Bulletin of Glaciological Research ,0 (,**2) .+ῌ.1
ῌJapanese Society of Snow and Ice

Hot water drilling and glaciological observations at the terminal part ofRhonegletscher, Switzerland in ,**1
Shin SUGIYAMA+, Shun TSUTAKI+, Daisuke NISHIMURA+, Heinz BLATTER,,
Andreas BAUDER- and Martin FUNK+ Institute of Low Temperature Science, Hokkaido University, Nishi-2 Kita-+3 Sapporo, Japan, Institute for Atmospheric and Climate Science, ETH Zurich, Universitätstrasse +0, CH-2*3, Zurich, Switzerland- Laboratory of Hydraulics, Hydrology and Glaciology, ETH Zurich, Gloriastrasse -1/-3, CH-2*3, Zurich, Switzerland(Received July 1, ,**2; Revised manuscript accepted September ,,, ,**2)

Abstract
After the retreat of Rhonegletscher over a bedrock ridge, a proglacial lake has been forming at
the glacier forefront since ,**/. The lake and the glacier draw glaciological attention because theretreat may be accelerated by calving into the lake. As an initial investigation into studying theimpact of the lake formation on the glacier, hot water drilling and preliminary observations werecarried out at the terminal part of Rhonegletscher in the summer of ,**1. Eight boreholes were
drilled down to the bed and accurate ice thickness was determined from the borehole depth. The ice
was +-/ m thick at a point 1** m from the terminus and it gradually decreased downglacier. Thisobservation revealed that the ice in this region was signiﬁcantly thinner than previous estimation by
an ice-radar survey. The water level in the +-/-m borehole showed diurnal variations. The daily
minima were steady during the study period at about +* m higher than the surface elevation of the
proglacial lake. After injecting a water jet into the bottom of the borehole, ﬁne sediment wasobserved in the upwelling water. At some of the other drilling sites, however, scratches on the
drilling nozzle suggested that the ice was underlain by solid rock. Thus, it is likely that the glacier
bed conditions are inhomogeneous. The rate of change in ice thickness from ,*** to ,**1 wasdetermined to be ῌ,.2 m aῌ+, which indicates that glacier thinning has accelerated in the ,+st century.

+.

Introduction

Rhonegletscher in the Swiss Alps is one of themost well-documented glaciers in the world. Thechanges in the terminus position have been recordedin photographs and maps since the +2th century, fromwhich a drastic retreat can be seen over the last +/*years. The changes in glacier length and ice volume
from +21. to ,*** are reported as ῌ+2** m and ῌ*.0
km-, respectively (Cryospheric Commission of the
Swiss Academy of Sciences, +22+ῌ,**2; Zhano, ,**.;
Bauder et al., ,**1). Despite the recent warming
trend, the retreat rate has decreased in the late ,*th
century because the glacier retreated over a changing
bed slope. The terminus retreated over a steep slope
in the ,*th century, and then became nearly stagnant
on a transverse rock ridge.
Although the changes in glacier length have been
well recorded, only a few comprehensive glaciologicalobservations have been carried out in Rhonegletscher.

In the late +3th century, Mercanton (+3+0) conductedvarious ﬁeld observations, including mass balance
and ice ﬂow speed measurements. Mass balance distribution over the glacier was studied in detail during
the period of +313ῌ+321 (Funk, +32/; Chen and Funk,+33*) and the mass balance was parameterized to theregional climate. A brief summary of the ﬁeld observations of the past was given by Carlen (,**/). Theseobservational data have been used for numerical modelling to reconstruct mass balance history (Huss et al.,,**2) and observed changes in the length and surface
elevation of the glacier (Stroeven et al., +323; Wallinga
and van de Wal, +332; Sugiyama et al., ,**1).
The ice thickness was measured in detail with an
ice radar along +, proﬁles across the glacier and a bed
rock elevation map was constructed by interpolating
the proﬁles (Zahno, ,**.). One of the interesting features in the bedrock topography is an over deepening
beneath the terminal part of the glacier. There is abedrock ridge in front of the current terminus, andbehind this point the bed elevation decreases upglac-

42

Bulletin of Glaciological Researchier, suggesting formation of a proglacial lake once theglacier retreats further. In fact, the terminus detached from the bedrock ridge in ,**/, forming a lake
between the glacier front and the ridge. The lakeformation draws glaciological interest because the ice
calving process may accelerate glacier retreat as wasobserved at the nearby Triftgletscher (Müller, ,**.).
To investigate the response of Rhonegletscher tothe formation of the proglacial lake, a research project
was initiated with the collaboration of Hokkaido University and the Swiss Federal Institute of Technology.
This report describes the preliminary results obtainedin the ,**1 summer ﬁeld study.

,.

Methods,.+ Study site
Rhonegletscher is a temperate valley glacier in
the Swiss Alps with a length of 1.2/ km and a surface
area of +/.3 km, (Cryospheric Commission of the Swiss
Academy of Sciences, +22+ῌ,**2; Bauder et al., ,**1)(Fig. +a). The glacier snout currently lies at an elevation of ,,** m a.s.l. (Fig. +b). Because of the interestin the inﬂuence of the lake formation on the terminal
part of the glacier, the ﬁeld activity in summer ,**1
was focused on the region within +./ km from theterminus. According to the bedrock elevation map
constructed by Zahno (,**.), the maximum ice thickness in the study area was about ,0* m at approximately + km from the terminus.
There are two ice-marginal lakes in the terminal
part of Rhonegletscher (Figs. +b and ,), one at thewestern margin (Lake A) and another one in front(south) of the terminus (Lake B). Lake A has existed

Fig. ,. (a) The terminal part of Rhonegletscher and (b)the ice-marginal lakes viewed from the southeast.
(c) The same part of the glacier looking down fromthe east. Approximate location and ﬂow directionof GPS- are indicated. The photographs (a) and(b) were taken on +. July and (c) on 3 September,**1.

Fig. +. (a) Map of Rhonegletscher with surface
contour lines with intervals of +** m. (b) Map of
the study site showing the borehole locations (῍),
GPS survey sites (ῌ) and ﬂow velocity vectors.
The contour lines indicate surface (black) and bed(grey) elevations with intervals of +** m. The
surface and bed elevations are based on the DEM
in ,*** and the ice-radar survey by Zahno (,**.),
respectively.

for at least +* years (since the late +33*s); whereas,
Lake B formed in ,**/. As it is observed in Fig. ,, ice
blocks were breaking into Lake B during the summer,**1. Although the primary interest focuses on Lake
B, Lake A is also important for the study because
these will probably merge to form one lake in the
future. The surface level of Lake B is expected to benearly constant as the water spills over the rock ridge,
which serves as a dam (Fig. ,b).
,.,

Hot water drilling
We used a hot water drilling system consistingSugiyama et al.

43

of a commercially available pump and heater unit
(Kärcher HDS+***), a -***-liter water basin, *./-inch
diameter drilling hoses, water jet nozzles with diameters of +.0ῌ,./ mm, a tripod, and a pulley equipped witha hose-length gauge. From +- to ,3 July, ,**1, eight
boreholes were drilled within + km of the glacier terminus, six along the central ﬂow line and two closer tothe side margins (Fig. +b). Drilling speed was in therange of -*ῌ/* m h῍+ for the upper +** m. Drilling
was continued until the nozzle reached the glacierbed, which was recognized by a change in the hosetension. The borehole depth was determined by thelength of the hose used for drilling. Because the hosestretched during the drilling, measured borehole
depth (or ice thickness) may have an error of ῌ(+ῌ,) mdepending on the drilling depth and water level in theborehole.
,.-

Water pressure measurement
A vibrating wire pressure sensor (Geokon Model./**) was installed at the bottom of a +-2-m deep
borehole (BH+ in Fig. +b) to measure the subglacial
water pressure. The pressure was recorded every +*minutes using a data logger (Campbell CR+***) withan accuracy equivalent to a water level of ῎*.-/ m.
The pressure sensor data were periodically calibrated
by measuring the water level in the borehole with ameasuring tape and a ﬂoat sensor.
,..

GPS survey
A pair of GPS (Global Positioning System) receivers and antennae (Leica System +,**) was used tomeasure ice ﬂow velocity and to survey glacier andlake surface elevation. One of the GPS antennae wasﬁxed on solid rock at the western ﬂank of the glacier(Fig. +b), which served as a reference station for thestatic and RTK (Real Time Kinematic) measurements.
The ﬂow velocity was measured at three locations(GPS+ῌ- in Fig. +b) by surveying .-m long aluminum
stakes drilled into the ice. The other GPS antenna wasmounted on the top of the stakes to record the L+ and L,
phase GPS satellite signals for /* min. We used threedimensional coordinates surveyed on +/ and -+ July to
calculate the velocities. The accuracy of the static
GPS measurement is within several millimeters in thehorizontal direction (Sugiyama and Gudmundsson,,**.), which is less than +ῌ of the total displacementof the stakes during the survey interval.
The ice surface elevation was measured along
two proﬁles on -+ July by using the RTK technique.
One of the proﬁles follows along the central ﬂow line
(longitudinal proﬁle) and the other one was across theglacier passing through the boreholes BH0, BH+ and
BH/ (transverse proﬁle) (Fig. -a). The accuracy of the
RTK measurements was monitored during the survey,
which was less than - mm for the majority of the time.
However, due to the roughness of the ice surface andFig. -. (a) The GPS survey proﬁles (thick lines) andthe borehole locations (ῑ). The contour lines
indicate surface (black) and bed (grey) elevations(Zahno, ,**.) with intervals of /* m. (b) Longitudinal and (c) transverse cross sections along the
GPS survey proﬁles with surface elevations
surveyed on -+ July ,**1 (ῐ) and based on the
DEM in ,*** (dashed line), the bed elevation after
Zahno (,**.) and the borehole bottoms (ῑ).

tilt of the survey pole, positioning error was expected
to be several centimeters. The surface elevation wasused to obtain recent ice thickness changes by comparing it to the Digital Elevation Model (DEM) of ,***.
The DEM was generated by photogrammetric analyses of aerial photographs with an accuracy in elevation of ῏*.- m. (Zahno, ,**.; Bauder et al., ,**1). We
employed the same RTK technique to measure thelake surface elevation on -+ July and 2 September.
The surface of Lake A was also measured by the static
technique on +. July.

44

-.

Bulletin of Glaciological Research

Results-.+ Ice thickness
The ice thicknesses obtained by the borehole drilling were signiﬁcantly di#erent than those expectedfrom the ice radar data (Table +, Figs. -b and c). The
borehole depth of BH+ was +-2 m, which was ,3 m lessthan the ice thickness calculated from the bedrock
map. The discrepancy increases upglacier and it ismore than +** m at the upper-most borehole, BH1. In
the lower reaches, at the boreholes BH- and BH2, theborehole depth agrees with the ice thickness from theice-radar data within / m. Accordingly, the longitudinal bedrock proﬁle is nearly parallel to the ice surface in the studied region and the bedrock depression
expected beneath the boreholes BH+, BH. and BH1
does not exist. The overestimation of the ice thickness in the ice radar data is also found in the transverse proﬁle (Fig. -c). The discrepancy is slightly
greater on the western side of the glacier.
-., Bed condition
When the drilling at BH+ was completed to thebed, water continued running out of the borehole evenafter the water jet was stopped, which indicated very

high subglacial water pressure. Interestingly, the upwelling water was turbid with ﬁne white particles,
which were most likely subglacial sediments raisedby the hot water injection. After the drilling, thesurface of the brass drilling nozzle was scratched as if
it had been sand blasted (compare Figs. .a and b).
From these observations, it is evident that an unconsolidated sediment layer exists beneath the drillingsite.
Water continued running out after the drilling at
BH, as well. However, sediments were not observedin the water. In the case of BH., the edge of thenozzle used for the drilling was scraped (Fig. .c), suggesting that the nozzle had hit on solid rock at thebottom of the borehole. Similar indications on thenozzle were found after the drilling at BH/. It was
not possible to judge the bed condition at every drilling site, because water did not ﬂow out of other boreholes and the nozzles were not always replaced before
the next drilling. Nevertheless, these observations
indicate that the glacier is underlain by patches ofsediment and bedrock in this region.
-.- Borehole water level
During the period from +- to -+ July, the waterlevel in BH+ showed diurnal variations (Fig. /). The

Table +. Locations and depths of the boreholes and ice thicknesses estimated for the drillingsites from the bedrock map (Zahno, ,**.).

Fig. ..

Water jet nozzles (a) before the drilling and (b) after the drilling of BH+ and (c) BH..

Sugiyama et al.

45

Table ,. Lake surface elevation and the time of measurements.

Table -. Components of the ﬂow velocity to the east(ux), to the north (uy) and upward (uz), and thehorizontal ﬂow speed (uh).

Fig. /. Water level in borehole BH+. The solid anddashed lines correspond to the ice overburdenpressure level and the surface elevation of Lake B,
respectively.

level began to rise in the morning at *3**ῌ+*** hr andpeaked in the afternoon at +.**ῌ+0** hr. It dropped
during the night at a rate slower than that of therising. The water level exceeded the ﬂotation level
several times, including the events on ,0 and -+ Julythat the borehole was overﬂowing. In contrast to thevariable peak water levels, the daily minima in theearly morning were nearly constant during the period. The minimum levels were approximately +* mhigher than the surface level of the lakes.
-..

Lake surface level
According to visual observations in July, the water level of Lake A varied diurnally with an amplitudeof several decimeters and reached peak levels in thelate afternoon. Since the level of Lake B is nearly
constant, as described in the previous section, the twolakes were not completely connected in July. Givena constant surface level from July to September for
Lake B, the level of Lake A in July was ,-ῌ,/ cm
higher than that of Lake B (Table ,). Because themeasurements at Lake A were carried out in the lateafternoon, it is likely that the surfaces of the two lakes
were at a similar level during the night. On 2 September in the afternoon, the surface level of Lake A
was +* cm lower than that of Lake B.

Fig. 0. Rates of ice thickness change from ,*** to ,**1(a) along the longitudinal proﬁle and (b) transverse
proﬁle.

vertical ﬂow speeds are negative (downward ice motion) at the three GPS survey sites.
-.0

-./

Ice ﬂow speed
The horizontal ice ﬂow speed decreases downglacier as it is usually observed near the terminus of avalley glacier (Fig. +b and Table -). However, theﬂow speed of ,+., m aῌ+ at GPS- is a fairly large value
for ice motion observed within +** m from the terminus. At this site, ice ﬂows west approximately to thepoint where the glacier reaches the upglacier side of
the bedrock ridge, forming an ice barrier between the
two lakes (Fig. ,c). Thus, this relatively fast ice motion prevents the connection of the two lakes. The

Surface elevation
The surface elevations measured by GPS along
the longitudinal and transverse proﬁles are shownwith those obtained from the DEM in ,*** (Figs. -b
and c). The elevation changes since ,*** indicate thatthe ice has thinned by approximately ,* m. The magnitude of the ice thickness change increases down
glacier in the longitudinal proﬁle (Fig. 0a) and towards the west margin in the transverse proﬁle (Fig.
0b). The thickness changes averaged over the proﬁles
are ῌ+3., m (ῌ,.1/ m aῌ+) and ῌ+3.0 m (ῌ,.2+ m aῌ+) forthe longitudinal and transverse proﬁles, respectively.

46

..

Bulletin of Glaciological Research

Discussion..+

Implications of the bedrock elevation proﬁleThe borehole depth disagreed with the previously
reported ice thickness, probably because of an error in
the bedrock elevation map introduced by interpolation of the ice radar data (Zahno, ,**.). Since most of
our boreholes were located in between the cross glacier radar proﬁles, uncertainty in the bedrock map
may be signiﬁcant. The agreement is better in thelower reaches (BH- and BH2) where one of the radar
proﬁles exists. It is not likely that the drilling nozzle
hit a rock in the ice before it reached the glacier bed as
the borehole depth was systematically less than theice thickness from the ice radar.
The longitudinal bedrock proﬁle revealed in thisstudy (Fig. -b) is of great importance in predicting thefuture development of the proglacial lake. Since bedrock depression does not exist at῎/** m from theterminus, the maximum area and depth of the lake
after glacier retreat would be less than predictedbased on the ice-radar data. The lake depth along thevalley center will reach approximately /* m; the glacier will be separated from the lake when the terminus
retreats 2** m upglacier from the present position.
Thus, the inﬂuence of the lake on the glacier is limited.
The new bedrock proﬁle is also important to predict the retreat of Rhonegletscher during the next few
decades. Due to the lack of bedrock depression, ice
ﬂux into the terminal part is less than expected. Accordingly, the retreat rate of the glacier would begreater than previous prediction by numerical modelling based on the ice-radar data (Sugiyama et al., ,**1).
Further ﬁeld measurements for a more complete bedrock elevation map and numerical investigations using the new information are required.

..,Hydrological conditions
The diurnal variations in the borehole water levelin BH+ indicate that the subglacial water pressure wascontrolled by the water supply from the surface andsubglacial drainage. Due to the snow and ice melt
during the daytime, the bed was highly pressurized in
the afternoon. The pressure immediately dropped
when the melt rate decreased in the evening, suggesting that a subglacial drainage system had alreadydeveloped.
A well-established drainage system was also implied by the fact that the daily minimum water levels
did not change during the period. The hydraulicgradient between the borehole and the lake was nearly constant because the conduit had already developed and reached a steady size. It is important thatthe borehole level was relatively high, even at night,
under the inﬂuence of the lake water. Since the pressure head in the studied region never drops below thelake surface, subglacial water pressure was maintained at a high level. This e#ect is more signiﬁcantnear the terminus, where the ice surface level is closer
to the lake level. The high water pressure is thelikely reason for the relatively fast ice ﬂow at GPS-.
The di#erence in lake level between Lakes A and
B provides information on the hydraulic connection
between the two lakes. In July, the water level was
higher in Lake A, probably because water inﬂow into
Lake A was not e$ciently drained into Lake B. The
levels were nearly the same in September, suggestingthat the two lakes had become more connected. Decrease in the melt water runo# may have reduced thelake level di#erence, but some di#erence should havebeen observed if the connection of the two lakes had
not changed. The slightly higher level in Lake B
may be due to more inﬂow from upper reaches into
Lake B through a subglacial drainage conduit.
..-

Ice thickness change from ,*** to ,**1
The ice thickness change in Rhonegletscher from
+21. to ,*** was studied by Zahno (,**.) by comparing
DEMs generated from old maps and photogrammetricanalyses of aerial photographs. According to the results, the rates of thickness change at the elevation of,,/*ῌ,-/* m for the periods +21.ῌ+3,3, +3,3ῌ+3/3, +3/3ῌ
+32*, +32*ῌ+33+ and +33+ῌ,*** are ῌ*..3, ῌ*.3,, ῌ*.+,,
ῌ*.3, and ῌ+.21 m aῌ+, respectively. Therefore, thechanging rates from ,*** to ,**1 observed in thisstudy (ῌ,.1/ m aῌ+ and ῌ,.2+ m aῌ+ along the longitudinal and transverse proﬁles) indicate that glacierthinning has accelerated during the last decade.
The recent increase in the thinning rate is consistent with mass balance changes in Swiss glaciers. Husset al. (,**2) computed seasonal mass balances of four
glaciers in the Swiss Alps including Rhonegletscher
for the period of +20/ῌ,**0. The rate of mass loss
averaged over the four glaciers has progressively increased after the period of mass gain from +31. to +32+.
The period +332ῌ,**0 was characterized by strongly
negative mass balances as the result of high summer
temperatures.
If a constant thinning rate of ῌ,.2 m aῌ+ is assumed, the lake will expand upglacier to the locationof BH+ and BH1 in ,*.* and ,*/,, respectively. Nevertheless, formation and development of the lake may
accelerate the thinning and retreat of Rhonegletscher.
The sharp increase in the thinning rate near the terminus (῍,** m in Fig. 0a) suggests that the e#ects of thelake on the glacier terminus, e.g., calving and ice ﬂow
acceleration, are already inﬂuencing the glacier. The
reason for the thinning rate variation across the glacier (Fig. 0b) is unclear. It may be the result of the curving
valley shape which a#ects ice ﬂow regime and solar radiation. To predict the change of Rhonegletscher under
the inﬂuence of lake development, detailed ﬁeld observations of the ice ﬂow and surface melt distributions

Sugiyama et al.

47

in addition to the calving process are required.

References/.

Conclusion

Hot water drilling and preliminary observations
were conducted in Rhonegletscher with focus on thee#ects of the newly formed proglacial lake on theterminal part of the glacier. Important information
obtained by this ﬁeld research includes:+. In the region /**ῌ2** m from the terminus, the ice
is up to +** m thinner than the previous estimation. The development of the proglacial lake inthe future is limited to 2** m upglacier from thecurrent terminus.
,. Subglacial sediment exists in at least a part of thestudied region. The glacier bed probably consistsof patches of sediment and bedrock.
-. Subglacial water pressure varied diurnally. The
borehole water level drops at night to about +* mhigher than the lake surface level.
.. Horizontal ﬂow speed near the terminus is relatively fast (,+ m aῌ+), suggesting that basal motion
is enhanced by the lake.
/. The glacier is thinning at a rate of ,.2 m aῌ+ in thestudied region. The thinning rate has increasedin the ,+ century.

Acknowledgments
We thank T. Wyder and members of VAW-ETHZ
for their help in the ﬁeld campaign. K. Chubachi
designed and constructed the mechanical parts for the
hot water drilling system. Kärcher Inc. provided information and spare parts for the hot water machine.
We thank N. Naito for his helpful review comments
and Scientiﬁc Editor K. Fujita for handling the paper.
This research was funded by the Japanese Ministry of
Education, Science, Sports and Culture, Grant-in-Aid
for Young Scientists (Start-up), +22.***,, ,**0ῌ,**1
and Grant-in-Aid for Scientiﬁc Research (C), ,*/.*.+2,,**2ῌ,*+* and by a funding program of Institute of
Low Temperature Science.

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