The presented work aims to reconstruct the frequency of past, undocumented events of the debris flow torrent Ritigraben (Valais, Switzerland). Based on a detailed geomorphological map (scale 1:1000) of the cone, sample selection for tree-ring analyses started on trees (Picea abies L. (Karst), Larix decidua Mill., Pinus cembra ssp. sibirica) that were obviously influenced by deposited material. In a total, 2450 samples of about 1200 trees were analyzed regarding disturbances in growth that could be related to the respective deposits. For the determination of disturbances, trees of the same species growing outside the sphere of influence of the torrent were cored to estimate common, undisturbed growth variations in this area. The resulting "reference curve" for each tree species was used to determine and to date growth reactions of disturbed trees on the cone. In addition trees growing in former flow paths or on top of old deposits were sampled to determine minimum ages of past activity.
The resulting geomorphological map of the cone outlines the distribution of 12 former flow paths and numerous depositions of former events. Dating of growth anomalies in distributed trees allowed a detailed correlation of depositions to single events. The existing knowledge on the frequency of debris flow events (1922 – 1994) was, thus, extended back to 1700. The results show that a combination of tree-ring and detailed geomorphic analyses of an area are highly useful to get a realistic impression of recurrence intervals of a debris flow torrent. Despite all limitations Dendrogeomorphology is perhaps the most powerful tool to reconstruct past debris flow activities.