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: ulloacontreras@gmail.com

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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