Tree-Ring Reconstruction of Debris-Flow Events Leading to Overbank Sedimentation on the Illgraben Cone (Valais Alps, Switzerland)

Dendrogeomorphological analyses of trees affected by debris flows have regularly been used to date past events. However, while previous studies in the Swiss Alps have focused primarily on granitic and gneissic debris-flow material and on Larix decidua Mill. and Picea abies (L.) Karst. trees, they have – at the same time – widely disregarded torrents dominated by finely fractured calcareous and dolomitic lithologies and forests populated with Pinus sylvestris L. In this paper, we report on results obtained from a debris-flow cone in the Rhone valley (Valais, Switzerland) where very fine and muddy material (limestone, dolomite, quartzite and calcite) occasionally affects P. sylvestris trees. Based on the results of a geomorphic map, 1004 increment cores from 451 disturbed P. sylvestris, 37 L. decidua and five P. abies trees were sampled, allowing reconstruction of 15 events between AD 1793 and 2005 as well as the determination of breakout locations of events. From the data, it also appears that debris-flow material only rarely left the incised channel over the last 200 years and that overbank sedimentation events did not occur after 1961, when a rockslide delivered large amounts of erodible material and subsequent debris flows caused an important incision of the canyon on the cone.