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This paper presents a method for determining the impact of dam reservoirs on the ice cover of rivers below their locations based on a long measurement period (1950-2020) and synthetic-aperture radar (SAR) data. Two rivers and two sets of dam reservoirs located in the Carpathian Mountains (central Europe) were selected for this study. In order to estimate the influence of their reservoirs, a logistic regression model was built to describe the relationship between the course of air temperature and the occurrence of ice cover (i.e., total ice cover and border ice) at water gauge cross sections above and below the reservoirs. The influence of reservoirs was then defined as the differences between the values predicted from air temperature and those observed at the water gauge cross sections. Additionally, the extent of the impact of the reservoirs was estimated based on SAR data (Sentinel-1) by identifying river sections below the reservoirs on which total ice cover did not form despite the persistence of very low air temperatures. This study demonstrates that dam reservoirs are an important factor in transforming the ice regime of rivers. We found that decreases in the incidence of ice cover as a result of their operation could exceed 80% in the sections immediately below their location, and that this reduction decreased as the distance from the reservoir increases. According to SAR data, it was estimated that total ice cover did not form in sections 26-60 kilometers below the reservoirs, despite the presence of favorable thermal conditions. Based on the research results presented here, it is reasonable to assume that the rapid increase in the number of dam reservoirs worldwide in the second half of the 20th century is an important factor transforming the ice regime of rivers. This study also demonstrates that the logistic regression model and SAR data are useful tools for assessing the impact of dam reservoirs on river ice cover. [ABSTRACT FROM AUTHOR]