Abstract

Strengthening the resilience of communities affected by volcanic eruptions and monitoring the associated hydrologic and geomorphic processes have prime societal relevance in a country such as Chile. The country is patched by ca. 90 active volcanoes (i.e. with geological evidence of eruptive activity in the last 10 ky) and ranks 5th worldwide. Clearly, studying the medium- to long-term impacts of recent eruptions is of utmost priority to underpin an accurate volcanic hazard assessment and management. Quantifying the resilience response to the impacts of explosive eruptions and the subsequent hydromorphological processes depend on the modes and rates at which communities adjust to the altered surrounding landscapes and on their recovery following disturbance; when compared to background fluxes, sediment and biomass budgets are indispensable to achieve a sustainable management of the short-term impacts and to understand the long-term legacies of landscape-scale disturbances on forest and water resources, land use, and biogeochemical cycles. The few detailed mass balances of basins impacted by pyroclastic eruptions underline the extremely intense, though ephemeral, feeding of volcanoclastic sediment to rivers, culminating in the highest sediment yields ever recorded. The low frequency of eruptions is offset by the high effectiveness of sediment transport and the potential for causing long-lasting landform change. Such landform changes include the remoulding of river-channel geometry, which complicates estimates of flood frequencies, and alters the stream competence and capacity for evacuating sediments and large wood (LW). Tephra blankets disturb forested floodplains and fans, causing die-back, and feed massive sediment transport involving sizeable fractions of biomass during rainfall-triggered lahars and floods. Excess LW triggers channel instability culminating in avulsions on fans and floodplains. Rapid changes on channel geometry cause feedbacks with riparian vegetation, with higher recruitment of LW in the process. What scientists and practitioners are looking for in this context are metrics that gauge the buffering capacity of vegetation against such volcanically driven disturbance cascades. Additional challenges in understanding post-eruption channel evolution and recovery arise from differences on the intensity and type of volcanic disturbances (i.e. tephra fall and pyroclastic density currents) affecting river basins, on the grain size and thickness of the volcanic deposits, and on the role of the riparian vegetation and large wood accumulation. Very little was known in Chile on the geomorphic impacts of explosive eruptions on river systems and patterns of channel morphology, sediment and large wood transport evolution until the Chaitén eruption in 2008. Given the enormous volumes of sediment still available in main valley of the Río Blanco (Chaitén) and at the high elevations in the Calbuco area, we aim at extending our knowledge on the posteruption fluvial responses and recovery on these representative rivers, which would allow us to determine the controlling evolutionary factors of these highly altered fluvial systems. For this, we hypothesise that infrequent but extremely high magnitude explosive volcanic eruptions severely distort the rivers’ sediment budget and alter wood fluxes in impacted watersheds. To test this, we will quantify changes in sediment storage and biomass stocks in selected river segments following recent eruptions in the temperate rainforests of the Andean Southern Volcanic Zone of south-central Chile. If distortion of mass fluxes is discernible from sedimentary archives, then we will be able to infer that explosive eruptions are fundamental physical disturbances that impose geomorphic recoveries of tens to hundreds of years of the fluvial landscapes and associated ecosystems. River’s sediment budget controls the downstream morphological evolution and recovery of channel segments affected by the eruption. The main goal of the project is thus to build-up sediment budgets at both catchment and channel scales in rivers affected by volcanic eruptions and to relate them with i) the morphological evolution of riverchannels, and ii) the downstream vegetation recovery after the impact. This will be achieved by studying water flow, sediments and large wood fluxes in selected basins, whereas the quantitative and qualitative results will be the bases to develop a conceptual model of Eco-Morphological recovery in eruption-affected rivers. The originality of this project is the unprecedented quantification of sediment and LW budgets following pyroclastic eruptions in Chilean Mountain Rivers. This study extends the few available rate estimates on the eco-hydro-geomorphic response and recovery to regional volcanic disturbance. For this, dGPS and electromagnetic geophysical explorations, detailed 2D and 3D modelling of the subsurface will be obtained to elucidate the thickness and sub-surface structure of in-channel and floodplain sediment deposits, as well as the buried LW quantification and its subsurface spatial distribution. Moreover, our focus on the role of vegetation in modulating fluvial processes, and hence the rate at which rivers recover from massive sediment input, promises important insights for practical forest and watershed manageme