Deglacial Mass-Movement Floods as a Potential Source of Catastrophic Flood Traditions: A Testable Geomythological Hypothesis

Tomoyasu Morita 

Catastrophic flood traditions occur in many geographically separated cultures, but their possible geological substrates need not be uniform. Existing explanations include river floods, earthquake and volcanic tsunamis, glacial-lake outburst floods, postglacial sea-level rise, and cultural transmission. This paper proposes a complementary, explicitly testable hypothesis: during the terminal Pleistocene and early Holocene, deglaciation and associated paraglacial adjustment increased opportunities for catastrophic glacier, ice-rock, and rock-slope failures in many formerly glaciated mountain regions; where such failures entered lakes, fjords, or confined river valleys, they could generate extreme displacement waves, debris-rich floods, secondary damming and outburst floods, and long-lived burial or destruction of valley-floor habitat. Modern analogues demonstrate the required magnitudes. The 2015 Taan Fiord landslide generated 193 m run-up, while the 2020 Elliot Creek rock avalanche produced greater than 100 m run-up before evolving into a destructive outburst flood. A 2026 glacier-associated slope failure in Nepal generated a debris-laden flood that travelled about 100 km, illustrating the continuing relevance of this hazard class, although its detailed mechanism remains under study. Geomythological and ethnographic evidence shows that mass-movement-generated floods can enter cultural memory: a Tlingit account recorded in 1909 explicitly describes an avalanche entering a lake, forcing water out, and overwhelming a settlement. The proposed Deglacial Mass-Movement Flood Tradition (DMFT) hypothesis therefore predicts a non-random association between formerly glaciated mountain settings and narrative features such as sudden onset, extreme local run-up, debris transport, anomalous flow direction, high-ground survival, and permanent loss of inhabited land. The hypothesis is not a claim of a simultaneous worldwide flood. It is a framework for testing whether regionally asynchronous but climatically clustered catastrophic events contributed to a subset of extraordinary flood traditions.