Chile faces high vulnerability to mountain hazards along the Andean Cordillera. As climate change and urban development intensify, the frequency and impact of destructive debris flows are anticipated to rise. To inform mitigation and adaptation strategies, it is imperative to understand the characteristics of historical events in this region. A notable example is the Parraguirre rock avalanche that occurred on November 29, 1987, which transformed into a catastrophic debris flow, propagating 50 kilometers down-valley and causing severe damage and loss of human lives. The high destructive power is attributed to the considerable amount of water involved. Yet, the source of this water remains largely unidentified. Further unknowns are the initial trigger volume and the total mass transfer down the valley.

In this study, we revisit the past event by integrating new insights from remote sensing, climate and hydrological records as well as process-based modelling. Our results suggest important corrections for the trigger volume, the total fluid flood volume and a first estimate of the solid mass transfer out of the Parraguirre catchment. Moreover, we find that the elevated water content cannot be solely attributed to the entrainment of soil moisture and snow cover. It requires a considerable contribution from another source – likely in form of glacier ice. Furthermore, our simulations corroborate the damming hypothesis of Río Colorado, thereby reconciling the observations of multiple waves as well as on arrival times and run-out distance.