4.9

BulletinofGlaciologicalResearch21（2004）4955⑥JapaneseSocietyofSnowandIce

Variation andlakeexpansion ofChubdaGlacier，Bhutan HimaJayas，duringthelast35years
Jiro KOMORll，Deo Raj Gurungl，2，ShujilWATAland HironoriYABUKl3
1DepartmentofGeography，TokyoMetropolitanUniversity，Tokyo1920397Japan

2GeologicalSurveyofBhutan，P．0．Box173，Thimphu，Bhutan
3FrontierObservationalResearchSystemforGlobalChange，Kanagawa2360001Japan（ReceivedSeptember30，2003；RevisedmanuscriptreceivedNovember6，2003）

Abstract

ChubdaTsho（ChubdaGlacialLake），locatedintheheadwaterofChamkharRiver，isoneofthe
largestglaciallakesinthe BhutanHimalayas．To assesstheGlacialLakeOutburstFlood（GLOF），
mapandsatelliteimageanalyses，andfieldinvestigationswerecarriedout．Theresultsshowedthe

pastandpresentfeaturesoftheglacierandtheglaciallake．Thehorizontallocationofthefrontof

ChubdaGlacierhasnotchanged，atleastduringthelast35years，andthepresentsurfaceelevation
ofthedebris−COVeredpartoftheglacierhasobviouslydropped．TheareaofChubdaTshoexpanded

byaboutthreefoldthroughtheyearsfrom1967to2001．Thisexpansionrateisslightlyslowerthan
those of other glaciallakesin the Eastern Himalayas．Assuming that the pace of expansion wasconstant，theinitialwaterareawouldhaveappearedinthelate1950stotheearly1960s．Thefeatures
Of thelower end of the glacier are gravitationally stable under current conditions．However，theexistenceofcollapsescarsandtheremainingicecoreintheterminussuggestthepotentialdangerof

failure．Inaddition，thesituationoftherocksandglacialiceontheupperpartofthelake，Whichcan
beatriggerofGLOF，hasnotbeenunderstood．Therefore，detailedinvestigationsofboththelower

partoftheglacial1akeandtheupstreamoftheglacierarerequired．

1．lntroduction

Severefloodscausedbyoutburstsofglaciallakes
have occurredat frequencyinmanymountain areas（Karma et al．，1999）．Thereportrecommendedinves−tigating the glacier and glaciallakein more detai］．

ThelakewascalledChurapoktoTsho（codenumber

GLC−5）bytheGeologicalSurveyofBhutan（1999）and
Chubda Tshoby Karma et al．（1999）．Inthe present
matic changes（e．g．Lliboutry et al．，1977；Vuichard Study，thelake was called Chubda Tsho．If Chubda
Of the world during thelast half−Century due to cli−

Tshois a dangerous glaciallake，the fertile basin
and Zimmermann，1987；Ⅹu and Feng，1994）．Glacial
downstream and Chamkhar Town，a major towninlake outburst floods（GLOF）have also occurredin
Bhutan，remain exposed to risk of severe disaster．
Bhutanintheeastern Himalayas（e．g．Gansser，1970，
1983；GeologicalSurvey of Bhutan，1999）．Thelast Urgent assessment ofthe danger ofGLOFisimpor−
OutburstfloodinBhutanoriginatedfromLuggeTsho
tantnotonlyforChamkharbasinbutalsothroughout（LuggeGlacialLake）intheLunanaregion，headwater Bhutan．Thepresentstudywasperformedtoelucidate
OfPhoChhu（PhoRiver），inOctober1994．TheGLOFthevariationintheChubda Glacier andChubdaTsh（）killedover20people，andcauseddamagetoproperty
duringthelast35yearsbasedontheinformationfrom
on the river（Watanabe and Rothacher，1996）．Theresatellite data and field investigations to assess theare many dangerous glacial lakes similar to Lugge

possibility of an outburst．Previously，the authors

Bhutan Himalayas．
Alargeanddangerousglaciallakeisplottedin
theupperpartoftheChamkharChhubasin，Central

other satellite data．

reportedthevariationsinChubdaTshoduringthelast
Tshoin Bhutan（e．g．Gansser，1970；Takada，1992；
AgetaandIwata，1999；Mooletal．，2001a；Iwataetal．，
llyears（Iwata et al．，2003）．Thepresentreportpro−
2002）．Hence，investigationoftheglaciersandglacial
vides additional results regarding variations over alakesis necessary for disaster preventionin thelongerperiodinChubdaglacierandglaciallakeusing

Bhutan（GeologicalSurvey of Bhutan，1999；Mooletal．，2001a）．TheGeologicalSurveyofBhutanreported

theresultsofthefirstfieldinvestigationofthelake

2．Research area and method

TheChamkharChhu，alsocalledBumthangChhu，

flows from north to south in central Bhutan andiTO

BulletinofGlaciologicalResearchgraphstakeninthelate1950sandthegroundsurvey
Carried outin1961−1963by the Surveyor Generalof

India（Karma et al．，2003）．Thesemapswereused asbase maps for this survey．Features of thelake andglacier were investigated using CORONA satellite

photographs（40−30m digitized resolution，takenin
January1967／November1968）and SPOT satellite
images（nominalresolutionof20m，takeninDecem−
ber1991／December 2001）in stereoscopic views．
Furthermore，thedetailshorelineoftheChubdaTsho

WaS Obtained from SPOTimages and digitallyscan−Fig．1．Map showing the study area and main riversin
Bhutanned CORONA photos（scanning resolution：720dpi），
usingtheDenebaCanvas7image−editingprogram．

The fieldinvestigation was carried outinlate

September2002，WiththecooperationoftheGeologi一

confluences with Mangde Chhu in southern central

CalSurveyofBhutan．TheBaseCampwassetupfor

isborderedonthenorthbyTibet．ThepresentglacierTown（ca．2600 m a．s．1．midstream of Chamkhar
Chhu）．ThefieldinvestigationinChubdaglacier was
Carriedoutontwodays，September24and26．Ittook

Bhutan（Fig．1）．Thenorthernlimit oftheChamkhar 5days，September23u27，at Domjyon（ct7．4400m
Chhubasin，Whichiscomprisedofover6000mpeaks， a．s．1．），5dayswalkfromthenorthernendofChamkhar
frontsin this area arelocated at4200−4500 m a．s．1．
（Gansser，1983）．There are a totalof94glaciers and557glaciallakesinthe basin（Moolet al．，2001a）．
Chubda Glacier（area：26km2）thelargest glacierin

the eastern branch of the basin，COntains Chubda

Tsho，thelargestlakeinthebasin．
Topographicmaps ofthe area（1：50，000inscale，

40mcontourinterval）wereplottedfromaerialphotoT

3−4hourstowalkfromBaseCamptothelowerendof

the glacier．Throughout the two days，the weather
WaS nOt gOOd，Cloudy，andit sometimes rained or
SnOWed．However，the geologicaland topographical

SurVeyforthelowerpartoftheglacierwascomplet−ed．

Fig．2．ChubdaGlacierandthesurroundingarea，theheadwaterofChamkharChhu．
Therectilinearandcrank−ShapedglaciersjoinformingChubda Glacier（center）．
Chubda Tshoislocatedonthedebris−mantled area ofChubda Glacier．Mappedfrom SPOTimages taken on December23，2001．A，Snow andIce area；B，Debris−mantledarea andglacier；C，Lake；D，Ridge；E，mOraine；F，Stream．

5J

Komoriet al．

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.52

BulletinofGlaciologicalResearch

3．Results and discussion
3．J．Cゐ〟∂（あGぬc才gγ

Chubda Glacieris a compoundglacier with a

middle part．Thus，thelake area enlargementis

thoughttohaveoccurredduetoloweringofthelake
bottomcausedbyicemeltingratherthananincreasein waterlevel．

ThefurthestdownstreampartofChubdaGlacier

rectilinear−Shapedtributaryglacier（GC4−1，COdedby
Karma et alリ1999）that stretches down from thethat functions as threshold of Chubda Tsho shows（GC4−2）thatstretchesdownfromthenortheast（Fig．
2）．Theupstreampartisthedebris−freeglacier，Whilelake have gentIe debris−mantled slopes with hum−northTnOrtheastandjoinsacrank−Shapedmainglacier
thelower partiscoveredwith a thindebrismantle．
Theglaciericeisexposedinthefacesofsmallcliffs
in the downstream part of the right bank of theglacier（Fig．3）withathincoveringofsurface−debris
O．5−1．O min thickness（Fig．4）．The debrisin thedebris−COVeredareawhichwereobservedbythefield

investigation，are COmprised oflight−gray fresh

granitic and gneiss boulderswith no plant orlichen

cover．The debris−COVeredarea couldbeidentifiedon

SPOTimages as alight gray area，With distinct
bordersfromthesurroundingareacomposedofdark−colored basement rocks and older moraines．Chubda

Tsho occupiesthecentralpart ofthedebris−COVered
area，Whichisanearlyflatglaciertonguewithsome

VeryCOmplicatedfeatures．It has a shallowchannel−

1ike water area with manyinlets．Both sides of the

mockyhillsranginginheightfromafewmeterstoten

meters（Figs．3and5）．Thelower end ofthelakeis

boundedby an almostflat narrowsurface ofdebris
mantle．Thisfiatsurfaceboundsontheterminalouterslope．Figure8showsthatthelight−graybouldersof

the outer side overlie the sparsely vegetated anddarkTCOlored older moraine．An outlet channelflows

On the flat surface and stretches to the outer slope．

Theoutletwaterseepsintothedebrisattheendoftheflatsurface，andemergesagainatthelowerendofthe
terminalslope．Shallow debris channels wereidentified on the terminal slope below the point of
disappearance，and the channelbedswere composed

Ofunstable andslipperyboulders．Thesound ofsub−hummockyhills（Figs．3and5）．Theglacialfront of mergedwaterflowcouldclearlybeheardbeneaththe

Chubda Glacier forms a steep slope covered withsurface debris．

Based on comparison of the topographic maps

（1ate1950s），CORONAphotos（1967／1968）and SPOT
images（1991／2001）thelower end of Chubda Glacierbouldersin the dry channel．The emergence point
COrreSpOndstotheboundaryofthepresentglacierandtheoldermoraine（Fig．8）．Karmaetal．（1999）reported

the continuoussurfaceflowontheterminalslopein

Summer，1999．The difference in the outlet flow

has not changedits position between thelate1950s

between1999and2002maybe due to differencesin

couldbeidentifiedfrom these data．

riverbed debrisfrom these alluvialcones was most

ユニ ぐ／…ふ尤r r訂（＝川（ノりJ†Jんイ．／7州・

mass movement，SuCh as debris flow and smallcol−

and2001．Thus，duringthelast40years，theterminus theoutletdischargeamount．Twoalluvialconesexist
at thelower end of the terminalslope（Fig．8）．The
hasnotexperiencedanymajoradvanceorretreatthat

The CORONA photos takenin1967 and1968
Show that Chubda Tsho already existed as a small

likelytransportedbyoutletflowsfromthelake and1apseduetofailuresofthethreshold．

lakeatthistime．Analysisofsatellitedatafrom1967， 3．3．AsぶβSS椚g瑠J〆（お聯γ〆GLOF

1968，1991and2001showed that Chubda Tsho has
expandedinsizebythreefold overthelast35years（Fig．6）．Thelake has mainly expanded upstream．

Extrapolationoftheexpansionrateduringthelast35
yearsindicatedthattheinitialwaterareaofthelake

appearedinthelate1950stoearly1960s．Theexpan−
sion rate of Chubda Tsho（0．027km2arl）is slightly

Thefollowingpointsareimportantindiscussion

Of the risk of an outburst from Chubda Tsho：1）
Stabilityofthelowerendoftheglacierandlake，2）thepossibility of bedrock，debris，glacierice and snow
avalanchesintothelakefromthecatchmentareaand

icecalvingattheupperendofthelake．

1）1．The features of theglacialfront and thelowerendoftheChubdaTshosuggestthatthetermi，nalpartoftheglacierisnotamorainedambutalake
Himalayas（Fig．6）．Thetimingoftheappearance ofthresholdcomposed ofdebris−mantled deadice．The
theinltialwaterareasofeachglacia11akeconverge
Wide threshold of Chubda Lake shows a relatively
with1950s to early1960s similar to Chubda Tsho．
stableformcomparedtoothermorainedams．Thatis，ComparisonbetweenSPOTimagestakenin1991andfreshdebrishascoveredthegentleslopeonbothsides
thosetakenin2001indicateddifferencesintheshore−
OfthelakeindicatingthatChubdaTsho，Oratleastits
1inesatboththelowerandupperpartsofthelake．On
lowerpart，isnotanunstablemoraine−dammedlake，
the other hand，nO Changes were observedin the
SlowerthanthoseofotherglaciallakesintheEastern

middlepartofthelakein1991to2001（Fig．7）．Ifthe butsupra−glaciallake．Expansionofthelakeoccursat

waterlevelof thelakeincreased during thelastlOyears，theshorelinesshouldhavechangednotonlyin
theupperandlowerpartsofthelake，butalsointhe

theupperandlowerend，buttherateatthelowerpart
isnothigh．Itcansafelybesaidthatthefluctuationof
thelake wateris stable．Althoughthe thresholdis

,5,7

Komoriどょβgl

15．Nov．1968
aroa：0．33km2、●●−−−1！Waterar8a妄nl粥8 l＿＿−＿J■Ⅵhter8reaO・11yin−99−

1960

柑80

2000 year−−▲−・乃ム○月bわ■（M00lさ亡■J．2001b）−……◇−｝‥
碑ざ肋間ね如（m8さSU怜d触mF；＆＄inAg8taef戚，2∝め）
二＝jニニ＝二㍍短めぬ乃め￠ 紬isstudy〉

一…汁…一 山磯野乃血（moasurodfromF盲g．3inAgebo亡戚．2000）
・・…九…・拗ね力○什abuki．セ002）−−一●一伽勅○爪／ねム○（moasur8d什omFig．4in4阿ね■−れ20（泊）Fig．6．Top：ChangesofChubda Tshobetween1968199land2001．Theshorelines
WeredeterminedfromCORONAphotosandSPOTimages．Bottomleft：Super−
imposedisp王ay ofChubdaTsbointbesameperiods，Boモモomrig壬1t：T‡leSCattergraphofarea…timevariationoftheiakesintheEastern‡iimalayas，Thearea
Of Raphsthreng Tsho，Lugge Tsho and ThorthomiTsho，giaciallakesin the
Llユnanaregioriaredigitizedandmeasureと重fromtbefiguresinÅgetag才αg，浄狛払
Tsho Rolpa andImjaTshoin NepaiⅢmalayasarereferredfrom Mooiet al．
（2001b）and Yabuki（2002）．

占■J

BulletinofGlaciologicalResearchgravitational1y stable under current condition，theexistenceofpastcollapsescarsontheterminalslope
andice coreremalnlngin the terminus suggest that
thereisapotentialdangerofthresholdfailure．
1）r2．Inthe area wherethe debrisis thick，thesurfaceflowdisappears．Thesubmergedoutletwaterthatgentlyemergesfromtheboundarybetweenthe

debris−mantled and older morainesflowsbeneaththe
glaciersurfacejustbelowthedebris・mOraine（Fig．8）．

The audible sound of submerged water flow andFig．7．ThemiddleandlowerpartsofChubdaTsho，1991and2001．Thepeninsulas（PRland2in1991）changedtosmallislands（IS，AandBin2001）andtwoponds（Pool−B，CandD）becamesmallbays（BRl，2andBR2．5）inthelowerpartofthelake．Incontrast，inthemiddlepartofthelake（upperrightoffigures）nochanges
weredetectedintheshorelinesoverthelasttenyears．

Fig．8．ViewofthefrontalpartofChubdaGlacierformingterminalslopesandflat
threshold．Twocollapsedscarsandassociateddepositscanbeseenonthecenter
andrightbanksideofthethreshold．Seepagewateremergesbetweentheolder
moraine（dark−COlored）andyoungerdebris．PhototakenonSeptember26，2002．A
sketch ofthe same outlineisshown on thelower right．W，emergeS pOint；（w），
emergedpointinSeptember1999（Karma et al．，1999）．

55

KomorigJαJ．

gentle emergence as the outlet indicate that the

groundwaterisnotpressurized．Thus，theoutletwater

ReIerences

COuldnotbeadirecttriggeroffailureofthethresh−

Ageta，Y．andIwata，S．（eds．）（1999）：ReportofJapanBhutan
Joint Research19980n the Assessment of Glacier Lake
old．
Outburst Flood（GLOF）in Bhutan．Institute for2）．Thedistributionsofthebedrock，debrisandHydrospheric−Atomospheric Science，Nagoya University，
glacialiceinthesurroundingareaofthelowerpartof
161pp．
Chubda Glacierindicate that they may not fallinto
Ageta，Y，，Iwata，SリYabuki，H．，Naito，N．，Sakai，A．，N
C．andKarma（2000）：Expansionofglacierlakesinrecent
thelakedirectly．Anicefallintheeasterntributaryof
decadesin the Bhutan Himalayas．Proceedings of the
ChubdaTsho（GC4−2）wasrecognizedinafieldsur・
Seattle Workshop，2000；Debris−Covered Glaciers．IAHS，Vey（Fig．5）．AvalancheandiceTblockfallseemtopose
Publicationno．264，165−175．
lessofadangerbecauseoftheconsiderabledistance
Gansser，A．（1970）：Lunana：the peaks，glaciers andlakes ofbetweentheicefallandthelake．However，thereare
northern Bhutan．The Mountain World1968／69，Swiss
FoundationforAlpineResearch，117−131．
largetalusesonthelakebankattheridgebetweenthe
Gansser，A．（1983）：Geologyof the Bhutan Himalaya．Birk−
GC4−1and GC4−2glaciers（Fig．5）indicating thathauserVerlag，Basel，181pp．
frequentrock fallshave occurred．Moreover，glacial
GeologicalSurveyofBhutan（1999）：GlaciersandGlacierLakes
icefacingtheuppermostpart ofthelakedirectlyas
inBhutan．GeologicalSurveyOfBhutan，83pp．
anicewallwasrecognizedonSPOTimages．Alarge
Iwata，SリAgeta，Y．，Naito，N．，Sakai，A．，Narama，Karma（2002）：GlacialLakes and Their Outburst Floodrock falland calvingin the upstream may become
Assessmentin the BhutanHimalaya．GlobalEnvironmen−
triggersofalargesurgewaveand／orasuddenrisein
talResearch，6，3−17．
thelakelevel．
Iwata，S．，Gurung，D．R．andKomori，J．（2003）：ReportonChubda
Tshointhe headwaters of Chamkhar Chhu，Bumthang，
4．Conc山Sions
2002．ReportofJapan−BhutanJointResearch20020nthe
assessment of Glacier Lake Outburst Flood（GLOF）in
Bhutan，InstituteofLowTemperatureScience，Hokkaido
Theresultsofthepresentstudyindicatedthatthe
University；Graduate Schoolof EnvironmentalStudies，
position of the terminus of Chubda Glacier has not
Nagoya University；GeologicalSurveyofBhutan，5466．
Changedduring atleast thelast40years，While theKarma，Ageta，Y．，Naito，NリIwata，S．andYabuki，H．（2003）：
area of Chubda Tsho has expanded constantly，Glacierdistributionin the Himalayas and glacier shrink−
agefrom1963to1993intheBhutanHimalayas．Bulletinof
increasingin size by threefold，during thelast35
GlaciologicalResearch，20，29−40．
years．Thelowerendofthelakeisrelativelystable
Karma，Thapa，T．andGhalley，K．S．（1999）：PreliminaryReportundercurrentconditionsasthethresholdoftheChub−
On Chubda Tshoin the Headwaters of Chamkhar Chhu，
daTsho．However，theexistenceofcollapsescarsand
Bhumthang，Engineering Geology and Glaciology Wing
theremainingtheicecoreintheterminusindicatethe
GeologicalSurveyofBhutan（unpublished），11pp．
potentialdanger of failure．Therefore，thelack of Lliboutry，L．，Morales，B．，Pautre，A．andSchneider，B．（1977）：
Glaciological problems set by the control of dangerous
detailedinformation about therocks andglacialicelakesin Cordillera Blanca，Peru．Ⅰ．Historicalfailures ofaroundtheupperpartofthelakeneedstobeaddres−
morainic dams，their causes and prevention．Journalof
Sed．In conclusion，Chubda Tsho does not pose an
Glaciology，18，239−254．
urgentrisk ofGLOF，butdetailedinspections ofthe
Mool，P．K．，Bajracharya，S．R．andJoshi，S．P．（2001b）：Inventory
threshold and upstream region of the lake by
Ofglaciers，glaciallakesandglaciallakeoutburstfloods、
Nepal．ICIMOD，Kathmandu，363pp．
glaciologists with knowledge of debris glaciers are
Mool，P．K．，Wangda，D．，Bajrachaya，S．R．，Karma，Gurung
neCeSSary．
andJoshi，S．P．（2001a）：Inventoryofglaciers，glaciallakes
andglaciallake outburstfloods，Bhutan．ICIMOD，KathN
Acknowledgments
mandu，247pp．
Takada，M．（1992）：LandformsoftheBhutanHimalaya：Several
issues related to fluvialterracesinintermountane basins．
The authors are high1yindebted to Mr．Dorji
JournalofGeography，101，283297（InJapanese）．
Wangda，DirectoroftheDepartmentofGeologyand
Vuichard，D．and Zimmermann，M．（1987）：The1985catas−
Mines，for his keeninterestinthe study．Wewould
trophic drainage of a moraine．dammedlake，Khumbu
like to express our gratitude to the staff of theHimal，Nepal：CaSeandconsequences．MountainResearch

GeologicalSurvey of Bhutan，andlocalstaffin thefield，fortheirsupportintheinvestigation．

ThepresentresultsrepresentpartoftheBhutan
LJapanJoint Project on Quaternary Geologyin the

Chamkhar Basin，2002．This project was financially
SuppOrtedby a Grant・in−Aidfor Scientific Research（ProjectNo．13373006）fromtheJapaneseMinistryof
Education，Culture，Sports，Science，andTechnology．

andDevelopment，7，91−110．

Watanabe，T．andRothacher，D．（1996）：The1994LuggeTsho
glaciallake outburst flood，Bhutan Himalaya．MountainResearchandDevelopment，16，77−81．
Xu，D．and FengQ．（1994）：Dangerousglacierlakes andtheir
Outburst featuresin the Tibetan Himalayas．Bul1etin of
GlacierResearch，12，1−8．
Yabuki，H．（2002）：InvestigationforexpansionofImjaglacierlake using satelliteimages．Meeting report of theJoint
Research Program of theInstitute of Low Temperature
Science，Hokkaido University，24T30（InJapanese）．