In line with global warming trends, warming of surface waters is expected. As part of the thesis, we analysed existing data of river water temperature in Slovenia. Most analysed hydrological gauging stations proved statistically significant upward trend, using Mann–Kendall statistical test. We noted that causes for statistically insignificant results on some stations, are deficiencies and irregularities in sets of temperature data and anthropogenic impacts on watercourses. Water temperature in rivers is heavily influenced by weather conditions, in particular air temperature. Water temperature data of selected water gauging stations were compared with air temperature data of space comparable meteorological station. We examined the Pearson correlation coefficient between water and air temperature. Correlation is sufficient, despite the problems with different quality of the air and water temperature data. The results of seasonal analysis of stream water temperature trends, showed an alarming rise of water temperature in the summer months. The temperature of the water courses vary, not only temporally but also geographically. We examined how the average water temperature is changing along the stream of rivers Sava and Savinja. We also examined the connection of air and water temperatures with the index of North Atlantic and Mediterranean oscillations. The results showed poor matching. The majority of correlation indexes between water temperature and oscillation indexes are statistically insignificant. Air temperature data correlate with indexes of oscillations better than water temperature, especially in the winter months. We have found that for the higher quality of the analyses, we would need longer, connected and homogenized series of water temperature data.