LONG-TERM WATER TEMPERATURE TRENDS IN AUSTRIAN RIVERS

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 ov er the study period for seven river stations, but the magnitude of the rise Varied from 0.47 to 1.26 degrees C and its pattern was not unifo rm between catchments. River temperature trends for individual months also ranged from increases of >2.5 degrees C, through no significant c hange, to decreases of > 1.0 degrees C. Rises in water temperature dur ing the twentieth century appear to reflect, at least in part, the inc reasing impact of human activity on Austrian rivers. Progressive regul ation of main rivers, such as the Danube and Inn, has resulted in stro ng increases in mean water temperatures during the low flow period of autumn but not in the summer months when high flows generally occur. D ifferent trends were evident for sites closer to glacial runoff source s, for catchments containing a large proportion of lakes, and for stat ions influenced by point source discharges and other local factors. Ri sing mean water temperatures at the end of the study period were relat ed more to the occurrence of mild winters than to the incidence of par ticularly hot summers.