Monthly mean data collected for a 90 year period (1901–1990) have been used to study long term water temperature trends at 10 river sites in Austria. Significant increases in monthly mean values were evident over the study period for seven river stations, but the magnitude of the rise varied from 0.47 to 1.26°C and its pattern was not uniform between catchments. River temperature trends for individual months also ranged from increases of >2.5°C, through no significant change, to decreases of >1.0°C. Rises in water temperature during the twentieth century appear to reflect, at least in part, the increasing impact of human activity on Austrian rivers. Progressive regulation of main rivers, such as the Danube and Inn, has resulted in strong increases in mean water temperatures during the low flow period of autumn but not in the summer months when high flows generally occur. Different trends were evident for sites closer to glacial runoff sources, for catchments containing a large proportion of lakes, and for stations influenced by point source discharges and other local factors. Rising mean water temperatures at the end of the study period were related more to the occurrence of mild winters than to the incidence of particularly hot summers.