Abstract:Changes in water temperature along stretches of the Kupa, Sava, Drava and Danube Rivers measured in Croatia during the last 20-60 years were investigated. Characteristic (minimum, mean and maximum) annual water temperatures measured at 15 discharge and water temperature stations are analysed. Massive construction on the Danube River basin and on the rivers themselves during the last centuries, as well as recent climate change and/or variability has caused many different and possibly dangerous changes to its water temperature regime. Water temperature, as one of the most important physical characteristics of river water, strongly affects all other physical, chemical and biological processes in the river system. The investigation focuses on changes that have occurred during the last 20-odd years, probably caused by climate change and/or variability. Methods of rescaled adjusted partial sums (RAPS) as well as regression and correlation analyses are used in order to explain changes in water temperature regimes. The water-air temperature relationship is also discussed in the article. In all analysed cases, water temperature is strongly and directly affected by air temperature. There is evidence in the time series of rises in the minimum and mean annual water temperatures of the River Danube and its main tributaries in Croatia (the Kupa, Sava and Drava Rivers). The rising of mean annual air as well as water temperatures is shown to have started in 1988.

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During the last 20 years numerous water supply, hydroelectric and other large structures have been built along the Drava River and in its catchment area in Slovenia, Croatia, Hungary and Austria. Their construction has greatly influenced the water regime of the Drava River, especially during low water periods. The changes in the low flow regime along the stretch from Varazdin (285 km) to its confluence with the Danube (0 km) are examined. The analysis includes changes in the water levels, discharges and the amounts of suspended sediment and bed load. The intensity of changes, the time of their occurrence and their trends are examined. KEY WORDS Flow regime Water level Suspended sediment Channel erosion OS86-9375/92/040349-09$09.50

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