Dendrogeomorphology and dating of debris flows in the Valle del Gallo, Central Alps, Italy
The debris flow is a mass movement frequent in the Italian Alps, causing the formation of alluvial fans. Some of the past centuries' episodes were dated by means of dendrochronology on a debris flow fan (Valle della Casina fan) in the Valle del Gallo (upper Valtellina – upper Valle dello Spöl, Central Italian Alps). The dendrogeomorphological study concerns the latest history of the Valle della Casina fan, while its former activity is documented by radiocarbon dating and geomorphologic and stratigraphic data spanning the late Holocene. A main channel from which some secondary channels originate runs through the fan. They are both active only during heavy rain. A buried fragment of charred wood dated 1720 ± 50 14C yr B.P. and two stratigraphic sections indicate that after the late Roman Age the debris flows were located near the fan apex and along the main channel and its ramifications. The debris flows were identified and dated by using dendrogeomorphological techniques applied to Pinus montana. Two hundred trees damaged by the impact with debris were sampled; they showed corrasion scars or slanting stems with compression wood. 53 damages caused by debris flows were dated. The oldest dated event occurred in 1888 and partially covered the middle of the fan. The debris flows dated between 1927 and 1929, and between 1936 and 1941 followed the same course of the previous one, while the latest episodes (1953, 1959, 1964, 1978, 1986 and 1992) were mostly confined to the fan channels. However, exceptional flows, such as the one, which occurred in 1978, can form new courses when they exceed the channel maximum capacity. The most recent debris flows coincide with heavy rain events recorded by the nearby meteorological station of Cancano. Moreover, a mountain pine tree-ring chronology was created for dating a dead tree.