Supplementary

Paleohydrological reconstruction is a core task in paleoflood hydrology, requires reliable flood model and geomorphic and sedimentary evidence. Identifying the triggering conditions and transport processes of megafloods is conducive to summarizing the occurrence patterns. In this work, we present a whole system model that describes the chronology and paleohydrology of catastrophic drainage following the Millennium Eruption (ME) in 946 CE in Changbaishan-Tianchi Volcano. The model encompasses the dynamics of the crater lake, the propagation of flood waves downstream, and an approximation of the breach erosion and outburst process. The dam erosion rate and the paleo-lake level were treated as unknowns in the system, and the boulder competence method was used to constrain by reasonable flow parameter, while eliminating the uncertainty caused by a single geological paleostage indicators. The paleohydrologic analysis indicated that at least 1 km³ of water was released from the caldera, with the vertical breach erosion rates as high as 34 m/h. Volcanic activity in ME II may have directly contributed to triggering of the flood event. Not all inundation records of lahar were from the crater lake outburst event. Reconstructions of megafloods based on downstream constraints on flood stage, velocity and discharge can help to infer and constrain the dynamics of dam failure mechanisms, and also contribute to our understanding of these complex paleohydrologic events.