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

Abstract

The 2008 eruption of Chaitén volcano in southern Chile severely impacted several densely forested river catch-\ud ments by supplying excess pyroclastic sediment to the channel networks. Our aim is to substantiate whether\ud and how channel geometry and forest stands changed in the Rayas River following the sudden input of pyroclas-\ud tic sediment. We measured the resulting changes to channel geometry and riparian forest stands along 17.6 km of\ud the impacted gravel-bed Rayas River (294 km\ud 2\ud ) from multiple high-resolution satellite images, aerial photo-\ud graphs, and fieldwork to quantify yield volume characteristics of the forest stands. Limited channel changes dur-\ud ing the last 60 years before the eruption reflect a dynamic equilibrium condition of the river corridor, despite the\ud high annual precipitation and the sediment supply from Chaitén and Michinmahuida volcanoes in the headwa-\ud ters. Images taken in 1945, 2004, and 2005 show that total size of the vegetated channel islands nearly doubled\ud between 1945 and 2004 and remained unchanged between 2004 and 2005. Pyroclastic sediment entering the\ud Rayas River after the 2008 eruption caused only minor average channel widening (7%), but killed all island veg-\ud etation in the study reach. Substantial shifts in the size distribution of in-channel vegetation patches reflect losses\ud in total island area of 46% from 2005 to 2009 and an additional 34% from 2009 to 2012. The estimated pulsed re-\ud lease of organic carbon into the channel, mainly in the form of large wood from obliterated island and floodplain\ud forests, was 78–400 tC/km/y and surpasses most documented yields from small mountainous catchments with\ud similar rainfall, forest cover, and disturbance history, while making up between 20% and 60% of the annual carbon\ud burial rate of fluvial sediments in the northern Patagonian fjords. We conclude that the carbon footprint of the\ud 2008 Chaitén eruption on the Rayas River was more significant than the measured geomorphic impacts on chan-\ud nel geometry for the first five years following disturbance. The modest post-eruptive geomorphic response in this\ud river is a poor indicator of its biogeochemical response

Pyroclastic Eruption Boosts Organic Carbon Fluxes Into Patagonian Fjords

“…Our data are based on new estimates of organic carbon stocks in the living (above and below ground) biomass as high as 850 t ha −1 in the temperate rainforests around Chaitén. Our biomass estimate exceeds previous ones for Chilean Patagonia (Perez‐Quezada et al, ; Urrutia‐Jalabert et al, ) by up to a factor of 2 (Ulloa, Iroumé, Picco, et al, ) but is consistent with the higher estimates for temperate rainforests (Keith, Mackey, & Lindenmayer, ): above‐ground biomass can attain 660 t ha −1 and 870 t ha −1 in mixed and Nothofagus ‐dominated forests in the southern Chilean Andes (Schlegel & Donoso, ). Deadwood may add as much as ~15% to the total above‐ground biomass of temperate rainforests (Richardson et al, ) but is difficult to quantify where buried beneath volcaniclastic deposits.…”

“…The lahars were triggered by intense rainfall in the aftermath of the initial eruptive phase (Pierson et al, ). The subsequent reworking of volcaniclastic sediments buried river channels and floodplain forests beneath up to 8–11 m of volcanic debris in places, setting in motion a cascade of channel avulsions, bank erosion, and log jams (Major et al, ; Major & Lara, ; Pierson et al, ; Swanson et al, ; Ulloa, Iroumé, Picco, et al, ). The Blanco River avulsed across Chaitén Township, buried large parts of the town beneath of sediments (Figure ), and formed a posteruptive fan delta at the head of the fjord (Major et al, ).…”

Characterization of wood‐laden flows in rivers

“…A fire before the flood is mentioned in the metadata for the July 2017 flood in Payson, Arizona (USA), in which nine people died. (6) Lahars or flows developed during or after volcanic eruptions may deliver large quantities of wood as well, such as occurred at Mount St Helens in Washington, USA or Chaitén Volcano in Chile (Lisle, 1995;Swanson and Major, 2005;Swanson et al, 2013;Ulloa et al, 2015). (4) A jökulhlaup or a glacial lake outburst flood (GLOF), causing substantial bank erosion and wood recruitment or supply along its route (Oswald and Wohl, 2008), although none of the sites where the videos were recorded are glaciated.…”

Large wood recruitment and transport during large floods: A review

“…al., 2013;Dixon and Sear, et al, 2014;Boivin et al, 2015; Ravazzolo et al, 2015b;Ruiz-Villanueva et al, 2015b;Ulloa et al, 2015), but only a few studies analyzed quantitatively LW transport during large floods. Such studies are of paramount importance to gain insights into the 'real' wood dynamics during high-magnitude events.After the 2005 flood in Switzerland (discussed in the previous section) and in addition to wood recruitment processes, locations and volumes of LW depositions also were identified and quantified in the four investigated catchments (Kander, Kl.…”