Patterns and drivers of long-term changes in ice phenomena along the Vistula River (Central Europe)

Abstract. Changes in river ice regimes are evolving rapidly in response to climate warming, and local anthropogenic influences may further modify their spatial and temporal variability. However, both the combined and individual effects of climate change and human activities on river ice phenomena remain poorly understood, particularly at the scale of entire river systems. In this study, we investigated long-term changes in river ice occurrence along the entire course of the Vistula River during the period 1950–2025, using hydrological observations from 54 gauge stations, Landsat satellite imagery and ERA5 climate reanalysis data. The results revealed a significant increasing trend in mean winter air temperature of 0.42 °C per decade, accompanied by decreases in both freezing degree days and the number of frost days. Concurrently, the frequency of river ice phenomena along the Vistula River decreased at an average rate of 6.2 days per decade, while the mean annual number of days with river ice phenomena declined from 49.4 in 1950–1985 to 26.5 in 1986–2020. Satellite observations further showed an average 17.7 percentage points decrease in river ice extent between the periods 1985–2005 and 2006–2025. Despite the overall declining trends on the whole-river scale, the spatial distribution of river ice phenomena exhibited pronounced heterogeneity. River reaches subject to the least human disturbance were characterized by the most frequent river ice occurrence and the strongest relationship with air temperature. In contrast, reaches heavily affected by reservoirs, thermal and chemical pollution, saline mine water discharge, urban areas, and channel regulation exhibited a substantially weaker relationship between ice occurrence and air temperature. Overall, these findings indicate that the Vistula River’s contemporary ice regime is shaped by the combined effects of climate warming and local anthropogenic pressures. Although climate change determines the broader direction of long-term decreases in river ice phenomena, human activities substantially modify their spatial patterns, highlighting the need to account for anthropogenic influences when assessing the response of river ice regimes to ongoing climate change.