Tree-Ring Based Record of Debris-Flow Dynamics and Triggering Rain Storms at Ritigraben (Swiss Alps) since AD 1570

The Ritigraben torrent is located on the west-facing slope of the Mattertal valley (Valais, 46°11′N., 7°49′E.) and spans a vertical range of 2,000 m from its confluence with the Vispa River at 1,080 m asl to the summit of the Seetalhorn (3,100 m asl). A rock glacier occupies most of the headwater basin (1.4 km²) between 2,500 and 2,800 m asl (Fig. 1a), constituting the main starting zone of debris flows. On its downward course to the Mattervispa river, the Ritigraben torrent passes a forested cone (32 ha; 4.3 × 10⁶ m³; Fig. 1b) on a structural terrace (1,500–1,800 m asl), where debris-flow material affects trees composed of European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Cembran pine (Pinus cembra L.). Virtually all trees on the cone show growth disturbances (GD) related to past debris-flow activity. The study of 2246 tree-ring sequences sampled from 1102 L. decidua, P. abies and P. cembra trees allowed reconstruction of 124 events since AD 1566. Based on tree-ring records of disturbed trees growing in or next to the deposits, 86% of the 291 lobes identified on the present-day surface could be dated. A majority of the dated material was deposited over the past century. Signs of pre-20th century events are often recognizable in the tree-ring record of survivor trees, but the material that caused the growth anomaly in trees has been completely overridden or eroded by more recent debris-flow activity. Tree-ring records suggest that cool summers with frequent snowfalls at higher elevations regularly prevented the release of debris flows between the 1570s and 1860s; the warming trend combined with greater precipitation totals in summer and autumn between 1864 and 1895 provided conditions that were increasingly favorable for releasing events from the source zone. Enhanced debris-flow activity continued well into the 20th century and reconstructions show a clustering of events in the period 1916–1935 when warm-wet conditions prevailed during summer in the Swiss Alps. In contrast, very low activity is observed for the last 10-yr period (1996–2005) with only one debris-flow event recorded on August 27, 2002. Since sediment availability is not a limiting factor, this temporal absence of debris-flow activity is due to an absence of triggering events, which not only shifted from June and July to August and September over the 20th century, but also seemed to be initiated primarily by persistent precipitation rather than summer thunderstorms.