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 weredrilled 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 theproglacial 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 thedrilling 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*.0km-, respectively (Cryospheric Commission of theSwiss Academy of Sciences, +22+ῌ,**2; Zhano, ,**.;
Bauder et al., ,**1). Despite the recent warming
trend, the retreat rate has decreased in the late ,*thcentury 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 anice 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-