Alpine megafans, outsize fans, and lesser anomalous fans - a first comprehensive inventory for the European Alps

The first comprehensive inventory of anomalously large debris fans for a whole mountain range identifies a population of 286 in the European Alps with fan heights >100 m. Almost 40% fall within eight clusters. The Vinschgau cluster reported in 2011 by Jarman, Agliardi, and Crosta is confirmed as the largest, possessing five of the 16 Alpine megafans. Valtellina is next, with two. There are 78 outsize fans and 192 lesser anomalous fans. This study provides the first systematic range-wide review of Alpine fan spatial incidence and morphology. Spatial statistics demonstrate significant clustering across all fan sizes. Many anomalous fans exhibit cavity–debouchure–fan associations, and in several cases, source volumes comparable to fan volumes are reconstructable, consistent with event-dominated emplacement within the spectrum of Rock Slope Failure (RSF). A continuum from incremental to event-dominated origins is inferred, with hybrid forms probably common. No obvious prime causes are identified for a spatial distribution that is both focussed and scattered across almost the entire range. A possible contributory role for localised amplification of glacial erosion, including near breaches of drainage divides, is explored. The fan population is assumed to be an early to mid-Holocene paraglacial response to deglaciation and to subsequent climate fluctuations, with no entirely new cases in recorded history. Several fans have interacted with the formation of landslide dams. The Blatten catastrophe of May 2025 is discussed as a minor increment to a pre-existing lesser anomalous fan. The Alps display the largest known megafans and population globally, a ‘Goldilocks’ case? • First comprehensive inventory of anomalously large debris fans for the European Alps • 286 exceptionally large conoidal fan-forms mapped over eight widely-spaced clusters • In total 16 alpine megafans, 78 outsized fans and 192 lesser fans • Incremental-catastrophic continuum recognizes many large-event candidates • European Alps display the largest known megafans and population in global context