Modelling late‐holocene snow‐avalanche activity: Incorporating a new approach to lichenometry

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Abstract
Where snow avalanches descend steeply into large rivers, displacing bedload, avalanche boulder‐ramparts may accumulate, retaining a record of late‐Holocene snow‐avalanche frequency. The age frequency of surface boulders on 12 such features in upper Jostedalen, southern Norway, was investigated using the size‐frequency distribution of lichens. A model was constructed to simulate the influence of variations in avalanche frequency sinceAD1325 on lichen‐size‐frequency distributions.Using this modelling approach it is not possible to define a unique pattern of avalanche activity to account for the observed lichen‐size‐frequency distributions, but it is possible to place strict limits on the range of scenarios that are acceptable. The results suggest that maximum avalanche activity occurred during the 19th century, after the peak of the Little Ice Age. This conflicts with historical records of avalanche damage to property at lower altitude in nearby valleys, which may reflect avalanche activity only during a short period of extreme climatic conditions. Close agreement between the records of snow avalanches and debris flows suggests that both reflect periods of high winter snowfall and rapid spring melting rather than low temperature. Future ‘greenhouse warming’ may therefore result in increased avalanche activity in southern Norway.