Non-eruptive debris flows at Mount Shasta: historical record, drivers, and implications for future hazards

Assessments of water-driven mass flows at snow and ice-covered volcanoes are often based on the assumption that the most impactful flows will happen during eruptions (lahars). However, as eruptions are a relatively low-probability event, non-eruptive debris flows tied to precipitation or glacial outburst floods may be more common. This is especially relevant at Mount Shasta, one of the southernmost glaciated stratovolcanoes in the Cascade volcanic arc, where past hazards assessments focused on the possibility of larger flows triggered by eruptive activity. Recent years (2021–2025) at Mount Shasta, however, have seen smaller non-eruptive lahars (locally called ‘debris flows’), which have had significant impacts on the communities and infrastructure surrounding the volcano, disrupting major transportation routes and threatening municipal water supplies. Given Mount Shasta’s current quiescence, it is prudent to focus volcanic hazard assessment on these non-eruptive lahars. In this study, we expand the historical record of debris flow occurrence and determine whether there is a discernible link between debris flow frequency and broader, meteorological forces. We classify debris flows into two broad groups based on associated meteorological parameters: (1) flows triggered by rainfall during storms, and (2) flows without associated precipitation occurring during hot summer days in years with widespread drought, including below average snow accumulation in the previous winter. These findings are consistent with historical anecdotal observations by local residents – that debris flow occurrence outside of precipitation is more frequent in hot, dry summers – and provide important information for evaluating which meteorological conditions may lead to an increase in debris flow occurrence. We also highlight the fact that debris flows may be a hazard at Mount Shasta even when there is no precipitation. These results provide important data for evaluating debris flow hazards in the face of ongoing changes in weather patterns across northern California and near other snow-and-ice-clad volcanoes worldwide.