Large wood dynamics in a mountain river disturbed by a volcanic eruption

Abstract

Volcanic eruptions can dramatically alter river basins and channel networks by delivering huge amounts of sediments into the channels and affecting the surrounding vegetated area. These disturbances modify the process chain of large wood (LW) recruitment, storage, and transport. We investigated the effects of the 2015 Calbuco volcanic eruption in southern Chile on the dynamics of LW along two reaches of the Blanco Este River, which drains the north-eastern flanks of the volcano. The two reaches, R1 and R2, were surveyed for almost four years, combining field and remote analyses to better comprehend LW dynamics. We framed our study around three research questions: i) what are the processes explaining the spatial and temporal variations of LW load into the active channel of the Blanco Este River? ii) can the longitudinal (dis)connectivity be considered a controlling factor rather than flood magnitude? iii) is it possible to infer a relationship between recruitment sources, and flood events, with LW load fluctuations? The abundance of individual LW pieces and wood jams (WJs) were always higher in the downstream reach (R2). LW source areas to R1 were limited to forested bank erosions, while in R2 LW was recruited from bank erosions with the addition of wood pieces entering in the main channel from a lateral affluent and from eroded old lahar deposits that re-connected this source to the active channel. WJs were highly unstable and there were not statistically significant relationships between LW fluctuations and water discharge. In the Blanco Este River, LW delivery and export can be classified as event-driven processes, but alternating periods with longitudinal (dis)connectivity led to events delivering wood into the channel which seemed to occur asynchronously with those exporting LW. Finally, LW load in R1 appeared to be in equilibrium, but this condition might change in the future since the area of upper forested streambanks is rapidly declining.