Air and Water Temperature Relationships in Major Polish Rivers and Their Long-Term Changes

Large rivers are crucial for the functioning of individual regions and entire countries. However, despite their importance, relevant data allowing for a full assessment of the processes and phenomena occurring within them are often lacking. In the contemporary context, climate change has become a key factor affecting river ecosystems, leading to transformations in one of their fundamental characteristics—thermal conditions. This study analyzes the relationships between air temperature and water temperature in the four largest rivers in Poland, using a diversified methodological approach (wavelet analysis, lag non-linear models, copula), which broadens the current understanding of rivers in Central Europe. All three analytical paths confirmed a strong relationship between the two media, forming a basis for further research aimed at addressing data gaps and determining the direction and rate of changes over the period 1965–2024. All cases showed a statistically significant positive trend, with rates varying slightly from 0.32 to 0.38 °C · decade−1 (an average increase of 0.35 °C · decade−1). The ongoing transformation of the thermal regime—its scale and rate—should be taken into account in interdisciplinary studies of river ecosystems, as well as by agencies responsible for water resource management. This issue is particularly important in the context of sustainable development, where the economic use and ecological condition of river systems are closely linked to water temperature.