The Occurrence of Widespread Slush Flow Events as an Indicator of Accelerating Climate Change in the Northwestern Italian Alps

Slush flows are rapid mass movements of water-saturated snow and debris that develop when the liquid water content of the snowpack exceeds the critical threshold for the fully funicular regime, resulting in viscoplastic flow behavior fundamentally distinct from that of dry snow avalanches. These phenomena have been extensively documented globally as responsible for fatalities and economic damage comparable to those of snow avalanches. In the northwestern Italian Alps, the situation is markedly different. Until 2025, rain-on-snow (ROS) events in this region had produced amplified effects on the ground in the form of increased landslide activity, debris flow mobilization, and flooding, as typical consequences of snowmelt superimposed on pre-saturated soils but had not generated widespread slush flow phenomena. The 2025 season marked a critical threshold: for the first time, diffuse and unambiguous slush flow events were documented in Italian northwestern Alpine sectors, signaling a qualitative shift in the hazard regime rather than a mere quantitative intensification of known processes. Documented emergence of slush flows in the Italian Alps must be interpreted not as an anomaly but as a measurable indicator of climate change and of its progressive effects on the Alpine environment.