Massive biomass flushing despite modest channel response in the Rayas River following the 2008 eruption of Chait\ue9n volcano, Chile

The 2008 eruption of Chait\ue9n volcano in southern Chile severely impacted several densely forested river catch- ments by supplying excess pyroclastic sediment to the channel networks. Our aim is to substantiate whether and how channel geometry and forest stands changed in the Rayas River following the sudden input of pyroclas- tic sediment. We measured the resulting changes to channel geometry and riparian forest stands along 17.6 km of the impacted gravel-bed Rayas River (294 km 2 ) from multiple high-resolution satellite images, aerial photo- graphs, and \ufb01eldwork to quantify yield volume characteristics of the forest stands. Limited channel changes dur- ing the last 60 years before the eruption re\ufb02ect a dynamic equilibrium condition of the river corridor, despite the high annual precipitation and the sediment supply from Chait\ue9n and Michinmahuida volcanoes in the headwa- ters. Images taken in 1945, 2004, and 2005 show that total size of the vegetated channel islands nearly doubled between 1945 and 2004 and remained unchanged between 2004 and 2005. Pyroclastic sediment entering the Rayas River after the 2008 eruption caused only minor average channel widening (7%), but killed all island veg- etation in the study reach. Substantial shifts in the size distribution of in-channel vegetation patches re\ufb02ect losses in total island area of 46% from 2005 to 2009 and an additional 34% from 2009 to 2012. The estimated pulsed re- lease of organic carbon into the channel, mainly in the form of large wood from obliterated island and \ufb02oodplain forests, was 78\u2013400 tC/km/y and surpasses most documented yields from small mountainous catchments with similar rainfall, forest cover, and disturbance history, while making up between 20% and 60% of the annual carbon burial rate of \ufb02uvial sediments in the northern Patagonian fjords. We conclude that the carbon footprint of the 2008 Chait\ue9n eruption on the Rayas River was more signi\ufb01cant than the measured geomorphic impacts on chan- nel geometry for the \ufb01rst \ufb01ve years following disturbance. The modest post-eruptive geomorphic response in this river is a poor indicator of its biogeochemical response