Ice regime of natural rivers is influenced by climate, water regime and channel morphology. Since channel morphology and water regime are influenced by river dams, changes in ice conditions can be expected. Ice appears commonly on the Hungarian rivers uninfluenced by human activities. Ice observations has a long tradition and data on ice are available from several sources. Important basic data from observations between 1875 and 1986 were derived. Characteristic parameters can be established from the time series for particular gaging sections. The time series of Danube stations indicate the modification of the ice regime. A sharp decline in the number of ice-run days and even more dramatic drop in that of ice cover days is observed in the frequency curves due to the effect of the upstream hydropower stations and thermal effluents. On the basis of the experiences of operating river barrages certain rules have been derived for winter operation of the planned river barrages. The changes of ice conditions due to these dams were characterized. Thickness of the ice in the reservoirs, length of period of ice cover, sensitivity for ice jam formation were predicted.
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