Destructive debris flows occur frequently at glacierized Mount Rainier volcano, Washington, USA. 23 such flows have occurred in the Tahoma Creek valley since 1967. Hydrologic and geomorphic evidence indicate that all or nearly all of these flows began as outburst floods from South Tahoma Glacier. Flood waters are stored subglacially. The volume of stored water discharged during a typical outburst flood would form a layer several centimeters thick over the bed of the entire glacier, although it is more likely that large linked cavities account for most of the storage. We suggest that outburst floods are triggered by rapid water input to the glacier bed, causing water-pressure transients that destabilize the linked-cavity system. -from Authors