–2000, 1981–2010, and 1991–2020, we noted a steady increase of 0.4 °C in average summer water temperatures. The highest average monthly water temperatures in recent years occurred during the summer seasons of 2018, 2019, 2021, and 2023. Notably, we also recorded some of the highest water temperatures in July 1951, June 1932, and July 1947 and 2010. Furthermore, we found a significant relationship (R 2 = 0.68) between maximum water temperature during low flow events and the average discharge. The results of this study indicate a warming trend in the waters of the Bug River.