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Massive biomass flushing despite modest channel response in the Rayas
River following the 2008 eruption of Chaitén volcano, Chile
Héctor Ulloa1,*, Andrés Iroumé2, Lorenzo Picco3, Oliver Korup4, Mario Aristide Lenzi3,
Luca Mao5, Diego Ravazzolo3
1Universidad Austral de Chile, Graduate School, Faculty of Forest Sciences and Natural
Resources, Valdivia, Chile
2Universidad Austral de Chile, Faculty of Forest Sciences and Natural Resources, Valdivia,
Chile
3University of Padova, Department of Land, Environment, Agriculture and Forestry, Italy
4University of Potsdam, Institute of Earth and Environmental Sciences, Germany
5Pontificia Universidad Católica de Chile, Department of Ecosystems and Environment,
Santiago, Chile
*Corresponding author: Héctor Ulloa, Universidad Austral de Chile, Graduate School,
Faculty of Forest Sciences and Natural Resources, Independencia 631, 5110566 Valdivia,
Chile. Tel.: +56-63-2293004; E-mail address: [email protected]
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Abstract
The 2008 eruption of Chaitén volcano in southern Chile severely impacted several densely
forested river catchments 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 pyroclastic 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 km2) from multiple high-resolution
satellite images, aerial photographs, and fieldwork to quantify yield volume characteristics
of the forest stands. Limited channel changes during the last 60 years before the eruption
reflect a dynamic equilibrium condition of the river corridor, despite the high annual
precipitation and the sediment supply from Chaitén and Michinmahuida volcanoes in the
headwaters. 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 vegetation in the
study reach. Substantial shifts in the size distribution of in-channel vegetation patches
reflect losses in total island area of 46% from 2005 to 2009 and an additional 34% from
2009 to 2012. The estimated pulsed release of organic carbon into the channel, mainly in
the form of large wood from obliterated island and floodplain forests, was 78-400 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 fluvial sediments in the northern Patagonian fjords. We
conclude that the carbon footprint of the 2008 Chaitén eruption on the Rayas River was
more significant than the measured geomorphic impacts on channel geometry for the first
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five years following disturbance. The modest post-eruptive geomorphic response in this
river is a poor indicator of its biogeochemical response.
Keywords: Chaitén volcano; forest disturbance; channel island; organic carbon
1. Introduction
An increasing number of studies emphasize how vegetation physically influences gravel-
bed river dynamics through increasing flow resistance, prompting sedimentation,
stabilizing banks, or delivering large wood (Comiti et al., 2011; Labbe et al., 2011; Gurnell,
2014; Surian et al., 2015). In this context, the pattern of vegetated islands and channel
banks reflects changes in discharge and sediment supply, including catastrophic floodplain
aggradation and dissection (Gurnell et al., 2001; Ashmore et al., 2011; Zheng et al., 2014).
Among other natural disturbances, explosive volcanic eruptions have had some of the most
decisive impacts on channel and valley-floor geometry, spawning some of the highest
fluvial sediment yields reported (Pierson et al., 2011; Korup, 2012; Pierson and Major,
2014). Reports on the effects on volcanic eruptions of mounts Saint Helens, Pinatubo, Etna,
El Chichón, Popocatépetl, Taupo, Hills, and Misti concentrate mainly in fluvial
geomorphology and in geologic and ecologic processes (see Ulloa et al., 2015). While these
reports also indicate important effects on the forest cover in adjacent catchments, the fate of
such disturbances on riparian and in-channel vegetation stands remains comparatively
obscure, although channel islands in particular are prime loci for studying the feedbacks
between natural disturbances and biogeomorphic response in river systems. Here we
investigate such effects caused by a sequence of eruptions of Chaitén volcano, south-central
Chile, that began on 2 May 2008 (Lara, 2009; Pallister et al., 2013) and that severely
altered the morphology, hydrology, and forest cover in several adjacent catchments. Tephra
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fallout, pyroclastic flows, lateral blasts, and lahars killed large stands of forest vegetation
along river channels and floodplains (Lara, 2009). Particularly, tephra fallout can suffocate
and kill the vegetation. Subsequent increases in runoff from ash-sealed soils, landsliding
from slopes sustaining dead trees, and gully erosion delivered large amounts of tephra and
large wood to the drainage network, triggered a major channel avulsion in the Blanco River
that partly obliterated the town of Chaitén (Major et al., 2013; Pierson et al., 2013;
Swanson et al., 2013; Ulloa et al., 2015).
We focus on the effects of the 2008 eruption on the Rayas River, a braided gravel-bed river
draining the northern flanks of Chaitén volcano. We use remote sensing and field evidence
to substantiate whether and how channel geometry and forest stands had changed following
the sudden input of pyroclastic sediment. We test the hypothesis that the loss of biomass of
riparian and island trees following the eruption can be approximately estimated from the
degree of changes in channel morphology.
2. Study area
The Rayas River catchment (294 km2) lies 250 km south of Puerto Montt in the Chilean
Región de Los Lagos and drains the northern slopes of the Chaitén (1100 m asl) and the
partly ice-capped Michinmahuida (2450 m asl) volcanoes (Fig. 1). Pleistocene volcanic
sediments cover a basement of Miocene granitoids and Paleozoic schists and gneisses
(Piña-Gauthier et al., 2013). About 84% of the catchment features old-growth forests
dominated by evergreen species and Fitzroya cupressoides (Donoso, 1981), and 16% is
permanent snow and ice (CONAF-CONAMA, 1999). Exploitation of these forests began in
the late nineteenth century as part of expanding settlement in the region, and early settlers
selectively harvested high-quality wood species and burned forests for pasture (Urbina,
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2011; Torrejón et al., 2011). Log driving or rafting was common along the remote rivers of
southern Chile such as the Rayas (Urbina, 2011; Torrejón et al., 2011). Forestry was the
main economic activity in the area, but logging decreased in the early twentieth century and
ceased in the 1970s by Decree 490 of 1977 by the Chilean Ministry of Agriculture
(http://bcn.cl/19k8m), which prohibits the cutting of Fitzroya cupressoides. The Rayas
River catchment has largely escaped any natural (e.g., wildfires, earthquakes) or human
(e.g., forest interventions) disturbances since.
The Rayas River is a fifth-order stream (Strahler, 1954) with peak flows and snowfall at
higher altitudes during winter months (May to September). Discharge and sediment
transport data are not available. A 16-year-long record from 1998-2013 indicates that mean
annual rainfall in the nearby town of Chaitén is 3200 mm with peaks exceeding 4200 mm,
but annual totals > 5800 mm are documented 45 km south of the town (Dirección General
de Aguas, 2014; http://dgasatel.mop.cl/).
We study a 17.6-km-long reach of the lower Rayas River (which we term the ‘Main reach’,
Fig.1), where it has multiple channels and a braided gravel bed with an average bed slope
of 0.008 and flows through a glacially carved valley. Upstream of the Main reach, the upper
Rayas catchment is ~ 114 km2, has an average gradient of 0.44 and a relief of ~ 2300 m.
The temperate evergreen rainforest covering the upper catchment is dominated by
Eucryphia cordifolia, Laureliopsis phillippiana, Nothofagus dombeyi, N. nitida,
Weinmannia trichosperma, and Caldcluvia paniculata and has an estimated density of 250-
500 trees/ha and a basal area of 57-124 m2/ha (Swanson et al., 2013). Floodplain forests are
the youngest and have a higher tree density (2100-2600 trees/ha) but lower basal area (76-
82 m2/ha) (Swanson et al., 2013). Patches of these forests were felled or damaged by the
2008 eruption of Chaitén volcano. Based on the map by Swanson et al. (2013), we estimate
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that 72% of the upper Rayas catchment was covered by > 10 cm of coarse (gravel) tephra
or lapilli (Fig. 2). About 4 km2 were damaged by a directed blast down the north flank of
the volcano (Swanson et al., 2013), and 63% of the forest area in the upper Rayas
catchment had severely damaged foliage four years after the eruption (Fig. 2). Swanson et
al. (2013) reported that remobilized tephra buried 5 km2 of floodplain forests in the lower
19 km of the Rayas River beneath up to 1 m (Figs. 3, 4), whereas pyroclastic deposits in the
upper Rayas channel were up to 2 m thick.
3. Materials and methods
Satellite images from May 2005 (Panchromatic + Multispectral four-band, QuickBird),
January 2009 (Panchromatic, WorldView-1), and January 2013 (Multispectral, QuickBird)
with 0.6/2.4-m, 0.5-m, and 2.4-m resolution, respectively, capture the extent of channel
geometry and forest cover along the 17.6-km long reach (Main reach, Fig. 1).We also
measured changes along another 5.6-km-long reach (Chana reach, named after the nearby
village, Fig. 1) to study the long-term dynamics of the Rayas River before the eruption.,
using a 1:35,000-scale panchromatic aerial photo from 1945 that we scanned to a resolution
of 720 dpi and georeferenced using ESRI ArcGIS 9.3 to obtain an average nominal
resolution of 1.5-m, and multispectral (QuickBird) 2.4-m resolution satellite images
captured in February 2004 and May 2005 (the same as above). Satellite images were
supplied georeferenced and corrected. All images show similar low-flow conditions and
allowed us to map the active channel defined by fringing riparian forests (Comiti et al.,
2011; Ulloa et al., 2015), comprising inundated areas and unvegetated sediment bars.
We characterized the active channel in the Main and Chana reaches at 27 and 14 cross
sections, respectively, which were between 400 and 900 m apart (see locations in Fig.1).
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Mean channel widths were calculated averaging the measurement of these cross sections in
every reach and time period. We classified vegetated channel islands into pioneer, building,
and established (Mikuś et al., 2013; Picco et al., 2014) using vegetation maturity and size as
the key criteria; and we also differentiated between arboreal and shrubby vegetation by
estimating vegetation height from canopy texture, shape, and shadows (Kollman et al.,
1999; Picco et al., 2014). Pioneer islands feature patchy in-channel vegetation 3-5 m high;
building islands have a more heterogeneous crown texture; and established islands sustain a
mature, high, and dense vegetation cover with well-developed crown texture (Gurnell and
Petts, 2002; Mikuś et al., 2013). From the photo-interpretation it was possible to define that
the structure of island forests were similar to riparian forests, a fact that was confirmed
during our field surveys performed in channel segments that were unaffected by the
eruption.
During fieldwork in January 2014 we recorded the thickness of fresh volcaniclastic
sediment and measured diameter at breast height and the height of trees growing on islands
of different sizes to estimate forest biomass losses and organic carbon export from the study
reaches (see location of sampling area on Fig. 1). We surveyed four pioneer, three building,
and three established islands that, judging from the 2005 satellite image, had prevailed after
the eruption and were only slightly affected by sedimentation processes. We recorded
species regenerating and diameter at breast height (DBH) and the height for all trees with
DBH > 10 cm to capture wood volumes in up to three 10 x 10 m plots in the upstream,
downstream, and central portions of each island. On smaller islands we only established
one plot. We used these data to estimate the total net losses of island forest vegetation to
fluvial export following post-eruptive forest die-back. We extrapolated individual tree
volumes v (m3) as:
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v = 0.000019 DBH2 h + 0.00024 DBH2 (1)
where DBH has units (cm) and h is tree height (m); the mean squared error for Eq.(1) is
~31% (Drake et al., 2003). We estimated the stand volume V (m3/ha) via multiplying v by
the tree density for each island type.
We also computed the volume V* of the floodplain forests (m3/ha) using the data of basal
area (m2/ha) and tree density N (stems/ha) from Swanson et al. (2013) and a yield volume
relationship from Drake et al. (2003):
V* = –162.0267 + 4.2006 BA + 17.9919 H – 0.08394 N (2)
where BA is the basal area (m2/ha), and H is the height of dominant trees (m); the mean
squared error for Eq.(2) is ~41% (Drake et al., 2003). We used a stratified sampling design
to sample the characteristics of the vegetation of the different categories of islands,
considering each category as a different stratum and calculating sampling errors
accordingly (Cochran, 1977). We applied Eq.(2) using Swanson et al. (2013) basal area and
tree density data to estimate floodplain biomass, and we assigned to this data the same
sampling error given to assess the biomass volume in established islands, considering that
the characteristics of the forests are similar in these two morphologic units.
Equations (1) and (2) were compiled by Drake et al. (2013) for the species and stands
belonging to the evergreen rainforest type that characterize the study area.
4. Results
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4.1. Post-eruptive channel changes
Remote sensing data document that the active channel area and width in the Rio Rayas
remained remarkably in a dynamic equilibrium condition between 1945 and 2004 before
the eruption, at least judging from our observations in the Chana reach. During this pre-
eruption period, the Chana reach active channel widened by only 3% and increased its
channel area by 5%, although such minor changes approach the scale of image resolution
and hence the margin of detectability (Fig. 5). The number of channel islands per unit area
of active channel also remained comparable between 1945 and 2004 at 25-26/km2 (Fig.
6A). However, the total and individual areas of islands nearly doubled in the 60 years
before the eruption (Figs. 6A, 6B, 7).
The 2008 eruption caused an average channel widening of some 7%, from 326 to 348 m in
the Main reach in the year after the eruption and another 5% between 2009 and 2013 (Fig.
5). The active channel area grew by a similar fraction (from 5.4 km2 before the eruption to
5.8 and 6.1 km2 in 2009 and 2013, respectively), widening locally by > 40% through bank
undercutting at average rates of 4 to 22 m/y (Fig. 5). More dramatic changes occurred to
channel island abundance and size (Figs. 6C, 6D, 7). Overall, 158 of the 193 islands that
existed in 2005 had been modified (n = 82) or had disappeared (n = 76) in the year after the
eruption; similar losses continued until 2013 (Table 1). The number of islands per unit area
of active channel had shrunk significantly (p < 0.05) by ~17% in the first year after the
eruption, while up to 43% were lost in subsequent years (Figs. 6C, 6D, 7). Total and
relative island areas lost 46% from 2005 to 2009 and another 34% from 2009 to 2013.
Mean individual island size decreased by 40% in the first year after the eruption but then
increased by 12% thereafter (Fig. 6D).
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Before the eruption, in 2005 pioneer islands were more frequent than building and
established islands but occupied a lower portion of the active channel (Fig. 8). The number
of small pioneer islands decreased by 48% between 2005 and 2009 and an additional 64%
between 2009 and 2013. The number of building islands increased by 56% from 2005 and
2009 mainly because of fluvial scour that dissected the formerly larger islands and
subsequently decreased by 57% between 2009 and 2013; whereas established islands were
amongst the most resilient to the volcanic disturbance (Figs. 8, 9, 10). In 2005 pioneer
islands had a median area of 0.11 ha, whereas building and established islands were
roughly three and seven times larger, respectively (Fig. 8C). The year after the eruption
concentrated major changes and island size decreased for all categories; however changes
continued between 2009 and 2013 (Fig. 8-C).
Between 2005 and 2009, most pioneer islands were modified or disappeared; while the
majority of the established islands remained unmodified, maintaining their shape and size
(Figs. 9A, 9C). In the following four post-eruption years, pioneer and building islands were
the most affected showing similar percentages of disappeared islands (Figs. 9B, 9D).
Following the eruption, the building islands had mainly disappeared, while others were
either dissected or unmodified (Figs. 7, 9, 10).
4.2. Post-eruptive biomass losses
The remote sensing images show that the 2008 eruption killed all island vegetation and
patches of floodplain forests in the study reach, mainly by lateral channel erosion and
deposition of 1-2 m of reworked tephra (Swanson et al., 2013). Our vegetation surveys on
the channel islands yielded stem densities of 725-1067 stems/ha (DBH > 10 cm) of dead
trees remaining in a growth position (Table 2). Although tree mortality was 100% on all
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islands, natural regeneration featuring wind-dispersed species such as Nothofagus dombeyi,
N. nitida, Weinmannia trichosperma, Caldcluvia paniculata, and Embothrium coccineum
had already begun on 70% of the surveyed islands.
From the observed tree densities, we estimate an average pre-eruption biomass volume of
140 ± 70, 240 ± 70, and 360 ± 120 m3/ha, for pioneer, building, and established islands,
respectively. Using Eq.(2) and data by Swanson et al. (2013), we estimate an average
biomass volume for floodplain forests of 390 ± 130 m3/ha. Assuming a total island area of
80.7 ha, we reconstruct the minimum forest biomass volume at 21,100 ± 5300 m3, or 1200
± 300 m3 per kilometer of channel length, before the eruption (Table 3).
About 34.8 ha of floodplain forests were obliterated the year after the eruption, and another
33.6 ha were lost between 2009 and 2013 (Table 3), so 13,800 ± 4700 and 13,300 ± 4500
m3 of biomass were flushed into the study Main reach, respectively. Adding to these values,
the biomass from eroded islands leads to minimum estimates of 21,600 ± 9000 and 17,200
± 7200 m3 for these two periods (Table 3). Channel islands contributed between 23% and
37% of these yields. Assuming a mean wood density of 0.65 t/m3 (Diaz-vaz et al., 2002)
and an organic carbon mass fraction of 50% (Seo et al., 2008), the corresponding average
specific yields of coarse particulate organic correspond to 400 ± 160 tC/y/km-length and 78
± 30 tC/y/km-length for the first period and second period, respectively.
5. Discussion
5.1. Assessment of geomorphic impacts following the 2008 eruption
Chana Reach offers some insights into the channel dynamics of the Rayas River prior to the
Chaitén eruption in 2008. The nearly twofold growth of channel-island areas between 1945
and 2004 reflects the gradual spread of channel-stabilizing forest vegetation rather than the
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dissection of forested floodplains (Figs. 6, 7). This trend of island expansion characterize
largely undisturbed rivers (Belletti et al., 2013) or those recovering from large floods (Picco
et al., 2014). Between 1945 and 2004 the Rayas River maintained its average channel width
despite local floodplain erosion and accretion of islands to the floodplain, similar to channel
reaches with minor flood impact or high sediment supply rates (Belletti et al., 2013). These
observations indicate that shortly before the eruption in 2008, the Rayas River did not
experience any major channel-shaping floods apart from the winter floods that frequent this
part of Chile and was in a dynamic equilibrium condition.
Changes in the Rayas River channel width and island system and in the forest cover were
detected after the eruption. Because the channel was in a dynamic equilibrium condition
before the eruption, we assume these changes to be triggered by the eruption of Chaitén
volcano. More than 60% of the upstream catchment had damaged vegetation in 2013 (Fig.
2). The minute changes to channel width were most likely linked to the low gradient and
the buffering of low-volume pyroclastic density currents by dense forest stands (Major et
al., 2013; Swanson et al., 2013). In-channel vegetation was completely obliterated
following the eruption, particularly owing to the loss of smaller islands (Figs. 8, 9), which
is consistent with observed effects of excess sediment input to river reaches (Gurnell and
Petts, 2002). Sediment inputs from pyroclastic flows and lahars and subsequent increases in
runoff as described for the Blanco River (Major et al., 2013; Pierson et al., 2013; Swanson
et al., 2013) are undoubtedly the mechanisms that led to forest mortality, channel
morphologic changes, and the mobilization and transport of the wood. Similar processes are
reported associated to the Mount St Helens eruption by Meyer and Martinson (1989) and
Swanson and Major (2005).
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In the Rayas River pioneer and established islands were the most and least affected islands,
respectively (Figs. 8, 9), likely because of the younger and less well-developed root
systems of low-relief pioneer islands as opposed to the denser, root-reinforced vegetation of
established islands (Mikuś et al., 2013). The first year after the eruption more pioneer
islands but fewer building and established islands disappeared, a possible indication that
well-vegetated islands had resisted erosion (Fig. 9), yet more islands disappeared in the
following 2009-2013 period suggesting that the roots of the dead trees were no longer
capable of anchoring the islands against fluvial scour, as stated by Gurnell et al. (2012) and
Gurnell (2014). Five years after the eruption, we found that forest stands are regenerating in
70% of the surveyed islands, mainly featuring wind-dispersed species (Nothofagus
dombeyi, N. nitida, Weinmannia trichosperma, Caldcluvia paniculata, and Embothrium
coccineum) that quickly colonize bare substrates and supplied by the riparian forests
(Veblen, 1989). Regenerating species were absent from the active channel, but dead trees
create favorable microclimates for succession (Dale et al., 2005). Overall, we expect that
channel adjustment in the Rayas River is likely to be ongoing, judging from previous
reports of volcanic disturbance of river channels (Pierson and Major, 2014; Zheng et al.,
2014).
5.2. Export of large wood following the eruption
Yield data obtained during our field surveys showed that forests were less dense on islands
compared to those for floodplains reported by Swanson et al. (2013). Post-eruptive erosion
might have begun to thin island vegetation, although we did not survey the largest and
possibly most densely vegetated islands. Hence, we treat our field data as minimum
estimates that potentially underestimate the actual tree densities and diameters, given that
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the bulk of biomass lost to the channel largely came from eroding floodplains. The release
of organic carbon from obliterated island and floodplain forests into the channel of the
Rayas River was distinctly peaked in the first year following the eruption. We did not find
information from the volume of biomass released by obliterated islands and floodplains into
channels affected by volcanic eruptions, but this peak (1227 m3/km/y) is much higher than
the volume of wood introduced by floodplain erosion (estimated as the product of the
eroded floodplain areas and the standing wood volume on that floodplain) to the Rayas
River during the period 1945-2004 (9.5 m3/km/y) and to the Saint-Jean River in Canada
between 1963 and 2013 (11.3 m3/km/y; Boivin et al., 2015). However, the volume of wood
introduced in the Rayas River during the 2009-2013 period was 61 m3/km/y, indicating a
rapid decrease of wood supply. The post-eruption release of organic carbon from the Rayas
River is also very high compared to reported yields from Japanese mountain rivers
sustaining similar dense forest stands, experiencing comparable amounts of annual rainfall
but different disturbance regimes (Seo et al., 2008). Our estimates of the specific yields
from large wood are of the order of 46-232 tC/km/. When corrected for upstream
contributing catchment area we arrive at rates between 12 and 60 tC/km2/y which surpass
most documented particulate organic carbon yields from small mountain rivers in humid
climates (e.g., Beusen et al., 2005). Clearly, the volcanic disturbance of southern Chilean
temperate rainforest vegetation favors the catastrophic release of large amounts of organic
carbon into the nearby fjords that could be a significant regional carbon sink (Smith et al.,
2015). Our estimates of post-eruptive organic carbon yields from a single medium-sized
river rank between 20% and 62% of the projected annual burial rate of terrestrially derived
organic carbon (5000-16,000 tC/km2/y) across nearly 4300 km2 of the northern Patagonian
fjords (Sepúlveda et al., 2011). However, not all this organic carbon yield has been
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immediately delivered to the Gulf as large wood is stored on bars, islands, and floodplains
following our observations during field surveys.
Future research on volcanic disturbances of temperate rainforest vegetation (e.g., Jara and
Moreno, 2012) should include such substantial post-eruptive carbon footprints, especially
where they accompany only modest geomorphic impacts on channels and floodplains.
6. Conclusions
Our study shows that the 2008 Chaitén volcanic eruption generated moderate geomorphic
impacts on the channel and floodplains of the Rayas River compared to significant
destruction of forested in-channel and riparian islands, which roughly lost a third of the
total biomass from felled temperate rainforest stands that were rapidly flushed out of the
study reach as large wood. This post-eruptive release of wood generated organic carbon
yields that surpass most reported rates from small mountainous rivers in humid climates,
especially those with comparable rainfall, forest cover, and disturbance history while
making up between roughly 20% and 60% of the estimated organic carbon burial from
fluvial sediments in the northern Patagonian fjords. The biogeochemical implications of
this volcanogenic release of large amounts of organic carbon into a coastal river system
seem more profound than the measured changes in channel geometry. Although the
primary disturbances during the eruptive phase killed the vegetation cover on the islands,
dead vegetation is playing an important role as biologic legacy within the active channel.
Five years after the eruption,75% of the surveyed islands sustained regenerating tree
species. We conclude that the consequences on highly pulsed organic carbon fluxes should
be included when studying the vegetation disturbances from volcanic eruptions, even where
geomorphic fingerprints are comparatively minor, thus potentially masking the associated
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biogeochemical implications from catastrophic floodplain forest die-back. Assessing the
ecological impacts of an eruption could be a good proxy for determining the hazards of
such events.
Acknowledgements
This study was supported by Projects FONDECYT 1141064 and CONICYT-BMBF
PCCI20130045 awarded to A. Iroumé and O. Korup. H. Ulloa is supported by the Chilean
Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). This research
is part of a Doctor in Forest Sciences thesis, Universidad Austral de Chile. Comments by
Julia Jones and Fred Swanson contributed to improving early manuscript drafts. The
authors acknowledge the valuable comments and suggestions by three anonymous
reviewers.
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List of Figures
Fig. 1. Map of study area and reaches and specific observation zones (a, b, c), Rayas River,
south-central Chile. CV: Chaitén Volcano; MV: Michinmahuida Volcano. Dotted black
lines perpendicular to flow direction indicate the location of cross sections.
Fig. 2. Comparison of LANDSAT false-color images of forest vegetation near Chaitén
volcano in (A) 2005 and (B) 2013. Light blue colors are snow and ice. Healthy vegetation
is green, whereas vegetation impacted by the Chaitén eruption is burgundy. Isopach of total
tephra (> 10 cm) cover from Swanson et al. (2013) shown in red. CV: Chaitén Volcano.
Images courtesy of http://earthexplorer.usgs.gov/
Fig. 3. Part of the Main reach showing the Rayas River draining into the Pacific Ocean.
Both photos were taken from the western slopes of the Chaitén volcano and show some of
the impacts on the active channel and floodplain (red ellipse; zone b demarcated in Fig. 1).
A) January 2010 (H. Ulloa); (B) February 2014 (courtesy of D. Antileo, Universidad
Austral de Chile).
Fig. 4. Vegetation impacts by the volcanic eruption in the Rayas catchment in zone c
demarcated in Fig. 1. (A) Photo taken from Rio Rayas looking toward Chaitén: damaged
and dead forests on hillslopes and along the river corridor, and post-eruption landslides dot
steep hillslopes; (B) and (C) details of dead forests on channel islands. Photos taken in
January 2014 (H. Ulloa).
Fig. 5. Pre-eruption active channel dynamics in the Chana reach and post-eruption active
channel dynamics in the Main reach obtained from 14 and 27 cross sections. Condition
2005 represents pre-eruption, although the Chaitén volcanic eruption began 2 May 2008.
Fig. 6. Pre- and post-eruption dynamics of islands in the Rayas River. Total number and
island area to active channel area (%) in the Chana reach (A) and Main reach (C), and
22
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
4344
boxplots of log-transformed island sizes (ha) expressed in arbitrary units (a.u.); the black
line within each box indicate the median values, box ends are 25th and 75th percentiles, and
whiskers are the minimum and maximum values without considering outliers in the Chana
reach (B) and Main reach (D).
Fig. 7. Detail of mapped gradual spread of channel and floodplain forest vegetation before
the eruption (1945, 2004 and 2005) and post-eruption disturbances (2009 and 2013) in an
~2-km-long subreach within the Main reach (zone a demarcated in Fig. 1).
Fig. 8. Changes in island features in the Rayas River Main reach through time. (A) Number
per unit active channel area; (B) total island area per category to total active channel area
(%); (C) boxplots of log-transformed island sizes (ha) expressed in arbitrary units (a.u.);
black line within each box is median, boxes encompass the 25th and 75th percentiles;
whiskers span at 1.5 times the interquartile range; open circles are outliers.
Fig. 9. Variation of island abundance per category for the periods of (A) 2005-2009 and (B)
2009-2013; and variation in total island area for periods of (C) 2005-2009 and (D) 2009-
2013.
Fig. 10. Temporal sequence of changes to the island system and active channel extention in
the Main reach of Rayas River. ‘2005 to 2009’ shows changes during the first year after the
eruption (2008-2009) and ‘2009 to 2013’ shows changes in the subsequent period. (see Fig.
1 for location).
23
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
4546
Chaitén
Santiago
MV
Chaitén Town
Chana Village
CV
RiverPA
CIF
IC
OC
EAN
CV
a
b
Rayas basin Caldera rimObservation island zone
LegendMain reach limit
Chana reach limitDetailed channel observation zone
Figure 1. Map of study area and reaches and specific observation zones (a, b, c), Rayas
River, south-central Chile. CV: Chaitén Volcano; MV: Michinmahuida Volcano. Dotted
black lines perpendicular to flow direction indicate the location of cross sections.
24
533
534
535
536
537
538
539
4748
Rayas basinRayas upper basin
Legend
Total tephra (> 10 cm)Disturbance areaCaldera rim
Upstream Mainsegment limit
(A) (B)
CV
CV
Figure 2. Comparison of LANDSAT false-color images of forest vegetation near Chaitén
volcano in (A) 2005; and (B) 2013. Light blue colors are snow and ice. Healthy vegetation
is green, whereas vegetation impacted by the Chaitén eruption is burgundy. Isopach of total
tephra (> 10 cm) cover from Swanson et al. (2013) shown in red. CV: Chaitén Volcano.
Images courtesy of http://earthexplorer.usgs.gov/
25
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
4950
Figure 3. Part of the Main reach showing the Rayas River draining into the Pacific Ocean.
Both photos were taken from the western slopes of the Chaitén volcano and show some of
the impacts on the active channel and floodplain (red ellipse; zone b demarcated in Fig. 1).
(A) January 2010 (H. Ulloa); (B) February 2014 (courtesy of D. Antileo, Universidad
Austral de Chile).
26
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
5152
Figure 4. Vegetation impacts by the volcanic eruption in the Rayas catchment in zone c
demarcated in Fig. 1. (A) Photo taken from Rio Rayas looking toward Chaitén: damaged
and dead forests on hillslopes and along the river corridor, and post-eruption landslides dot
steep hillslopes; (B) and (C) details of dead forests on channel islands. Photos taken in
January 2014 (H. Ulloa).
27
575
576
577
578
579
580
581
582
583
584
585
586
5354
0
100
200
300
400
500
600
700
Mea
n 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Act
ive
chan
nel w
idth
(m)
Reach2005 2009 2013
Main reach
0
100
200
300
400
500
600
700
Mea
n 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Act
ive
chan
nel w
idth
(m)
Reach
1945 2004 2005
Chana reach
Figure 5. Pre-eruption active channel dynamics in the Chana reach and post-eruption active
channel dynamics in the Main reach obtained from 14 and 27 cross sections. Condition
2005 represents pre-eruption although the Chaitén volcanic eruption began 2 May 2008.
28
587
588
589
590
591
592
5556
0
5
10
15
20
0
10
20
30
40
1945 2004 2005
Are
a(%
)
N�i
slan
d/km
2
Year
Number of islands Island area (%)
0
5
10
15
20
0
10
20
30
40
2005 2009 2013
Are
a (%
)
Nº i
slan
d/km
2
Year
Number of islands Island area (%)
Year2005 2009 2013
0.50.0
-0.5
-1.0
-1.5
-2.0Log
(Isl
and
area
) (a.
u.) 1.0
(A)
Year1945 2004 2005
Log
(Isl
and
area
) (a.
u.)
0.5
0.0
-0.5
-1.0
-1.5
-2.0
1.0
(D)(C)
(B) n=44 n=48 n=48
n=193 n=175 n=102
Figure 6. Pre- and post-eruption dynamics of islands in the Rayas River. Total number and
island area to active channel area (%) in the Chana reach (A) and Main reach (C), and
boxplots of log-transformed island sizes (ha) expressed in arbitrary units (a.u.); the black
line within each box indicate the median values, box ends are 25th and 75th percentiles, and
whiskers are the minimum and maximum values without considering outliers in the Chana
reach (B) and Main reach (D).
29
593
594
595
596
597
598
599
600
601
602
603
604
605
606
5758
Figure 7. Detail of mapped gradual spread of channel and floodplain forest vegetation
before the eruption (1945, 2004 and 2005) and post-eruption disturbances (2009 and 2013)
in an ~2 km-long sub-reach within the Main reach (zone a demarcated in Fig. 1).
30
607
608
609
610
611
612
613
614
615
616
617
618
619
5960
05
10152025
2005 2009 2013
N/k
m2
Pioneer Building Established
0.0
2.0
4.0
6.0
2005 2009 2013
Are
a (%
)
Pioneer Building Established
(A) (B)
(C)
n=116
n=51 n=26
n=65
n=79
n=31
n=27n=37
n=38
Log(
Isla
nd a
rea)
[a.u
.]
Pre-eruptive Post-eruptive
2005 2009 2013
Figure 8. Changes in island features in the Rayas River Main reach through time. (A)
Number per unit active channel area; (B) total island area per category to total active
channel area (%); (C) boxplots of log-transformed island sizes (ha) expressed in arbitrary
units (a.u.); black line within each box is median, boxes encompass the 25th and 75th
percentiles; whiskers span at 1.5 times the interquartile range; open circles are outliers.
31
620
621
622
623
624
625
626
627
628
629
6162
0%
20%
40%
60%
80%
100%
Pioneer Building Established0%
20%
40%
60%
80%
100%
Pioneer Building Established
0%
20%
40%
60%
80%
100%
Pioneer Building Established0%
20%
40%
60%
80%
100%
Pioneer Building Established
Period 2005-2009 Period 2009-2013
Nº i
slan
dIs
land
are
a(A) (B)
(D)(C)
0%
20%
40%
60%
80%
100%
Unmodified Modified Disappeared
Pioneer Building Established
Unmodified Modified Disappeared
Type of island
Figure 9. Variation of island abundance per category for the periods of (A) 2005-2009 and
(B) 2009-2013; and variation in total island area for periods of (C) 2005-2009 and (D)
2009-2013.
32
630
631
632
633
634
635
636
637
638
639
640
641
642
643
6364
Legend
Disappeared islands
Unmodified islandsModified islands
Active channel 2005
Bank erosion
2005
2005 to 2009
2009 to 2013
Active channel 2009
Figure 10. Temporal sequence of changes to the island system and active channel extent in
the Main reach of Rayas River. ‘2005 to 2009’ shows changes during the first year after the
eruption (2008-2009) and ‘2009 to 2013’ shows additional changes in the subsequent
period. (See Fig. 1 for location).
33
644
645
646
647
648
649
650
651
652
653
654
655
656
657
6566
Table 1.
Number of islands that were unmodified or that disappeared in the periods 2005-2009 and
2009-2013
Period 2005-2009 2009-2013Type of
islandUnmodified
Modified Disappeared
New Unmodified
Modified Disappeared
NewTotal Divided Total Divided
Nº o
f isl
ands
Pioneer 7 40 18 69 - 17 8 2 40 -Building 9 36 33 6 1 22 13 2 44 -
Established 19 6 0 1 6 14 11 6 6 7Subtotal
island35 82 58 76 7 53 32 17 90 7
34
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
6768
Table 2.
Characteristics of the vegetation on surveyed islands, Rayas River
Type of island Pioneer Building EstablishedIsland ID 1 2 3 4 Mean 1 2 3 Mean 1 2 3 MeanDensity(stems ha-1) 700 400 1000 800 725 800 733 867 800 900 1567 733 1067Mean DBH (cm) 13 14 27 16 17 18 18 21 19 25 20 25 23
Range DBH (cm) 10-15
11-20
11- 38
10-30 - 10-
4810- 35
10- 60 - 10-
4310-57
10-46 -
Mean height (m) 7 6 10 9 8 10 10 15 12 16 13 15 15
Volume (m3 ha-1) 43 29 405 104 140 172 276 291 240 339 479 271 360
35
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
6970
Table 3.
Estimated volume of wood lost to the Main reach of the Rayas River from eroded islands
and floodplains in the periods 2005-2009 and 2009-2013
Morphological
unit
Surface
2005
Biomass
volume
2005
Eroded surface Wood loss
(ha) (m3) (ha) (m3) (m3/y)
2005-2009 2009-2013 2005-2009a 2009-2013 2009-2013
Pioneer island 21.7 3100 16.7 2.9 2400 400 100Building island 28.8 7100 15.5 7.4 3800 1800 450Established island 30.2 10900 4.5 4.7 1600 1700 425Total island 80.7 21100 36.8b 15.0b 7800 3900 975Floodplain - - 34.8c 33.6c 13800d 13300d 3325Total - - 71.6 48.6 21600 17200 4300a 2005-2009: Treated also as m3/y, given that the eruption was in 2008 and considering that no channel
changes were evident between 2005 and 2008.
b Island eroded surface is the difference of total island area between 2009 and 2005 and between 2013 and
2009. This surface comprises the area of disappeared islands and the reduction in area of modified islands.
c Floodplain eroded surface is the difference of active channel area between 2009 and 2005 and between 2013
and 2009.
d Floodplain biomass volume calculated using data of floodplain forests by Swanson et al. (2013).
36
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
7172