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Nat. Hazards Earth Syst. Sci., 14, 3243–3259, 2014
www.nat-hazards-earth-syst-sci.net/14/3243/2014/
doi:10.5194/nhess-14-3243-2014
© Author(s) 2014. CC Attribution 3.0 License.
Moraine-dammed lake failures in Patagonia and assessment
of outburst susceptibility in the Baker Basin
P. Iribarren Anacona1, K.P. Norton1, and A. Mackintosh1,2
1School of Geography Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand
2Antarctic Research Centre, Victoria University of Wellington, Wellington, New Zealand
Correspondence to: P. Iribarren Anacona (pablo.iribarren@vuw.ac.nz)
Received: 11 July 2014 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 29 July 2014
Revised: 15 October 2014 – Accepted: 19 October 2014 – Published: 5 December 2014
Abstract. Glacier retreat since the Little Ice Age has re-
sulted in the development or expansion of hundreds of glacial
lakes in Patagonia. Some of these lakes have produced large
(≥106m3) Glacial Lake Outburst Floods (GLOFs) damag-
ing inhabited areas. GLOF hazard studies in Patagonia have
been mainly based on the analysis of short-term series (≤50
years) of flood data and until now no attempt has been made
to identify the relative susceptibility of lakes to failure. Power
schemes and associated infrastructure are planned for Patag-
onian basins that have historically been affected by GLOFs,
and we now require a thorough understanding of the char-
acteristics of dangerous lakes in order to assist with haz-
ard assessment and planning. In this paper, the condition-
ing factors of 16 outbursts from moraine-dammed lakes in
Patagonia were analysed. These data were used to develop
a classification scheme designed to assess outburst suscep-
tibility, based on image classification techniques, flow rou-
tine algorithms and the Analytical Hierarchy Process. This
scheme was applied to the Baker Basin, Chile, where at least
seven moraine-dammed lakes have failed in historic time.
We identified 386 moraine-dammed lakes in the Baker Basin
of which 28 were classified with high or very high outburst
susceptibility. Commonly, lakes with high outburst suscepti-
bility are in contact with glaciers and have moderate (>8◦)
to steep (>15◦) dam outlet slopes, akin to failed lakes in
Patagonia. The proposed classification scheme is suitable for
first-order GLOF hazard assessments in this region. How-
ever, rapidly changing glaciers in Patagonia make detailed
analysis and monitoring of hazardous lakes and glaciated ar-
eas upstream from inhabited areas or critical infrastructure
necessary, in order to better prepare for hazards emerging
from an evolving cryosphere.
1 Introduction
Amongst the most frequent and damaging processes related
to glaciers are Glacial Lake Outburst Floods (GLOFs). The
failure of glacial lakes can release millions of cubic me-
tres of water in a short time (minutes to days) and pro-
duce floods with high peak discharges (104m3s−1) and re-
markable erosive and transport capacity (Costa and Schuster,
1988; Breien et al., 2008). GLOFs can occur through differ-
ent mechanisms. Moraine-dammed lakes commonly fail due
to overtopping and the progressive enlargement of a breach
in the dam. Rainfall, meltwater and waves produced by mass
movements, ice avalanches or calving often trigger the over-
flow and subsequent moraine-dam failures (Costa and Schus-
ter, 1988; Emmer and Cochachin, 2013). Piping after earth-
quakes, the mechanical failure of ice-cored moraines and
flow waves from upstream lake failures have also been re-
lated to GLOFs (Lliboutry et al., 1977; Buchroithner et al.,
1982).
In the Himalayas, European Alps and the Andes GLOFs
have affected mountain communities for centuries, result-
ing in thousands of casualties (Hewitt, 1982; Grove, 1987;
Reynolds, 1998). However, the generation of new glacial
lakes as a consequence of glacier retreat, and the eco-
nomic exploitation of previously uninhabited valleys make
the emergence of new endangered areas likely. For example,
in Chilean Patagonia, hydroelectric generation plants are be-
ing planned in areas that have historically been influenced by
GLOFs (Dussaillant et al., 2009; Vince, 2010). Thus, there is
now an urgent need to better understand and assess the GLOF
hazard in these regions where detailed analyses are lacking.
Published by Copernicus Publications on behalf of the European Geosciences Union.
3244 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
Figure 1. (a) Geographical setting and name (unofficial) of failed moraine-dammed lakes in Patagonia used to develop a GLOF susceptibility
classification scheme. (b) Failed dams are located in zones with annual precipitation ranging from 500 to 2000 mm and where mean monthly
temperature in winter is generally above 0◦C. (c) Note the decrease in the number and magnitude of earthquakes south of the 46◦S and
the high frequency of shallow earthquakes (hypocentre <30km). Climate data extracted from Hijmans et al. (2005) and from the National
Climatic Data Center (http://www.ncdc.noaa.gov). Seismic data (period 1973–2012) retrieved from the Northern California Earthquake Data
Center (http://www.ncedc.org/anss/catalog-search.html).
A first step towards the analysis of GLOF hazards is the
identification of glacier lakes. Remote sensing methods (e.g.
image classification techniques) are especially suitable for
this task allowing rapid analysis of large areas (hundreds
of square kilometres) in an inexpensive way (Huggel et al.,
2002; Kääb et al., 2005). Using these methods, hazardous
lakes can be identified, and subsequently, if they are found
to pose a potential risk to lives or infrastructure, more de-
tailed local studies (e.g. GLOF modelling) might be devel-
oped (Mergili and Schneider, 2011) (e.g. Bajracharya et al.,
2007; Worni et al., 2012). Hazardous lakes are identified
by comparing lake characteristics (e.g. dam geometry and
potential for ice avalanche impacts entering the lake) with
those of failed lakes and their surroundings (McKillop and
Clague, 2007; Bolch et al., 2011; Wang et al., 2011; Emmer
and Vilímek, 2013). In Patagonia, data about failed moraine-
dammed lakes have not been systematically analysed and the
contributing factors of most of the failed moraine-dammed
lakes are unknown.
Since the beginning of the 20th century, at least 16
moraine-dammed lakes have failed in Patagonia (Iribarren
Anacona et al., 2014). Seven of these lakes are located in
the Baker Basin where major hydroelectric generation plants
are planned. One of these GLOFs (Laguna del Cerro Largo)
is probably the largest outburst of a moraine-dammed lake
(in terms of water volume released, 229×106m3)reported
worldwide (Hauser, 1993; Clague and Evans, 2000). In the
Baker Basin GLOFs have destroyed houses, forced the relo-
cation of a village and have damaged inhabitant’s livelihoods
(Iribarren Anacona et al., 2014). Furthermore, a flood wave
with GLOF characteristics killed three people boating in the
Baker River in January 1977 (El Diario de Aysén, 1977a, b).
This makes the Baker Basin an important location to study
the GLOF hazards.
In spite of the damage and the increasing frequency of
GLOFs in Patagonia, and the Baker Basin, GLOFs haz-
ards studies have been limited and based mainly on sta-
tistical analysis of short series (<50 years) of flood data
(HidroAysén, 2008; Vince, 2010). The relative susceptibil-
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3245
Figure 2. Location of moraine-dammed lakes and settlements in the
Baker Basin.
ity to failure of moraine-dammed lakes in the Baker Basin is
currently unknown as well as the extent to which these lakes
pose a threat to infrastructure or human life.
In summary, GLOFs might pose a significant hazard to
lives and newly developing infrastructure in Patagonia, but
several questions remain unanswered concerning their past
behaviour. Thecurrent status of moraine-dammed lakes, one
of the prime sources of GLOFs, remains uncertain. We aim to
analyse previously failed moraine-dammed lakes in Patago-
nia to identify the conditioning factors that led to failure, and
to use these data to identify the moraine-dammed lakes most
susceptible to future failure in the Baker Basin.
1.1 Setting
Patagonia is a region located in the southernmost part of
South America (≥40◦S) in the territories of Chile and Ar-
gentina (Fig. 1). This region hosts some of the largest temper-
ate ice masses on Earth (Harrison, 2011). However, glaciers
in Patagonia have suffered significantlosses in mass since the
maximum Little Ice Age (LIA) expansion (between the 16th
and 19th centuries) (Masiokas et al., 2009a), resulting in the
formation or growth of several ice-, bedrock- and moraine-
dammed lakes (Loriaux and Casassa, 2013). North-facing,
land-terminating glaciers with surfaces <5km2have shown
the fastest retreat in the region (Davies and Glasser, 2012).
The Baker Basin is located between 46 and 48◦S, in the
eastern side of North Patagonian Icefield (NPI) and has a sur-
face area of about 20 500 km2of which ca. 1940km2are cov-
ered by ice (Fig. 2). Climate varies from arid continental, in
the East (precipitation ca. 200mm year−1) to maritime hy-
perhumid on the west side of the Andean main divide (pre-
cipitation ca. 2000 mm year−1). Seismicity in the Baker basin
is low. Seismic activity has been concentrated in the north as-
sociated with Hudson Volcano eruptions; however seismic-
ity decreases south of 46◦S (Barrientos, 2007) and no recent
seismicity (from 1973 onwards) has been recorded in the rest
of the basin. The Baker Basin is sparsely populated, mainly
by low-density rural settlements. The number of tourists that
visit the region is low (Muñoz et al., 2006). The basin hosts
pristine rainforest, lakes and glaciers. Hundreds of glacier
lakes exist in the Baker Basin where major hydroelectric
schemes are planned. This makes the basin an ideal site to
study the hazard posed by glacier lakes in Patagonia.
2 Data and methods
Data from historical outburst floods in Patagonia were used
to develop an outburst susceptibility classification scheme
which was applied in the Baker Basin. This section details
the data used and procedures followed to (a) characterize the
failed moraine dams in Patagonia (b) to select, measure and
weight the outburst susceptibility factors and to (c) define the
outburst susceptibility classes (Fig. 3).
2.1 Data
Morphometric characteristics of dams, glaciers and lake
catchments were extracted from Landsat TM and ETM+im-
ages. Both Landsat TM and ETM+images have a spatial
resolution of 30m (15 m for the ETM+panchromatic band)
and were acquired from http://glovis.usgs.gov/. Topographic
data were derived from the Advanced Spaceborne Thermal
Emission and Reflection Radiometer Global Digital Eleva-
tion Model (ASTER GDEM V2). The spatial resolution of
the ASTER GDEM V2 is 1 arcsecond (approximately 30m)
and the DEM has a vertical accuracy of 17m (Tachikawa et
al., 2011).
2.2 Characterization of failed moraine-dammed lakes
in Patagonia
The 16 moraine-dammed lakes that failed in historic time in
Patagonia were mapped manually using Landsat images. In
the oldest events (before 1985) the lake area and glacier ex-
tent prior to the outbursts were reconstructed using historical
documents or geomorphic features (e.g. trimlines and lake
shorelines). Morphometric parameters in these cases are less
accurate than in GLOFs which occurred after 1985 but they
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3246 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
Table 1. Data of 16 GLOFs in Patagonia and characteristics of the moraine-dammed lakes and their surroundings prior to the failure.
Site Date Lake area (km2) Ov (m3×106) Pd (m3s−1) Pl (km) H/L (◦) Dh (m) Dos (◦) Glc Ias Mms Plg
Before After
1. Gl. Frías 1942–1953 0.01 0 0.06 81 – – ≤10? 35 Y(b) Y Y N
2. Gl. Ventisquero Negro 21 May 2009 0.55 0.32 4.36 1301 7.1 1.1 30 18 Y N Y Y
3. Río Lacaya 2000–2001 0.33 0.15 3.14 1048 21.5 3 ≤10? 9 Y Y Y N
4. Monte Erasmo 1985–2000 0.71 0.69 0.16 150 6.0 2.1 ≤10? 8 Y N Y Y
5. Estero El Blanco 2000–2003 0.12 0.04 1.05 511 5.4 5.1 ≤10? 11 N N Y N
6. Río Engaño 11 Mar 1977 1.15 0.81 7.36 1839 6.5 2.1 50 24 Y Y Y Y
7. Estero El Pedregoso 1985–1987 0.12 0.09 0.28 214 5.0 8.4 – 4 Y(c) N Y Y
8. Río Los Leones 2000 0.02 0 0.16 150 2.3 7.4 40 22 N N Y N
9. Río Viviano 1987–1998 0.02 0.01 0.06 81 2.3 7.3 15 26 N Y Y N
10. Cerro Largo 16 Mar 1989 1.82 0.98 24.73 4092 13.0 1.1 160 26 Y Y Y N
11. Estero Las Lenguas 1987–1998 0.67 0.44 4.36 1301 23.8 1.3 110 21 Y Y Y N
12. Gl. Piedras Blancas 16 Dec 1913 0.21 – – – 4.5 2.6 80 21 Y Y Y Y
13. Seno Mayo 2001–2003 0.07 0.03 0.41 277 2.3 19 ≤10? 8 Y N Y Y
14. Gl. Olvidado 2003 0.53 – – – 5.8 2.6 20 10 Y N Y Y
15. Última Esperanza 1999–2006 0.09 0.08 0.06 81 10.6 4.0 ≤10? 5 Y(c) Y Y Y
16. Peninsula de las Montañas 2005–2006 0.07 0.06 0.06 81 2.0 8.4 ≤10? 20 Y N Y Y
Abbreviations: Outburst volume (Ov), Peak discharge (Pd), Path length (Pl), Angle of reach (H/L), Dam height (Dh), Dam outlet slope (Dos), Glacier lake contact (Glc), Ice avalanche susceptibility (Ias), Mass movement
susceptibility (Mms) and Potential lake growth (Plg). Ov was calculated using the lake area lost after the GLOFs in the lake’svolume formula. (B) Data inferred from the position of Glacier Frías front in 1935-1938 (see Villalba
et al., 1990). (C) Lake in contact with a debris-covered glacier. GLOF data sourced from Iribarren Anacona et al. (2014) and references therein.
Figure 3. Flow chart of procedures followed to classify lake out-
burst susceptibility in the Baker Basin.
still provide an approximation of the lake and glacier condi-
tions prior to the dam failures (Table 1). The basin and glacier
topography were extracted automatically from the ASTER
GDEM using standard spatial analysis tools (see Reuter and
Nelson, 2008).
The GLOF paths were mapped on Landsat images and,
when available, high-resolution satellite images (≤5 m) from
Google Earth. GLOF angle of reach was measured along the
flow path, from the dam breach to the lowest area of stripped
vegetation or sediment deposition. Thus, path lengths may
be underestimated in the oldest events due to vegetation re-
Figure 4. Empirical curves showing the relationship between area
and volume of glacial lakes. The green curve was obtained from
38 data of lake area and volume of 25 moraine-dammed lakes
worldwide. Note that the lake volume does not increase propor-
tionally with an increase in the lake area and that the major diver-
gence between curves occurs on the largest lakes where fewer data
are available. Data extracted from O’Connor et al. (2001), Huggel
et al. (2002), Allen et al. (2009), Rivas (2012) and Loriaux and
Casassa (2013).
growth. The dam height and outlet slope were derived from
topographic profiles drawn over the ASTER GDEM (30m of
spatial resolution) on undisturbed sections of the dam near
the original lake outlet. Breaks in the topographic profile
were not straightforward to map in the case of small dams
(probably ≤10m in height), making it difficult to estimate
the dam geometry.
Lake volume and outburst peak discharges were calculated
using empirical formulae. Several formulae exist that relate
lake area and volume. Formulations by Huggel et al. (2002)
and Loriaux and Casassa (2013) are based on data of ice- and
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3247
moraine-dammed lakes, whereas the O’Connor et al. (2001)
formula is based on a small number of moraine-dammed
lakes. Ice- and moraine-dammed lakes often have different
geometries (McKillop and Clague, 2007) and therefore dif-
ferent volumes. Thus, we collected data from literature of a
large number of moraine dammed lakes worldwide (38 mea-
surements of lake area and volume from 25 lakes; Fig. 4)
and derived the following empirical formula to calculate lake
volume:
V=31.249A1.3399,
where Vis the lake volume in m3×106and Ais the lake
area in km2.
We compared the measured volume of 38 data of moraine
dammed lakes with the volume estimated with the derived
empirical formula. The mean error of the volume estimates
was ±71%.
The peak discharge was calculated using the following for-
mula proposed by Walder and O’Connor (1997):
Q=0.054V0.66,
where Vis the lake volume in m3.
Outburst parameters estimated by regression-based meth-
ods have large uncertainty. Dam breach peak discharge esti-
mates can have uncertainties of up to ±1 order of magnitude
(Wahl, 2004).
2.3 Selection of outburst susceptibility factors
Lakes dammed by temperate glaciers may be considered in-
herently unstable since ice-dam characteristics (e.g. glacier
thickness, crevassing and bed adhesion) are subject to fre-
quent changes, affecting the ice-conduit dynamics (Tweed
and Russell, 1999). Consequently, we considered all the ice-
dammed lakes as hazardous. Thus, we centred our analy-
sis on selecting variables to identify moraine-dammed lakes
susceptible to failure. Several variables have been used to
identify hazardous moraine-dammed lakes (see Emmer and
Vilímek’s, 2013, review paper). The dam geometry (e.g.
width-to-height ratio, flank steepness and dam freeboard)
and internal structure (e.g. presence of ice and particle size
distribution) are probably the most important conditioning
factors of outburst floods (Richardson and Reynolds, 2000a).
However, most dam characteristics can only be measured ac-
curately in the field or by using high-resolution satellite im-
ages or DEMs. We chose six characteristics of lakes, dams
and their surroundings that can be measured and modelled
using medium-resolution satellite images and DEMs. These
variables comprise outburst conditioning and triggering fac-
tors and also give an idea of the outburst damaging potential.
Due to the low spatial and temporal resolution of meteorolog-
ical data in Patagonia, extreme meteorological events were
not included in the analysis. The selected outburst factors are
described below.
Figure5. (a)Results of ice avalanche modelling and automatic clas-
sification of glaciers. (b) Examples of lake outlet slope measure-
ments.
2.3.1 Lake area
Lake dimensions have been directly related to outburst vol-
ume, peak discharge and the flood damage potential (Costa
and Schuster, 1988). Accordingly, larger lakes are consid-
ered to be more hazardous than small lakes. Furthermore,
lakes with larger areas are generally deeper (see e.g. Diaz
et al., 2007 database), and may exert higher hydrostatic pres-
sures over the dams making them more susceptible to failure
(Richardson and Reynolds, 2000b). Larger lakes also have a
greater surface area potentially exposed to mass movement
and ice avalanche impacts, increasing their outburst suscep-
tibility.
2.3.2 Glacier–lake contact
Lakes in contact with glaciers can be affected by calving and
the sudden floating of dead ice. Both mechanisms can pro-
duce waves capable of overtopping dams starting a breach-
ing process and subsequent dam failure (Richardson and
Reynolds, 2000a). Icebergs also can block the lake outlet,
raising the water level potentially overtopping and breaching
the dam. Thus, lakes in contact with glaciers are considered
more hazardous than lakes detached from the glacier snout.
2.3.3 Slope of glacier terminus
A glacier with a low-angle terminus can be an indicator of
a negative mass balance. Consequently, lakes in contact with
flat glacier fronts (slopes less than 5◦) are likely to grow as
a consequence of glacier retreat (Frey et al., 2010a). Lakes
that are expected to grow are more hazardous than lakes
which are expected to remain stable or shrink (examples of
minor moraine dammed lake area reduction, not related to
GLOFs, have been observed in Patagonia; see Fig. 5d in Lo-
riaux and Casassa, 2013), since the potential area exposed
to mass movements or ice avalanches may increase and the
dams may be subject to higher hydrostatic pressures.
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3248 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
2.3.4 Lake outlet slope
Steep outlets can be more easily enlarged than low-gradient
outlets if an increase in the lake discharge occurs. Progres-
sive erosion can widen and deepen the outlet leading to
lake drainage. Consequently, dams with steep outlets are
more susceptible to failure (O’Connor et al., 2001). Further-
more, high dams which produce outbursts with high peak
discharges (Walder and O’Connor, 1997) usually have steep
outlets (see Table 1).
2.3.5 Glacier steepness above lake
Steep (≥25◦) temperate glaciers are a common source of
ice avalanches (Alean, 1985). Ice avalanches impacting lakes
can generate impulse waves capable of overtopping dams
starting catastrophic lake drainage. The likelihood of an ice
avalanche impacting a lake depends on the distance, slope
and roughness of the terrain between the glacier and the wa-
ter body. Ice avalanches are the most common cause of out-
burst floods in the Himalayas (Wang et al., 2011) and have
also been reported in the Tropical Andes (Lliboutry et al.,
1977).
2.3.6 Steepness of slopes above lake
Steep unvegetated slopes are a common source of mass
movements (Peduzzi, 2010) and can be indicators of high
geomorphic activity. Large and high-velocity landslides can
generate impulse waves of hundreds of metres of run-up that
can easily overtop dams starting progressive erosion and lake
drainage (Walder et al., 2003). Lakes can also be suddenly
drained by large waves without a dam-breaching process
(Clague and Evans, 2000). Mass movement impacts have
been related to outburst floods in Patagonia and other An-
dean regions (Hubbard et al., 2005; Harrison et al., 2006).
2.4 Measuring and modelling of selected factors
2.4.1 Glacier and lakes delimitation
Glaciers and lakes were delimited using multispectral clas-
sification techniques that exploit the maximum reflectance
difference of a surface (i.e. glaciers and lakes) in different
spectral channels to identify the desired object (Huggel et al.,
2002; Paul et al., 2002). Thresholded band ratios have been
successfully used in glacier inventories (e.g. Andreassen et
al., 2008; Svoboda and Paul, 2009). We mapped glaciers via
band rationing of the near-infrared and mid-infrared bands
of Landsat images in reflectance values (i.e. pixel values not
converted to radiance; Paul et al., 2002). The thresholds val-
ues to identify glaciers (bare ice) were defined comparing
visually the band ratio image with false composite Landsat
images (Table 2). Debris-covered glaciers were drawn man-
ually.
Lakes in the Baker Basin were mapped using the Normal-
ized Difference Water Index (NDWI) of Huggel (2002) ob-
tained from the following equation:
NDWI =(Near-Infrared Band−Blue Band)
/(Near-Infrared Band+Blue Band).
The NDWI was applied on Landsat images in Digital Num-
bers. A cast-shadow mask was used to eliminate shadowy
areas mistakenly classified as lakes (see Huggel et al., 2002).
A median filter of 3×3 kernels was applied to smooth
the glacier and lake surfaces incorporating or eliminating iso-
lated pixels in the classified image (Paul et al., 2002). Mis-
classified lakes in shadowy areas and debris-covered glaciers
were corrected manually. The error in lake and glacier de-
limitation is estimated to be one pixel (i.e. ±30 m) although it
can be larger in shadowy areas. The lake inventory was devel-
oped using images from the years 2000 and 2002. However,
Landsat images of the years 2012 and 2013 also were anal-
ysed to manually incorporate recently formed lakes in the
inventory and to assess the glacier–lake contact status. Only
the area of lakes in contact with glaciers was determined us-
ing 2012–2013 images since the area of other lakes probably
remained stable (see Loriaux and Casassa, 2013). The 2012–
2013 images were not used as base for the entire inventory
since about 20% of the data in each Landsat 7 image were
lost after the failure of the scan-line corrector in May 2003
(USGS et al., 2003). Google Earth images were used to clas-
sify the dams (i.e. moraine, bedrock or ice dams). Only lakes
located in valleys glaciated during the LIA were included in
the analysis since lakes situated far from the LIA expansion
(including very large moraine-dammed lakes such as Gen-
eral Carrera and small lakes dammed by bedrock) were con-
sidered to be stable. Published geomorphological maps were
used to identify the glacier extent during LIA (Glasser et al.,
2011; Glasser and Jansson, 2008), which was also inferred
from trimlines and terminal moraines.
2.4.2 Slope steepness above lake and mass movement
modelling
Mass movement paths were mapped using the Modified Sin-
gle Flow Direction (MSF) model of Huggel et al. (2003). The
MSF simulates the trajectory of mass movements from the
source area following the steepest descent with a maximum
deviation of 45◦. The mass movement stops (i.e. the end of
the path) when it reaches a predetermined ending condition
generally set as the angle of reach (i.e. the angle of the line
connecting the starting and the ending zone of a mass move-
ment; Hsu, 1975) (see Huggel et al., 2003 and Gruber et al.,
2008 for model details).
Published data of the angle of reach of mass movements
and typical angles of detachment zones were used as input
to model the flow paths. The angle of reach and the slope of
the starting zone of rock falls, debris flows and other com-
plex mass movements vary locally according to the geol-
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3249
Table 2. Satellite images and threshold used to identify glaciers, lakes and vegetation.
Glacier band Lakes NDWI Vegetation NDVI
Sensor Image date Path/row ratio threshold ≥threshold ≥threshold ≥
Landsat ETM+
8 Mar 2000 232/092 3 −0.5 0.1
8 Feb 2013 232/092 – – –
18 Feb 2002 231/093 2.5 −0.45 0.1
18 Feb 2002 231/092 1.5 −0.45 –
8 Mar 2000 232/093 3 −0.5 –
22 Feb 2012 232/093 – – –
Table 3. Pairwise comparison of outburst predictor variables and consistency ratio.
Glacier–lake Lake Glacier steepness Slope steepness Slope of Lake outlet
Matrix contact area above lake above lake glacier terminus slope
Glacier–lake contact 1 2 2 2 3 1
Lake area 1/2 1 1/2 2 2 1/3
Glacier steepness above lake 1/2 2 1 3 3 1/2
Slope steepness above lake 1/2 1/2 1/3 1 2 1/3
Slope of glacier terminus 1/3 1/2 1/3 1/2 1 1/5
Lake outlet slope 1 3 2 3 5 1
Consistency ratio: 0.026
Factor intensity: 1: Equal importance; 3: Moderate prevalence of one over another; 5: Strong or essential prevalence; 7: Very strong or demonstrated prevalence; 9:
Extremely high prevalence; 2, 4, 6 and 8: express intermediate values (after Saaty and Vargas 2001).
ogy, terrain roughness and vegetation coverage. Steep un-
vegetated slopes may indicate high geomorphic activity and
can be associated with loose, readily erodible material. Thus,
we assume potential starting zones for all mass movements
are unvegetated or sparsely vegetated slopes ≥30◦. We have
not distinguished between solid bedrock slopes and non-
cohesive slopes since this task can only be accurately ac-
complished by photo interpretation or fieldwork, which are
costly or time consuming, and consequently not suited to our
preliminary regional analysis.
Vegetation was mapped using the Normalized Vegetation
Index (NDVI) calculated using the following equation:
NDVI =(Near-Infrared Band−Red Band)
/(Near-Infrared Band+Red Band).
The angle of reach of mass movements was considered sim-
ilar to that of rock avalanches (about 15◦; see Nicoletti and
Sorriso-Valvo, 1991) which are the events most likely to start
catastrophic lake drainages.
2.4.3 Glacier steepness above lake and ice avalanche
modelling
The ice avalanche paths were also delineated using the MSF
model based on empirical data of ice avalanches source
and angle of reach (Fig. 5). According to Alean (1985)
ice avalanches commonly start at slopes ≥25◦in temperate
glaciers (such as Patagonian glaciers) and have an angle of
reach of 17◦(data derived from about 100 ice avalanches
mostly from the European Alps). Thus, we mapped glacier
surfaces with slopes ≥25◦and used these areas as ice
avalanche detachment zones. The stopping condition in the
MSF model was set at an angle of reach of 17◦.
2.4.4 Lake outlet slope measurement
The lake outlet and the outlet slope were identified and mea-
sured automatically following a series of GIS procedures.
First, we identified the lake outlet as the point with maxi-
mum flow accumulation in the lake (see Gruber and Peck-
ham, 2008). From this point the steepest descent path 200
metres downstream (with a maximum deviation of 45◦) was
calculated using the Path Distance tool of ArcGIS. We as-
sumed that moraine-dam widths are less than 200m. Dam
widths of the largest failed lakes in Patagonia are ≥300m.
However, we used a smaller value since most of the moraine-
dammed lakes in the Baker Basin are small and this value
(200m) does not significantly affect the average slope of
larger dams. Finally, the mean slope of the steepest descent
path was calculated.
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3250 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
Table 4. Weight of variables associated with moraine-dammed lake failures.
Variable weight
Lake outlet Glacier–lake Glacier steepness Lake area Stability of slopes Slope of
slope contact above lake above lake∗glacier terminus
30 25 18 12 9 6
Classes weight
≥15◦1 Yes 1 ≥25◦1>0.5km21 Yes 1 Yes 1
≥8◦and <15◦0.75 No 0 <25◦0>0.1 to 0.5km20.75 No 0 No 0
<8◦0 0.01 to 0.1 km20.5
∗Mass movements are a common cause of outburst floods. However, we assigned to this variable a low weight because the slope steepness and vegetation cover only provide a rough idea of
potential detachment zones.
2.5 Weighting process
After analysing 16 GLOFs in Patagonia, the six conditioning
factors were weighted using the Analytical Hierarchy Pro-
cess (AHP) (Saaty, 1980). We chose this method since it al-
lows evaluation of the consistency of the subjective judge-
ments which is not accomplished by other qualitative or
semi-quantitative GLOF hazard approaches (see Emmer and
Vilímek, 2013, for a review). The AHP is a multicriteria
decision-making technique which allows estimation of the
relative significance of factors contributing to an event based
on pairwise comparison, expert judgment and the linear al-
gebra transformation of the comparison factors matrix (Ta-
ble 3). The aim of the pairwise comparison is to assess the
significance of one factor compared to another. Values from
1 to 9 can be assigned to each factor, where a value of 1
means that both factors have equal importance and a value of
9 means that a factor has an extreme prevalence over another.
The AHP method has been used in natural hazard assess-
ments by several authors (e.g. Ayalew et al., 2005; Lari et
al., 2009). The AHP also allows the evaluation of the consis-
tency of the judgements based on the estimation of the eigen-
value of the factors matrix. Generally only consistency ratios
<0.1 are considered acceptable (Satty, 1980). We assigned
the higher weights to the GLOF factors more frequently as-
sociated with GLOFs in Patagonia. After weighting the six
GLOF factors, each factor was subdivided into classes for
which we assigned weights (Table 4). The total GLOF sus-
ceptibility score for each lake was obtained by multiplying
the weight of the factor by the weight of the classes and then
adding up the six factor scores. The highest possible score is
100.
To define the outbursts susceptibility classes we applied
the outburst classification scheme to the failed lakes in Patag-
onia. Five categories that represent five relative scales of out-
burst susceptibility were defined (Fig. 6). Twelve (75%) of
the 16 recorded failed lakes in Patagonia had scores ≥65.
Thus, a score of 65 was used as threshold to classify lakes
with high outburst susceptibility.
Figure 6. Outburst susceptibility score of the 16 moraine-dammed
lakes failed in Patagonia. These data were used to define five out-
burst susceptibility classes.
3 Results
This section summarizes characteristics of moraine-dammed
lakes failed in Patagonia. The data provide insights to iden-
tify moraine-dammed lakes with high outburst susceptibility
in the region.
3.1 Characteristics of GLOFs, failed lakes and their
surroundings
3.1.1 GLOFs characteristics
Moraine-dammed lakes that failed in historic time in Patago-
nia had areas ranging from 0.01 to 1.82km2(Table 1). Four-
teen lakes experienced just a partial emptying; including the
largest lake (Laguna del Cerro Largo) which released an esti-
mated water volume of ∼229×106m3(Hauser, 2000). The
outburst paths (patches of stripped vegetation and/or debris
deposition visible in Landsat images) range from few kilo-
metres up to ∼27km in length and commonly are less than
100m wide. However, GLOFs probably reached farther ar-
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3251
Figure 7. Accumulated percentage of GLOF paths within different
angles of reach (H/L). Most path sections with H/L ≥10◦(where de-
bris flows often occur) are located at distances ≤3000 m from failed
lakes. Sections with H/L≤10◦(89 % of the total path lengths) of-
ten have diffusive and rapidly attenuating flows. H/L was measured
every 10m from 15 failed moraine-dammed lakes in Patagonia.
eas since vegetation recovery might conceal evidence of flow
after just a few years. This has been corroborated by eye-
witness’s accounts (interviews held by the authors with in-
habitants of Los Leones valley and Bahía Murta Village)
which indicate that floating debris (large trees) were trans-
ported during GLOFs to the river outlets, kilometres from
the preserved geomorphic evidence, clearly posing a risk for
inhabited areas. GLOF paths are dominated by steep slopes
up to five kilometres from the lakes (Fig. 7). These sections
of the GLOF paths favour the development of debris flows
and the transport of coarse material from failed dams. In the
lowest areas, patches of stripped vegetation and bank erosion
are common, although the flows attenuate due to wider val-
leys and lower slopes.
3.1.2 Dam characteristics
Outbursts have affected moraine dams with different geome-
tries. Drained lakes were dammed by both, steep moraine
arcs and relatively flat ground moraines. No relationship be-
tween moraine heights and failure was evident, although
higher dams (associated with larger lakes) resulted in GLOFs
with higher peaks discharges. The heights of the failed dams
vary from a few metres to up to 160m. In eight cases the
dams were small making it difficult to accurately estimate
their dimensions. However, they were probably ≤10m high.
Nine dams were vegetated at the time of failure and one of
them (Piedras Blancas) was covered by mature forest dat-
ing from the early 1600s (Masiokas et al., 2009b). Most of
the failed lakes had moderate to steep outlet slopes. The out-
let slope of 14 lakes was ≥8◦and 8 lakes had outlet slopes
≥5◦. The dam’s internal composition is known in just one
case. The Ventisquero Negro dam was composed of non-
cohesive coarse material (boulders and blocks) in a matrix of
Figure 8. Types of moraine dams failed in Patagonia; (a) lake
perched over moraine deposits (b) lake dammed by ground
moraines and partially by bedrock (c) lake dammed by a small crest-
shaped dam and (d) lake behind large and steep vegetated dam. Im-
ages sourced from Google Earth.
sand and gravel. This dam was vegetated and also presented
an ice-core at the time of failure (Worni et al., 2012). Estero
el Pedregoso Lake was dammed by (or was embedded in)
glaciofluvial deposits and was partially dammed by bedrock
(Fig. 8).
3.1.3 Characteristics of upstream catchments
Moraine-dammed lakes that produced outburst floods in
Patagonia were located in different settings. Thirteen lakes
were in contact with a glacier at the time of failure and in
at least three cases (Ventisquero Negro, Olvidado and Penín-
sula de las Montañas) glaciers exhibited rapid retreat before
the outburst floods (Fig. 9). For example, between 2000 and
2003 the Olvidado Glacier retreated 271m per year (Rivera
and Casassa, 2004) significantly increasing the lake surface
before the failure. Half of the lakes were located in areas
prone to ice avalanches. In fact, the upper-edge of Lacaya and
Las Lenguas lakes were at the toe of reconstituted glaciers,
clear indicators of high snow and ice avalanche activity
(Fig. 9). All of the lakes were surrounded by steep (≥25◦)
valley walls or moraines. Rock-falls, snow avalanches and
debris flows are common in this setting. However, a mass
movement was definitely identified as the cause of the dam
failure in just one case (see Harrison et al., 2006). In the other
cases (excluding Piedras Blancas and Frías outbursts) no ev-
idence of large mass movements (i.e. fresh landslide scars
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3252 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
Figure 9. Settings of moraine-dams failed in Patagonia; (a) lake
in the head of a catchment at the toe of a reconstituted glacier,
(b) growing lake in contact with a retreating glacier and (c) lake
distant from the glacier tongue. Note the steep slopes and glaciers
surrounding the lakes.
or deposits) in the lake’s surroundings were identified when
comparing images before and after the outburst floods.
3.1.4 Triggering factors
The triggering of only 3 of the 16 GLOFs is known. However,
these events exemplify the variety of factors that can cause
outburst floods. These factors include gravitational processes
and meteorological events. The outburst of the Calafate Lake
in the Río Los Leones Valley was caused by the impact of
a rock-fall into the lake. The rock-fall detached from a re-
cently deglaciated slope and completely covered the lake’s
area (Harrison et al., 2006) generating impact waves that
probably caused an almost instantaneous lake emptying. The
Río Engaño outburst was caused by a different gravitational
process. Reconnaissance flights carried out few days after the
Río Engaño outburst indicate that the lake was impacted by
glacier ice (this might correspond to an ice avalanche or calv-
ing) that probably caused waves which overtopped the dam
and started the lake drainage (El Diario de Aysén, 1977c).
The Ventisquero Negro outburst occurred after prolonged
(180mm of rain in 6 days) and intense (50 mm of rain in
the 48h prior the outburst) rainfall that possibly caused an
overflow and subsequent dam breach and failure (Worni et
al., 2012).
3.2 Glacial lakes in the Baker Basin
Overall, 480 glacial lakes with surfaces ≥0.01km2were
identified in the Baker Basin. Distinguishing between
bedrock and moraine-dammed lakes proved to be difficult.
All uncertain cases (<10%) were classified as moraine-
dammed lakes in order to evaluate their outburst suscepti-
bility. A preliminary classification indicated that 85 lakes are
dammed by bedrock and 386 (80%) lakes are dammed by
moraines. Only three lakes are dammed by glaciers. Two of
them are dammed by the Colonia Glacier, the Lake Cachet
2 (drained 10 times between 2008 and 2012) and a smaller
(0.35km2) unnamed lake located 10 km to the north. A fourth
ice-dammed lake (Laguna Bonita) emptied at least two times
between 2002 and 2008. However, glacier retreat since the
last outburst now impedes the lake refilling (Iribarren Ana-
cona et al., 2014). Of the 386 moraine-dammed lakes at least
seven have produced outburst floods.
3.2.1 Lake outburst susceptibility in the Baker Basin
According to our classification scheme, the majority of the
moraine-dammed lakes in the Baker Basin have low outburst
susceptibility. The lakes have low-gradient outlets, are dis-
connected from glaciers or are small (<0.1km2)(Fig. 10).
However, such lakes may still produce outburst floods as
they are subject to ice avalanches or mass movement im-
pacts. Seven moraine-dammed lakes are in the range of very
high outburst susceptibility and 21 lakes are in the range of
high outburst susceptibility (Fig. 6). A closer look of these
lakes, however, shows that the largest lakes are located in
flat valleys and have superficial drainage through large (sev-
eral metres) low-gradient outlets making a catastrophic lake
drainage unlikely. This is the case for example of the Fiero,
Laguna Soler and Cachet 1 lakes. While these lakes are ex-
posed to ice avalanches or mass movements, impact waves
may be attenuated after travelling long distances (Slinger-
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3253
Figure 10. (a) Characteristics of 16 failed moraine-dammed lakes
in Patagonia and (b) characteristics of 386 moraine-dammed lakes
in the Baker Basin.
land and Voight, 1979), reducing the outburst susceptibility.
Low-gradient outlets also limit the transformation of even-
tual outburst floods into debris flows since this phenomenon
generally starts in slopes ≥10◦(Hungr et al., 1984). Smaller
lakes with high or very high outburst susceptibility which are
in the surface range (≤1.82km2)of failed lakes in Patagonia
more closely resemble their characteristics (i.e. lakes with
steep outlet slopes in contact with glaciers and exposed to
ice avalanches and mass movements) (Fig. 11). The com-
puted (hypothetical) peak discharge of GLOFs from the 28
lakes most susceptible to failure range from 70 to more than
10000 m3s−1in the worst scenario (100% of the lake vol-
ume drained) (Fig. 12). However, the complete drainage of
moraine-dammed lakes is uncommon.
The risk from GLOFs remains low in spite of the large
number of glacial lakes existing in the Baker Basin, with 28
lakes having high or very high outburst susceptibility. This is
because the population and infrastructure threatened by out-
burst floods is scarce, since the region is mostly uninhabited
(Fig. 13). Debris flows are the most damaging process trig-
gered by the sudden drainage of glacial lakes since they can
Figure 11. Examples of lakes classified with high or very high out-
burst susceptibility. Steep glaciers, moraines and rock-slopes sur-
round small and medium-sized lakes. Large growing lakes are in
contact with retreating glaciers and have vegetated dams (panel E).
Icebergs are common in proglacial lakes in contact with grounded
glaciers.
Figure 12. Potential peak discharge of GLOFs from lakes with high
or very high outburst susceptibility in the Baker Basin.
develop high-impact pressures, can obstruct rivers causing
back water flooding or floods from the sudden drainage of
these ephemeral lakes. However, not all outburst floods can
develop into debris flows, as they depend on sediment avail-
ability, channel morphology and slope gradient.
We modelled debris flow paths from the 28 moraine-
dammed lakes with higher outburst susceptibility in the
Baker Basin (using the MSF model described in Sect. 2.4.2,
and setting as a source zone the lake area and as stop-
ping condition an angle of reach of 10◦) and none of them
reached currently inhabited zones (Fig. 14). However, flood
waves travel larger distances and could potentially flood for-
est and agricultural lands, damaging local inhabitants’ liveli-
hoods (Table 5). Floods can also affect transport routes isolat-
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3254 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
Figure 13. Classification of lake outburst susceptibility in the Baker
Basin. Note that most of the lakes with high or very high outburst
susceptibility are located on the west side of the basin.
ing populated areas, as has been demonstrated by historical
events (Hauser, 2000; Worni et al., 2012).
4 Discussion
4.1 Documented outburst floods from
moraine-dammed lakes in Patagonia
The 16 documented lakes that produced outburst floods in
Patagonia are located in areas which became ice free as a
consequence of 20th and early 21th century ice retreat, and
most of the lakes (13, namely 81%) were in contact with
glaciers at the time of failure. Calving-induced waves, the
obstruction of the lakes outlets by icebergs, and the increase
in the hydrostatic pressure over the dams as a result of lake
growth/deepening may explain some of these outburst floods.
The melting of ice-cored moraines also may be related to
dam failures (through dam subsidence or the erosion of oth-
erwise ice-cemented debris (Richardson and Reynolds, 2000;
McKillop and Clague, 2007) since at least one of the failed
moraine-dammed lakes in Patagonia had an ice-core (Worni
et al., 2012). Other recently formed dams, close to glacier
fronts, may also contain buried ice. Thus, most of the out-
burst floods may be an expression of the adjustment of the
landscape to new and evolving glacial conditions after LIA
(Clague and Evans, 2000).
Most of the failed lakes had steep (≥15◦) dam outlet
slopes. The higher shear stress in these steep slopes prob-
ably favoured the dam’s erosion when overflows or an in-
crease in the lake discharge occurred. The four largest dams
(≥50m in height) were covered by mature forest at the time
of failure. However, the vegetation could not stop the pro-
gressive erosion of these steep dams and subsequent catas-
trophic lake drainages. In fact, trees were incorporated in
the flow increasing its damaging capacity. The largest dams
had narrow fronts, closely resembling classic examples of
failed moraine-dammed lakes worldwide (e.g. Lliboutry et
al., 1977). These lakes could be identified as potentially haz-
ardous through a quick examination of aerial photographs
or satellite images. However, two small failed lakes had low
dams with flat and broad surfaces and superficially appeared
stable. A possible factor contributing to their failure is that
lower dams can be easily overtopped by waves or a rise in
lake level since they have less potential freeboard (i.e. there
is less height difference between the lake surface and the low-
est point of the dam).
All the failed lakes were located in areas prone to mass
movements but only one outburst flood was certainly caused
by this phenomenon (Harrison et al., 2006). The dimen-
sions of impact waves, and hence the likelihood of a dam
overtopping, are directly related to the volume and veloc-
ity of the mass movements and the lake bathymetry (Walder
et al., 2003). Large and high-velocity mass movements are
more likely to trigger outburst floods (Walder et al., 2003).
Mass movement modelling shows that lakes in Patagonia
are exposed to this phenomenon. However, frequent low-
magnitude rock-falls, debris flows or snow avalanches are
probably not capable of generating large-impact waves, dam
overtopping, and catastrophic lake drainage.
There is evidence of just one outburst flood that might have
been triggered by an ice avalanche. However, ice avalanche
modelling shows that several failed lakes were located in ar-
eas prone to ice avalanching. The deposits of ice avalanches
can be rapidly obliterated hampering their identification af-
ter few months or years (Kellerer-Pirklbauer et al., 2012).
Thus, this process cannot be discarded as one of the triggers
of other outburst floods. The failure of lakes as a consequence
of an upstream outburst is another potential cause of large
floods (Xin et al., 2008). However, none of the failed lakes
in Patagonia is known to have occurred by this mechanism.
Large lakes (>0.5km2)in areas of low relief are common
in Patagonia and may delay or attenuate outburst floods as
has been demonstrated in the Cachet 2 events (Dussaillant et
al., 2009). Therefore, chain lake ruptures may be restricted
to smaller lakes in high-relief catchment heads which show
quick responses to large and rapid water influxes.
Only two lakes were completely emptied by outburst
floods. This is because moraine dams generally impound
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3255
Table 5. Potential damages caused by debris flows and floods originated from moraine-dammed lakes with high or very high outburst
susceptibility in the Baker Basin.
Forest and
Routes (m) Others bush (km2)
Debris flow – Vehicle track – Mining camp (disused) 16
(angle of reach 10◦)=300 Foot – 1 Bridge – 1 planned dam
paths =1600
Flood (angle – Route 7 =– 1 Bridge in
of reach 5◦) 1000 – Vehicle secondary route - 1.3
track =300 -1 Bridge in Route 7
Figure 14. GLOF modelling from lakes with high or very high out-
burst susceptibility closest to inhabited zones. Forestry land, routes
and a planned dam are in the path of potential debris flows (angle
of reach ≥10◦) and floods. The flow width in D is probably exag-
gerated in its unchannelized path.
only part of the lake’s water volume (the rest of the wa-
ter occurs below the moraine base in overdeepened valleys).
Hence, in spite of the existence of lakes of hundreds of me-
tres in depth in Patagonia (see e.g. Warren et al., 2001), com-
plete lake drainage is unlikely.
Failed moraine-dammed lakes in Patagonia ranged in area
from 0.01 to 1.82 km2. Although larger lakes exist, they have
not failed in historic time. A probable explanation for the fail-
ure of these, comparatively, smaller lakes is that the area and
volume of small lakes can grow quickly after small glacier
changes, dramatically altering the catchment hydrology. Fur-
thermore, large lake systems have had longer periods of ad-
justment (e.g. development of large low-gradient outlets) to
new climatic, glacial and hydrologic conditions since most of
the large lakes were formed during or before the LIA. This
adjustment may have included prehistoric outburst floods
that helped to shape lower and wider outlets.
4.2 Outburst susceptibility classification
Here we have carried out the first systematic analysis of the
conditioning and triggering factors of outburst floods from
moraine-dammed lakes in Patagonia. We weighted these fac-
tors (using the AHP method) to define outburst susceptibility
classes. In conjunction, these data were used to develop a
methodological scheme to assess the outburst susceptibility
of glacier lakes in Patagonia. The approach builds on simi-
lar analyses (e.g. Bolch et al., 2011), however, the weight-
ing of the outburst factors was based on empirical data from
past outburst floods in Patagonia and thus is representative
of the Patagonian geographical context. Thus, it can be used
as a first-order approach to identify hazardous lakes in this
region.
Twelve (75%) of the 16 failed lakes in Patagonia had
scores ≥65 (other failed lakes had scores ranging from 30
to 49) and thus we selected this score to identify lakes with
high outburst susceptibility. This score does not comprise all
the failed lakes in Patagonia but includes lakes with at least
three characteristics that make them susceptible to failure.
The suggested approach, however, has drawbacks – for ex-
ample, the omission of dam characteristics in the analysis
and the subjectivity of the weighting scheme. Furthermore,
the rapid nature of glacier changes in Patagonia (see Davies
and Glasser, 2012) means that this analysis needs to be up-
dated regularly.
The use of medium-resolution (15–30m) satellite images
and DEMs limit the inclusion of dam characteristics that
can be critical to explain outburst floods, such as dam free-
board and resistance to erosion. However, these resources al-
low a rapid extraction of data from hundreds of lakes in a
short time. The relatively coarse spatial resolution of the im-
agery means that distinguishing between lakes dammed by
moraines and bedrock was not straightforward in all cases.
In some examples, categorical identification of features is
not even possible using finer-resolution satellite images and
aerial photographs. Thus, detailed local-scale analyses of the
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3256 P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia
Figure 15. Geomorphic effects of an outburst flood (Los Leones
Valley) produced by the impact of a rock avalanche. The small lake,
detached from the glacier tongue at the time of failure, was clas-
sified with low outburst susceptibility in spite of the steep outlet
slope. Note the elevated traces (∼40m) of the impact wave and the
large boulders (>6m in diameter) transported by the flow.
lakes classified with high or very high outburst susceptibility
needs to be carried out to judge whether outburst preventive
or mitigation measures are required. The identification of po-
tential source of mass movements (slope steepness and veg-
etation coverage) can be refined using empirical data from
landslide inventories in glacial and periglacial belts in Patag-
onia, or geomorphic features such as fresh scars and landslide
deposits.
Although the weighting scheme used in the Baker Basin
is subjective, it has the advantage of being based on GLOFs
conditioning and triggering factors in Patagonia. It is thus
better suited to the identification of potentially hazardous
lakes in this region than approaches developed for other ge-
ographical contexts. Furthermore, the evaluation of the con-
sistency of the judgments in the weighting scheme (Table 3)
is an advantage of the AHP method in relation to other qual-
itative or semi-quantitative approaches used in GLOF hazard
assessments (see Emmer and Vilímek, 2013, for a review).
Glacier fluctuations can shift the source area of ice
avalanches and expand or generate new glacial lakes, result-
ing in a change in outburst susceptibility and hazard over
time (Huggel et al., 2001, 2004). This makes periodic mon-
itoring of glaciers, lakes and their surroundings necessary
in Patagonia, particularly near inhabited areas or critical in-
frastructure. The rapid growth of the Olvidado Lake three
years before the outburst in 2003 is an example of the speed
at which glacier and lake changes can occur in this region
(Rivera and Casassa, 2004).
In the Baker Basin 28 lakes were classified with high or
very high outburst susceptibility. Most of the lakes are lo-
cated in uninhabited valleys or dozens of kilometres from set-
tlements or infrastructure. However, modelled debris flows
and floods from hazardous lakes reached forestry land, a
planned dam, and transportation routes. Damage to access
routes by GLOFs can increase accessibility problems faced
by Patagonian settlements (Muñoz et al., 2006). These 28
lakes, based on the results of this study, are more suscepti-
ble to failure than other lakes. However, this does not imply
that other lakes cannot also fail. For example, large, albeit
infrequent, landslides or ice avalanches can cause the sudden
drainage of otherwise stable lakes (Fig. 15).
The approach used in this study has the advantage that can
be applied at regional-scale using publicly available satellite
images and DEMs allowing the analysis of hundreds of lakes
in an inexpensive way. Also, it is based on simple and robust
image classification and flow modelling techniques proven in
different geographical settings (Paul et al., 2002; Huggel et
al., 2003; Frey et al., 2010b; Bolch et al., 2011). Thus, it is
suitable for identifying the lakes most susceptible to fail in
Patagonia as a first approach to GLOF hazard assessments.
5 Conclusions
We analysed 16 historic outburst floods from moraine-
dammed lakes in Patagonia and our analysis shows that lakes
in contact with glaciers and having moderate (≥8◦) to steep
(≥15◦) outlet slopes are more likely to fail. The influence of
other factors, such as dam height and vegetation coverage,
on the lake outburst susceptibility is less clear. The dam ge-
ometry and vegetation coverage, however, had a direct influ-
ence on the flow hydrology (e.g. peak discharge and debris
transport) and hence the damage potential of flows. GLOF
paths in Patagonia display a rapid decrease in damage poten-
tial downstream of the lakes. Most of the steep path slopes
favouring debris flow occurrence and fast flows (the most
damaging processes linked with GLOFs) were at distances
≤3000m from failed dams. However, as has been demon-
strated by historical events, attenuated flows might still en-
danger widespread areas in unconfined valleys. Furthermore,
wood transport has been common during GLOFs and can af-
fect distant zones.
The characteristics of failed lakes in Patagonia were used
to develop an outburst susceptibility scheme (based on the
AHP method and remote sensing and GIS techniques) which
was applied in the Baker Basin, Chilean Patagonia. The
scheme allowed categorizing the outburst susceptibility of
hundreds of lakes in a short time in a qualitative, yet repro-
ducible way. The scheme integrated data from past GLOFs
in Patagonia making it suitable for wider application in the
region. The scheme might be used to complement GLOF
hazard assessments in Patagonia which until now have re-
lied mostly on statistical analysis of short-term series of flood
data. The identification of the lakes more susceptible to fail-
ure, and the empirical modelling of the floods, are first steps
toward a full GLOF hazard assessment which should ulti-
mately include data on potential flood intensity (e.g. flood
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P. Iribarren Anacona et al.: Moraine-dammed lake failures in Patagonia 3257
volume, velocity and sediment entrainment/deposition) and
GLOF probability in a determined time span.
Acknowledgements. We thank Rodrigo Iribarren for assistance in
the field. This work was improved by the constructive reviews of
Martin Mergili, Adam Emmer and an anonymous reviewer, and
by the comments of the Editor Paolo Tarolli. Michael Crozier is
also thanked for his comments on an earlier draft of this paper. P.
Iribarren Anacona is a Becas Chile fellow.
Edited by: P. Tarolli
Reviewed by: M. Mergili, A. Emmer, and one anonymous referee
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