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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, F04019, doi:10.1029/2007JF000920, 2008

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ArticleTriggering and drainage mechanisms of the 2004
glacier-dammed lake outburst in Gornergletscher,
Switzerland
S. Sugiyama,1,2 A. Bauder,2 M. Huss,2 P. Riesen,2 and M. Funk2*
Received 17 October 2007; revised 7 October 2008; accepted 27 October 2008; published 31 December 2008.

[1] To investigate the triggering and the drainage mechanisms of a glacier-dammed lake
outburst, we conducted high-frequency measurements of the ice surface motion in thevicinity of Gornersee, an ice marginal lake on Gornergletscher, Switzerland. During theoutburst event in July 2004, the ice surface within a distance of 400 m from thelakeshore moved vertically upward by up to 0.1 m. This vertical surface motion cannot be
explained by vertical straining of ice which was measured in one of the boreholes;
therefore, we suggest the separation of the glacier sole from the bed was caused bysubglacially drained lake water. Our observation indicates that the lake water drained as asheet-like flow through the space created by the basal separation. The upward surface
motion was greater in the region where the ice flotation level was exceeded by the lakelevel, implying that the ice barrier was breached when the lake water hydraulicallyconnected to the bed and lifted up the glacier. In addition to the centimeter-scale vertical
ice motion, three survey stakes located within 100 m from the lake showed extraordinarily
large vertical displacement of 0.5–3.0 m associated with abrupt changes in horizontal
flow direction. A plausible interpretation is that the marginal ice wedge bent upward
because of the buoyancy force generated by the drained water. Such bending is possible if
subglacial and englacial fractures formed at about 200 m from the glacier margin andacted as a hinge. The newly formed and preexisting englacial fractures probably took therole of inducing englacial water drainage which preceded the outburst.
Citation: Sugiyama, S., A. Bauder, M. Huss, P. Riesen, and M. Funk (2008), Triggering and drainage mechanisms of the 2004
glacier-dammed lake outburst in Gornergletscher, Switzerland, J. Geophys. Res., 113, F04019, doi:10.1029/2007JF000920.

1. Introduction[2] The outburst of a glacier-dammed lake is a sudden
release of meltwater impounded in ice marginal, subglacial,englacial, or supraglacial locations [Roberts, 2005]. It canhave substantial impact on the physical environment and canpose a serious hazard as it is difficult to predict the timing
and magnitude of floods [e.g., Haeberli, 1983; Björnsson,
1992; Richardson and Reynolds, 2000; Raymond et al.,2003]. Since a number of glacier-dammed lakes have been
newly formed as a result of recent glacier retreats, it isurgently necessary to acquire a better understanding of thetriggering and drainage mechanisms of lake outbursts.
[3] In general, glacier-dammed lakes start to drain when
the lake level reaches a critical threshold, which varies
depending on the triggering mechanism of the outburst
1
Institute of Low Temperature Science, Hokkaido University, Sapporo,
Japan.
2
Laboratory of Hydraulics, Hydrology, and Glaciology, ETH Zurich,
Zurich, Switzerland.

Copyright 2008 by the American Geophysical Union.
0148-0227/08/2007JF000920$09.00*The author order has been corrected here due to an error introduced in
production.

[Tweed and Russell, 1999]. Thorarinsson [1953] proposedthat the flotation of the ’ice dam’ (glacier adjacent to thelake) caused by the pressure of the lake water initiates thedrainage. In this case, the outburst is expected to occur
when the lake level exceeds the flotation level of the ice
barrier and the lake water breaks through the seal underneath. Although some observations suggested flotation as
the triggering mechanism [Sturm and Benson, 1985; Knight
and Russell, 1993], many other outburst events were initiated before the flotation condition was met. The outburst ofGrı́msvötn, a subglacial lake in Iceland, usually occurs at
the lake level which is 20– 50 m less than that required for
flotation [Björnsson, 1992, 2002]. Nye [1976] ascribed thisdiscrepancy to the flexure of the ice floating on the
subglacial lake, the so-called buoyant cantilever effect.
Buoyant force acting on the floating part of the glacierpries the grounded ice off its bed, resulting in the flotationof the ice barrier at a subglacial pressure slightly less than
the ice overburden pressure. Since the stress condition of
the ice dam is influenced by the stress coupling withneighboring ice, the lake level required to breach the seal
is not a simple function of the ice thickness at the point
where hydraulic potential barrier exists. In addition to thelake level, the timing of the outburst is controlled by ice
motion and crevasse formation, as well as certain character-

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F04019bed due to the lake water pressure exceeding the ice
overburden, and the subsequent sheet-like water flow
through the subglacial space were proposed as an alternativedrainage mechanism [Björnsson, 1997, 2002; Jóhannesson,
2002]. Numerous observations during the 1996 event,including the unusually high lake level, surface uplift of
the ice dam, and the development of supraglacial fountains,
were consistent with this hypothesis. Flowers et al. [2004]
showed that a model formulated by the combination of
conduits and sheet-like flow can explain the hydrograph of
the 1996 outburst from Grı́msvötn.
[5] To test the hypotheses described above, it is necessary
to gather comprehensive field data during a lake outburst
together with topographical information on the ice dam,e.g., ice thickness, bedrock and surface elevation. This
paper presents the results of field measurements madeduring the glacier-dammed lake outburst at Gornergletscher
in 2004 and discusses the triggering and drainage mechanisms implied by the observational data. We propose thatthe outburst was initiated by the flotation of the ice dam andthe lake water drained as a sheet-like flow rather than
through a single conduit, as indicated by the vertical surface
motion of the ice dam.

2. Field Measurements

Figure 1. (a) Map of Gornergletscher and Grenzgletscher
with the borehole (circle) and theodolite (triangle) locations
and surface contour lines at intervals of 50 m. The study site
is indicated by the box. (b) Locations of the survey stakes(crosses), boreholes (open circles), and englacial water
channels (solid circles) with surface contour lines atintervals of 10 m. The boreholes BH210 and BH150 are
in the same location. The shading on the glacier shows themedial moraine.
istics of the ice dam, e.g., hydraulic connection at the bed,
the possible existence of a subglacial sediment layer[Fowler and Ng, 1996], and debris content of the ice[Tweed, 2000]. For this reason it is very difficult to predictlake outburst timing.
[4] The water drainage through a glacier during a lake
outburst has been studied in a theoretical way by many
authors, who relied on the assumption that the lake waterdrains through a single conduit [Nye, 1976; Spring and
Hutter, 1981, 1982; Clarke, 1982, 2003; Fowler, 1999].
These theoretical investigations successfully reproduced theexponentially rising limb of hydrographs (slowly starting
and rapidly increasing discharge) measured at several
glaciers. It follows from these investigations that the opening of the conduit by frictional heat generated by the waterflow and its closure due to viscous ice deformation arelikely the controlling processes in many outburst events.
However, some outburst events show linearly increasing
discharge, which cannot be explained by conduit flow
alone. The outburst of Grı́msvötn in 1996 is one such event[Björnsson, 2002]. Separation of the glacier sole from the

2.1. Study Site
[6] Gornersee is an ice-marginal lake situated at theconfluence area of Gornergletscher and Grenzgletscher in
Valais, Switzerland (Figure 1a). Its annual formation inspring and subsequent drainage in summer provide an
opportunity to study the outburst mechanisms of a glacierdammed lake. Most of the lake is ice floored and the eastern
margin is dammed by the bedrock. The lake collects surface
snowmelt and icemelt over the course of the ablation season
and abruptly releases water from a subglacial outlet into theriver Gornera, 5 km downstream from the lake. Over the
past decades, the outburst has occurred regularly in June orJuly, whereas the maximum volume of lake water varies
significantly from year to year [Huss et al., 2007]. The
maximum water volume in 2004 was estimated to be (4.0 ±0.2)
106 m3 on the basis of the lake level and the
hypsometry of the lake floor, obtained by processing anaerial photograph taken after the lake was emptied [Huss etal., 2007]. A bed elevation map with 25 m resolution isavailable for Gornergletscher on the basis of radio-echosoundings carried out in 2004 and 2005 [Huss, 2005;
Riesen, 2007]. Details of the radio-echo soundings and dataprocessing methods are described in the auxiliary material.1
2.2. Ice Flow and Deformation Measurements[7] From May to July 2004, we measured ice motion by
surveying aluminum stakes installed in the glacier surface
either by an automatic theodolite or GPS (Global Positioning System). The theodolite (Leica TCA1800) installed on
the northern flank of the glacier (Figure 1a) was automatically set in operation every hour to survey the threedimensional positions of the reflectors mounted on stakes
33– 36, 41, and 43– 47 (Figure 1b). The survey data werecorrected by the reference measurement of reflectors fixed1
Auxiliary materials are available in the HTML. doi:10.1029/
2007JF000920.

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F04019SUGIYAMA ET AL.: GLACIER-DAMMED LAKE OUTBURST

Figure 2. Photographs showing (a) Gornersee viewed
from the northern flank of Gornergletscher on 30 June 2007
and (b and c) englacial water channels observed at P1 andP2 in Figure 1b. Figures 2b and 2c by P. Weiss.
on the bedrock. The errors in the angle and distance
10 6 ) mm
measurements were ±1 00 and ±(1 + d(d: distance in millimeters), which correspond to positional
errors from several to 10 millimeters. The accuracy of therelative stake movement in the vertical direction was from
±3 to ±8 mm, depending on the distance from the theodolite
to the survey stakes. Owing to the high-frequency measurement, the accuracy can be improved by filtering the data.
[8] Stakes 37 and 42 (Figure 1b) were surveyed by
GPS receivers (Leica System 500) mounted on top of the
stakes. The L1 and L2 phase signals were recorded eighttimes a day for static epochs of 1 h at regular intervals of
3 h. The GPS data were postprocessed with the data
recorded by the reference receiver installed on the bedrockat the northern flank of the glacier. The accuracy of the
GPS survey is estimated to be about 3 and 5 mm in thehorizontal and vertical directions, respectively [Sugiyama
and Gudmundsson, 2004].
[ 9 ] High-accuracy borehole length measurements[Gudmundsson, 2002; Sugiyama and Gudmundsson,
2003] were repeated from 14 June to 12 July to measure
the vertical strain of the ice. We drilled a 156 m deep
borehole (BH150 in Figure 1b), where the ice is 210 m
thick, with a hot water drilling system and installed a ringmagnet at the bottom of the borehole. By using a measuring
tape equipped with a magnetic sensor on its end, thedistance from the magnet to the reference bar installed on
the glacier surface was measured once or twice a day. The
accuracy of the measurement was estimated as 2 – 3 mmfrom repeated measurements [Gudmundsson, 2002;
Sugiyama and Gudmundsson, 2003].

F040192.3. Hydrological Measurements[10] Three boreholes (BH430 in Figure 1a, and BH230
and BH210 in Figure 1b) were drilled for subglacial waterpressure measurements. According to the length of thedrilling hose, the ice thickness at these drilling sites was
430, 230, and 210 m, respectively, with an accuracy ofseveral meters. The water levels in the boreholes wererecorded every 10 min by using vibrating wire pressuretransducers (Geokon Model 4500) and a data logger(Campbell CR10X). The measurement accuracy was
equivalent to a water level of ±0.35 m. The water level
data in BH210 was available only until 6 July because of atechnical problem with either the sensor or the logger.
[11] The lake water level was measured by a waterpressure transducer (Keller, PAA-36W) installed near the
deepest point of the lake and recorded every 10 min by thedata logger (Campbell CR10X) with an accuracy of ±6 mm.
The measurement was terminated when a floating ice block
cut the sensor cable during the outburst on 5 July. From thechange in the water level and the hypsometry of the lake
floor, the discharge rate from the lake was computed for theperiod of 2 – 5 July. The error in the discharge due to theuncertainty in the lake floor elevation was about ±5%. To
obtain an accurate figure for the lake discharge, changes in
the water level due to meltwater input were corrected byusing a surface melt model [Huss et al., 2007].
[12] Water discharge from the glacier was measured at
approximately 1 km down the valley from the glacierterminus. The Grande Dixence hydroelectric power company operates a water intake system and measures the riverdischarge every hour.

3. Results

3.1. Lake Outburst[13] In 2004, the lake began forming in the middle ofMay, and the lake level increased progressively until thelake basin was completely filled (Figures 2a and 3a). The
amount of stored water on 1 July was estimated as (4 ± 0.2)
106 m3 from the measured lake level and the known
hypsometry of the lake floor. The rate of the water volume
increase agrees well with the cumulative meltwater input
computed by the surface melt model [Huss et al., 2007],
suggesting no significant leakage before the outburst event.
The lake level overtopped the ice dam surface on 1 July andthe lake water began to flow over the glacier surface prior tothe main drainage. On the same day at 1110 h, the water level
suddenly dropped by 0.3 m within the measurement intervalof 10 min (Figure 3a inset). This incidence indicates thetransfer of (8.0 ± 0.4) 104 m3 of lake water to subglacial
and/or englacial space.
[14] The lake level rose again during the rest of the day,and then the outburst began on 2 July. The lake level began
to decrease gradually early in the morning, which was
followed by calving of marginal ice at the north of the lakeand subsequent increase in the discharge rate. Although thelake level data are available only until 5 July, daily images
taken by an automatic camera from Gornergrat (northernflank of Gornergletscher) and visual observations confirmedthat the lake was nearly empty on 7 July. The water was
draining subglacially at the eastern margin of the glacierduring the latter half of the drainage from 5 to 6 July. When

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F04019the lake emptied out, we found several water channels withdiameters of 2 – 3 m were excavated into the ice (Figures 2band 2c). These channels were located at horizontal distancesof about 5 – 10 m from the maximum lakeshore and about
2– 5 m below the maximum water level. These observations
indicate that the outburst occurred via a combination ofsubglacial and englacial drainage. Englacial water drainage
was also confirmed during the event by sensing the vibration of the sensor cable used for the borehole length
measurement. From 1800 on 1 July to 1800 on 5 July, astrong water current was detected by the sensor cablevibration as the sensor was lowered in a 120 m deep
borehole, which was located at about 5 m from BH150
and BH210, at the depth of 101– 103 m from the surface(107– 109 m from the bed). The sound of the water flow inthe borehole could also be heard. The discharge from theglacier terminus began to increase about a day after the
onset of the outburst and it reached a peak discharge on 6
July [Huss et al., 2007].

Figure 3. (a) Time series of the lake water level and lake
discharge, (b) the vertical displacement and the borehole
length change of BH150 (diamonds), and (c) the verticaldisplacement for the stakes which showed large
vertical motion. Six h running mean was taken for thevertical displacement measured by the theodolite and theerror estimation for each stake is indicated by the gray band.
The vertical gray band indicates the lake outburst period
from 2 to 7 July.

3.2. Motion of the Ice Dam[15] During the outburst, all the stakes showed upward
motion (Figures 3b and 3c). The magnitude of the verticaldisplacement was up to 0.1 m except for stakes 44– 46
located at the western lakeshore. The displacement at thesethree stakes was 0.5– 3.0 m, 1 order of magnitude greaterthan at the other stakes (Figure 3c). For the centimeter-scale
uplift at stakes 33– 37, 41– 43, and 47, the initiation of the
upward motion coincided with the onset of the outburst. The
upward displacement was followed by nearly the sameamount of downward displacement during the latter halfof the outburst. After the outburst, the general trend of theelevation change was more negative than before the event.
At stake 33, for example, the elevation was nearly constantfrom 20 June to 1 July, but it decreased about 0.05 m from
7 to 11 July. For the greater uplift at stakes 44– 46, theinitiation was several days earlier than the onset of theoutburst and the peak elevation occurred in the middle ofthe drainage on 4 July. The surface elevation dropped below
the preevent level when the lake emptied.
[ 16 ] The length of the borehole BH150 generally
increased until the lake outburst (Figure 3b). It then began
to decrease during the outburst on 4 July and decreased until
the end of the measurement process. The mean thickening
rate over the upper 156 m was 7.9 ± 0.3 mm day 1 (vertical
10 5 day 1) for the period
strain rate e_ zz = (5.0 ± 0.2)
from 20 June to 4 July, whereas it was 10.3 ± 0.5 mm
day 1 (_ezz = ( 6.6 ± 0.3) 10 5 day 1) from 4 to 11 July.
The temporal pattern and magnitude of the borehole lengthchanges are comparable to those of the surface verticalmotion at the same location (compare with the verticaldisplacement at stake 37 in Figure 3b), except for theoutburst period.
[17] To examine the details of the centimeter-scale upliftduring the outburst, linear trends for the period 1 – 7 July
were subtracted from the daily mean surface elevation at
stakes 33– 37 and 41– 43 (Figure 4). This procedure was notapplied to stake 47 because the difference between thetrends before and after the outburst was too large. The
magnitude of the uplift and the time of the peak elevation
were different depending on the distance from the lake. The
uplift was greater near the lake at stakes 37, 42, and 43.

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F04019stakes as well [Sugiyama et al., 2007], but at 44– 46 aremuch more pronounced. The ice flow directions weretoward the empty lake after 8 July, which was substantiallydifferent from before 2 July.
3.3. Subglacial Water Pressure[19] Water levels in boreholes BH430, BH230, and BH210
are shown in Figure 7a with the lake water level, the rate of
the lake discharge, and the vertical displacement of stake 45.
During the lake drainage period, the water level in BH430
remained at a high level, in contrast to the large diurnal
variations prior to the outburst. The water level rapidly
dropped on 7 July and it did not rise again to the level before
the event. Although these observations represent the clear
impact of the outburst on the subglacial water pressure in theconfluence area, the other two boreholes located directly in
the ice dam are the object of focus here.

Figure 4. Daily mean vertical displacement at stakes 33–
37 and 41 –43 from 1 to 7 July. The linear trend from 1 to 7
July is subtracted from the vertical displacement.
Stakes near the lake (41 – 43 and 37) peaked about a day
earlier than those at lower reaches (33 –35).
[18] The plan view of the stake motion revealed complex
ice flow changes at stakes 44– 46 during the outburst event(Figure 5). At the onset of the outburst on 2 July, thedirection of the ice flow at stake 44 suddenly changed to thesouthwest followed by a 180 backward motion lasting until
the lake emptied on 8 July (Figure 5a). The horizontal flow
speed during 2 – 8 July was about 1 order of magnitude
greater than before and after the lake drainage period [Weiss,
2005]. The flow direction after 8 July was northeast, which
was clearly different from the preevent direction. The flowchanges at stakes 45 and 46 are also very complex, as
shown in Figures 5b and 5c. The changes are not exactly thesame as those at stake 44, but there are similarities in termsof timing, flow direction and speed. At these three stakes,
the first change in the flow direction occurred at the onset ofthe outburst. The ice began to flow away from the lake until
the nearly 180 direction change at around noon on 4 July.
The timing of the flow change coincides with the peak inthe vertical displacement, which can be recognized by the
stake motion projected on the vertical planes (Figure 6). Iceflow speed increased particularly during the second half ofthe lake drainage from 4 to 8 July. The direction changes
and speed up of the ice motion were observed at otherFigure 5. Plan view of the stake motion from 20 June to
12 July at survey stakes (a) 44, (b) 45, and (c) 46. Local
time is given to the locations indicated by the open circles.

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F04019glacier surface is the result of vertical straining, subglacial
separation, and the sliding over an inclined bed [Hooke etal., 1989]. The sliding was expected to be enhanced during
the outburst, but it cannot explain the upward motion
because the bed inclination in the lake vicinity is negativein the flow direction. The effect of the vertical strain can beevaluated by comparing the length change of the borehole
BH150 and the vertical displacement at stake 37 (Figure 3b).
The lengthening and shortening of the borehole before
2 July and after 7 July show similar trends as the verticalFigure 6. Stake motion projected on (a) west-east and(b) south-north vertical planes intersecting stake 45 with ice
and bed surface geometry along the transects. Trajectories
before and after 4 July 1200 h (time of the highest
elevation) are drawn by the gray and black lines,
respectively. The initial positions of the trajectories
correspond to the elevation of the stakes.
[20] After the installation of the pressure transducers in
BH230 and BH210 the water levels were consistently highand close to the flotation level, probably because the boreholes were not very well connected to the subglacial
hydraulic system. Only small diurnal fluctuations wereobserved as the result of water input from the surface. In
these boreholes, water levels dropped on 1 July, the day
before the onset of the outburst event. This event approximately coincided with the sudden 0.3 m drop in the lakelevel. The borehole levels were elevated on 2 July at theonset of the outburst and remained very close to the
flotation level during the drainage period. When the lake
discharge ceased, the level subsided slowly in BH230,
whereas it dropped suddenly by more than 50 m in BH210.
[21] A closer look of the data shows that the features of the
precursory event on 1 July are similar in BH230 and BH210(Figure 7b). The water level in BH210 sharply increased at
0730 h up to several meters higher than the flotation level.
Water level in BH230 rose above the flotation level at thesame time, which was 2 h earlier than the level increase dueto the diurnal variations. The borehole levels dropped at
0940 h about 1.5 h before the 0.3 m lake level change.

4. Interpretation and Discussion

4.1. Vertical Surface Motion[22] The centimeter-scale uplift observed at stakes 33–
37, 41 – 43, and 47 provide clues to the triggering anddrainage mechanisms of the outburst. Vertical motion of a

Figure 7. (a) Time series of lake water level; lake
discharge; borehole water levels in BH430, BH230, andBH210; and vertical displacement at stake 45. The verticalgray line and the band indicate the timing of the 0.3 m lakelevel drop on 1 July and the lake outburst period from 2 to
7 July, respectively. The horizontal dashed lines are flotationlevels. (b) Detail of the water levels in BH230 and BH210 atthe onset of the outburst.

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F04019Figure 8. (a) A contour map of Dz as defined by equation (1) at the maximum lake level on 2 July.
Locations of the survey stakes and boreholes are indicated by crosses and circles, respectively. The
margin of Gornersee is shown by the black line. (b) Vertical displacement at stakes 33– 36 from 1 to 7
July. The linear trend from 1 to 7 July is subtracted from the vertical displacement. (c and d) Verticalglacier cross sections along the lines X and Y in Figure 8a with ice surface (thin black line) and bottom(thick black line), lake surface elevation (dashed line), ice flotation level (gray line), and zb0 as defined byequation (2) (dashed-dotted line). Locations of the survey stakes are indicated by the arrows and thepoints A – D are introduced for interpretation (see text). The inset of Figure 8d is a schematic diagram
showing the marginal ice bending upward.
displacement on the surface. This indicates that the verticalstraining of the ice is an adequate explanation of the verticalsurface motion before and after the outburst. Nevertheless,the surface vertical motion from 2 to 6 July is not consistent
with the borehole length change, suggesting the occurrenceof subglacial separation due to pressurized water. This
interpretation is also supported by the water levels in
BH230 and BH210 during this period, which were close
to the flotation level. The water level data indicate that the
subglacial water pressure was high enough to lift the ice inthis region. The magnitude of the vertical motion was of thesame order at all the survey stakes except for 44 –46. The
spatially uniform uplift implies that the lake water drained
as a subglacial sheet-like flow and the drainage pathextended over the studied region. Therefore, we propose
ice dam flotation as the triggering mechanism of theoutburst and the lake water drainage through the spacecreated between the ice and the bed.
[23] Flotation of the ice dam is likely because the lakewater level was higher than the ice surface at one point, thus
higher than the ice flotation level. Figure 8a is a contour plotof the difference Dz between the ice flotation level and thelake level zl on 2 July, described as follows:
Dz ¼ ðzs

zb Þ

ri
rw

ðzl

zb Þ;

ð1Þ

where zs and zb are the glacier surface and bed elevations(Figures S1 and S2), and ri and rw are ice and water densities.
The lake level exceeds the flotation level (Dz < 0) over part ofthe ice dam (90.7 km < northing < 91.0 km) and this condition
extends down glacier. At these regions, the subglacial waterpressure would have been high enough to lift the ice dam
when a hydraulic connection between the lake and the glacierbase was established. If this was the mechanism of theoutburst initiation, the width of the drainage pathway wasprobably greater than 300 m as the floating part of the ice dam
would have pried up the neighboring ice resulting in basal
separation over a broader region.
[24] To verify the above interpretation, spatial variationin the surface uplift was examined along the stake profile
33 – 36. This profile was chosen as it is approximately
perpendicular to the assumed drainage pathway. Although
the glacier surface motion was influenced by the verticalstraining, the greater part of the uplift and subsequent
lowering during the outburst was due to the basal separation
as shown by the vertical strain measurement in BH150(Figure 3b). Thus, we use the detrended vertical displacement in Figure 4 as an estimate of basal separation. The
uplift occurred evenly at the beginning of the outburst(Figure 8b), implying that the glacier sole was bridging
across a subglacial water sheet in a north-south direction.
During the second half of the outburst, the vertical dis-

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placement was peaked in space at stake 35, suggesting thatthe drainage was more enhanced at the middle of the profile.
These observations support the interpretation that the outburst was triggered by the ice dam flotation over a relatively
broad region between 90.7 and 91.0 km northing. If weassume that the maximum lake discharge of about 20 m3 s 1
on 3– 4 July (Figure 7) took the form of a subglacial sheet
300 m wide and 20 cm thick (Figure 8b), the peak water flow
speed can be estimated roughly as 0.3 m s 1.
[25] The location of the peak uplift can be explained by
the ice and bedrock geometry. In Figure 8c, the ice flotationlevel along the line X in Figure 8a is plotted on the glaciercross section together with the lake surface elevation on
2 July. Also indicated is the imaginary ice bottom elevation
which is required to satisfy the condition Dz = 0 for a given
ice surface and a lake level [Nye, 1976]
z0b ¼ zl

ðzs

zl Þ

rw

ri

ri

:

ð2Þ

In other words, z0b is the elevation where the hydraulicpotential referenced to the lake surface is equal to zero. The
comparison of z0b and zb gives an indication that the ice isfloating or grounded where z0b > zb or z0b < zb, respectively.
An interesting feature of z0b is the hydraulic barrier at the
point B. The ice overburden pressure is locally high becauseof the surface mound which is a medial moraine covered
with rocks and boulders up to about 1 m thick. The ridge of
Dz in Figure 8a along stakes 41 –43 and 33 is due to thismedial moraine (see Figures 1b and 2a). This ridge might
have formed a hydraulic barrier between the lake and thenorthern part of the glacier. Because this barrier pinned theice down to the bed, the uplift peak was not at the minimumin the hydraulic potential (stake 34) but at the center of thesection AB (stake 35). From the foregoing analysis, we inferthat the lake water first drained approximately throughsection AB in Figure 8c, and elevated water pressure lifted
the ice above and in the vicinity of this section. Then, morewater drained into the middle of the drainage pathway,
forming a drainage center which approximately corresponds
to line Y in Figure 8a.
[26] Our observation of surface uplift may be evidence ofthe triggering and drainage mechanisms proposed for the
1996 jökuhlaup from Grıgrave;msvötn [Jóhannesson, 2002;
Björnsson, 2002; Flowers et al., 2004]. The sharp increasein the lake discharge at the onset of the outburst (Figure 3) issimilar to the hydrograph modeled as a sheet discharge[Flowers et al., 2004]. Moreover, the slight increase indischarge from 4 to 5 July after the peak discharge suggests
the enlargement of conduits [Huss et al., 2007], which isalso consistent with the assumption used in Flowers’ model:
a sheet flow feeds a nascent system of conduits. The mostimportant feature commonly observed in those two events at
Gornersee and Grıgrave;msvötn is a very high lake waterlevel, which is consistent with outburst triggering by ice
dam flotation.
4.2. Surface Motion at Stakes 44– 46[27] The timing of the vertical motion at stakes 44– 46 issimilar to the other stakes, but the large magnitude of the
uplift (Figure 3c) and the abrupt direction changes in thehorizontal motion (Figure 5) require further interpretation.

F04019Figure 8d shows the glacier cross section along the line Y in
Figure 8a. The distance from stake 44 to the nearest stake
37 is only 80 m, but the uplift during the outburst is morethan 1 m at 44, while it is less than 0.1 m at 37. The increasein the ice surface uplift toward the lake suggests the bendingof the marginal ice due to the buoyancy force, as thelake level exceeded the ice flotation level in this region(Figure 8d). Nevertheless, the steep gradient in the verticaldisplacement from 37 to 44 cannot be explained by the elastic
flexure of ice as envisaged by Nye [1976]. For a floating ice
plate with a thickness of 150 m and a Young’s modulus of
1010 Pa, a characteristic length scale required to reduce themagnitude of the uplift in an exponential scale can be
estimated as 2000 m [Turcotte and Schubert, 1982; Walder
et al., 2006], which is far greater than the distance between
stakes 37 and 44.
[28] The vertical ice motion localized near the lake issimilar to that of the observation at Kennicott Glacier[Walder et al., 2005, 2006]. During the filling process of
the ice-marginal lake, they observed that the magnitude of
the uplift increased toward the lake discontinuously across a
50– 100 m wide band which lay approximately parallel tothe ice-lake margin. This ice motion was interpreted as the
movement of the marginal ice along a high-angle fault. The
trajectories of stakes 44– 46, however, are too complex to
attribute them to the sliding along a fault plane. The uplift
was accompanied by the reversal in horizontal motion, i.e.,southwest and northeast motion for the periods 2 – 4 Julyand 4 – 8 July, respectively (Figure 5). Thus, the three
dimensional trajectory of the stake motion is inclined froma vertical line (Figure 6). Because the azimuth and dip of the
trajectory are different for the three stakes, three independent fault systems would have to be assumed to interpretthem by faulting. The hysteresis in the trajectories is alsodifficult to explain by the motion along a fault. The timingof the uplift is clearly different from the observation at
Kennicott Glacier. In the case of Kennicott Glacier, theupward surface motion began several weeks before theoutburst and it was more correlated with the lake level,
suggesting that the ice near the glacier margin was afloat in
the lake water [Walder et al., 2005, 2006]. On the contrary,
the uplift at Gornergletscher occurred when the lake level
was falling and it appeared to be related to the lake
discharge with a lag of 1 – 2 days (Figure 7).
[29] A plausible interpretation of the large uplift at stakes
44– 46 is as follows. When the lake water intruded into the
bed approximately along the line Y in Figure 8a, themarginal ice went afloat, opposing the thicker part of
the glacier which was still grounded. As the resulting
stresses in the ice became greater than its mechanicalstrength, basal crevasses and englacial fractures wereformed. Accordingly, the densely crevassed and fractured
region enabled the marginal ice to bend steeply upward byacting as a hinge. In Figure 8d, z0b sharply decreases fromthe margin D to the west and it reaches a local minimum at
the point C. This indicates that the buoyancy force acting
on the ice above the section CD was much greater than thaton the west side of C. Thus, it is likely that many crevasses
and fractures were introduced at around the point C and thatthey caused the upward bending of section CD (Figure 8d
inset). The westward motion during the uplift at stakes 44,
45, and 46 (Figure 6a) is consistent with this hypothesis.

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F04019SUGIYAMA ET AL.: GLACIER-DAMMED LAKE OUTBURST

Figure 9. Schematic diagrams showing the triggering anddrainage mechanisms of the lake outburst. The diagrams
correspond to the conditions when (a) the borehole waterlevels increased on the morning of 1 July, (b) the waterdrained englacially on the evening of 1 July, and (c) the
subglacial sheet flow was initiated.
The glacier surface is always heavily crevassed near the lake
(Figure 2a) because ice flow toward the lake stretches theice dam in an east-west direction when the lake is empty.
These preexisting surface crevasses weakened the ice in thelake vicinity and enabled the large uplift as well.
[30] The magnitude of the uplift appears to be related tothe distance from the ice margin, the distance from thesuggested drainage center (line Y in Figure 8a), and the ice
thickness. The uplift at stake 45 (3.0 m) was the greatest
among the surveyed stakes, as it was close to the ice margin
and the drainage center, and also the glacier was relatively
shallow (110 m ice thickness). The thinner ice causes more
uplift as it is expected to be bent more by the buoyancy
force. Stake 44 was located approximately on the drainage
center, but the uplift was only 1.0 m probably because of thelarger distance from the margin and the relatively greater ice
thickness (150 m). Presumably, the uplift along the drainage
center progressively increases from C to D in Figure 8d.
The azimuth of the horizontal ice motion can be explainedby the location of the drainage center. The trajectories of
stake 45 and 46 are inclined more to the south than that of
44 (Figure 6b), suggesting the ice was pushed away fromthe line Y (Figure 8a) by the subglacial water intrusion.
4.3. Mechanism of the 2004 Outburst[31] The foregoing discussion assumes that Gornersee
predominantly drained subglacially. However, the water

F04019channels excavated into the ice floor indicate englacial
drainage as well. It is likely that this englacial and subglacial water flow interacted with the ice-dam motion, that is tosay, the ice dam flotation triggered the outburst and thedrained lake water induced the surface uplift. Here, we use
the observed vertical ice motion, and the lake and borehole
water levels to elucidate the details of the link between theoutburst and the ice dam motion.
[32] The observational facts relevant to the outburst
initiation are listed following in chronological order:(1) on 30 June uplift was detected at stake 45, (2) on 1
July 0730 h water level increased in BH210 and BH230,(3) on 1 July 0940 h water level dropped in BH210 and
BH230, (4) on 1 July 1110 h lake water level dropped by
0.3 m, (5) on 1 July 1800 h water flowed englacially near
BH210 at the depth of 101 – 103 m, (6) on 2 July 0230
h lake surface reached the maximum level, and (7) on 2 July
1200 h water level rose up to the flotation level in BH210
and BH230. On the basis of these observations, we propose
the mechanism of the 2004 outburst as described below andsketched in Figure 9.
[33] Although the lake level had already been higher
than the flotation level at a part of the ice dam, the sole ofthe ice was in contact with the bed until 30 June. We
assume this is because the hydraulic connection of the lake
to the glacier bed was insufficient and the stress couplingwith neighboring ice prevented the ice dam from flotation.
The surface uplift of the ice dam began on 30 June
(Observation 1) when the lake water penetrated under
the glacier and pried up the marginal ice (Figure 9a).
The upward ice-dam motion connected lake water to theglacier bed as indicated by the water level increase in theboreholes in the morning of 1 July (Observation 2). The
subsequent drop in the borehole levels (Observation 3)
could be attributed to the levering of the ice caused by theelevated subglacial water pressure (Figure 9b). The pressure dropped because of the space created beneath theglacier, which can also explain the sudden drop in the lakelevel 1.5 h later (Observation 4). Such pressure change
controlled by mechanical response of a glacier to ahydrological event was reported by Flowers and Clarke[2000] as a result of numerical modeling on a waterdrainage event in Trapridge Glacier [Stone and Clarke,
1996]. At this time, the boreholes were not connected tothe ambient drainage system as the water levels did notshow the expected diurnal variations (see BH430 in Figure7a) during the rest of the day. As the uplift rate of the ice
dam progressively increased, we hypothesize that cracks
and crevasses were formed within the ice dam, which
introduced lake water into the glacier body (Figure 9b).
The water current detected in the borehole at about 100 m
below the surface (Observation 5) confirms that thedrainage through englacial water channels preceded sheet
flow discharge along the bed. The channel was possibly
developed by connecting the englacial cracks and fractures. The hydraulic connection under the ice dam became
more pervasive as the uplift proceeded. The discharge
from the lake and the subglacial water pressure rapidly
increased on 2 July (Observation 7) when the ice dam
decoupled from the bed, inducing subglacial drainage as asheet flow (Figure 9c). The large uplift at stakes 44– 46
occurred at this time as the result of levering of the ice

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F04019SUGIYAMA ET AL.: GLACIER-DAMMED LAKE OUTBURST

wedge facilitated by crevasse formation and englacial
fracturing.
4.4. Outburst Events in 2005 and 2006[34] We repeated the field observations at Gornergletscher
in 2005 and 2006, and found strong annual variabilities inthe timing and drainage patterns of the lake outburst. In
2005, the outburst initiated when the lake water volume wasless than a third of the full capacity. Detailed examination of
the lake level indicated that the water started to leak one
week before the outburst and the discharge from the lake
increased exponentially [Huss et al., 2007]. We infer fromthe lake discharge pattern that the lake drained by theprocess of subglacial channel enlargement. In 2006, water
filled the lake without leakage until it started to drainsuperficially into a moulin located near the lake. The
discharge increased as the supraglacial water flow inciseda deep gorge on the glacier [Werder et al., 2007]. Thisdrainage process is similar to that observed at Black Rapids
Glacier in Alaska [Raymond and Nolan, 2000]. From theseobservations in 2005 and 2006, it is clear that our interpretations for the 2004 outburst are not applicable to theseyears.
[35] A plausible reason for the annually varying drainage
patterns of Gornersee is the condition of the ice dam, whichis affected by the previous outburst event. Because of thesudden and large ice motion during the outburst in 2004, it
is expected that the ice dam was mechanically damaged bycrevasses and cracks. These fractures may not have healed
enough to seal the lake water in 2005 and thus caused theleakage before the basin was filled. Accordingly, the impactof the 2005 outburst on the ice dam was relatively small andit might be the reason why the lake did not drain until it was
fully filled in 2006. Such an effect has been observed inGrı́msvötn, Iceland. The ice dam of Grı́msvötn was severely
damaged by the catastrophic outburst event in September
1996. After this event, the lake water continuously drained
and the lake level could not rise for several years [Björnsson
et al., 2001]. Variability in the mechanical condition of theice dam makes the prediction of the lake outburst moredifficult.
[36] Gornersee is floored predominantly by ice rather
than bedrock. This lake geometry may contribute to
the varying drainage patterns. Buoyancy force is alreadyapplied to the marginal ice during the initial stage of lake
filling, which may facilitate drainage before the lake iscompletely filled. The geometry of the lake floor is subjected to more change than that of bed-floored lakes. The
timing and location of the outburst may be influenced by thechange in the lake hypsometry. For example, if the ice
submerged beneath the lake water were thicker, the outburst
might be triggered earlier because greater buoyancy force
would be exerted on the ice.

5. Conclusions[37] Detailed measurements of ice-dam motion, combinedwith hydrological measurements in the lake and adjacentboreholes, are interpreted to describe a plausible mechanismof the 2004 outburst of Gornersee.
[38] The centimeter-scale surface uplift observed pervasively around the lake cannot be attributed to the vertical

F04019straining of ice or bed-parallel sliding, thus it suggests theseparation of the glacier sole from the bed, which we argue
induced a sheet-like outflow from the lake. The magnitudeof the uplift was greater at a part of the ice dam where
subglacial water pressure would have exceeded the ice
overburden pressure if the bed were hydraulically connectedto the lake. Thus, hydraulic potential at the bed of the ice dam
controlled the location and timing of the outburst. However,
the outburst did not just occur when the lake level exceeded
the flotation level of the ice dam, because hydraulic connection between the lake and the glacier bed would not beestablished immediately and the mechanical coupling withneighboring ice would prevent the flotation.
[39] The uplift reached 0.5– 3.0 m in the vicinity of thelake. It is likely that englacial fractures, and crevasses at thesurface and bottom acted as a hinge which enabled themarginal ice to bend upward. The horizontal ice motion
near the lake is consistent with the idea that the lake waterdrained into a part of the ice dam where the ice overburdenpressure was exceeded by the lake water pressure.
[40] Timing of the vertical ice motion and changes in thelake and borehole water levels suggest that the outburst was
initiated by drainage through englacial channels, followedby the sheet-like subglacial flow. Presumably, the englacial
drainage was induced by newly formed and preexisting
crevasses and cracks in the ice dam.
[41] Acknowledgments. The authors are grateful to the members of
the field campaign at Gornergletscher in 2004 for their help on the glacier.
Special thanks are due to P. Weiss, B. Müller, U. H. Fischer, T. Wyder, and
H. Bösch for their contribution to the field measurements and data analyses.
We thank G. E. Flowers and J. S. Walder for providing insightful reviews
and the scientific editor M. Church for handling the paper. H. Blatter
offered suggestions on the manuscript and S. Braun-Clarke corrected theEnglish. International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat kindly offered facilities for the study. This research
was funded by the Swiss National Science Foundation grant 200021103882/1 and 200020-111892/1.

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