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Quaternary Science Reviews 77 (2013) 156e166

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Quaternary Science Reviews

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A 1000-year history of large floods in the Upper Ganga catchment,central Himalaya, India

R.J. Wasson a,*, Y.P. Sundriyal b, Shipra Chaudhary b,1, Manoj K. Jaiswal c, P. Morthekai d,S.P. Sati b, Navin Juyal d

aAsia Research Institute, National University of Singapore, SingaporebDepartment of Geology, HNB Garhwal University, Srinagar, Uttarakhand, IndiacDepartment of Earth Sciences, Indian Institute of Science Education and Research, Kolkota 741252, IndiadGeoscience Division, Physical Research Laboratory, Navrangpura, Ahmedabad, India

a r t i c l e i n f o

Article history:Received 26 March 2013Received in revised form11 July 2013Accepted 19 July 2013Available online 22 August 2013

Keywords:Landslide lake outburst floodsFlood depositsClimate changeFlood historyHimalaya

* Corresponding author. Tel.: þ65 85351932.E-mail address: arirjw@nus.edu.sg (R.J. Wasson).

1 Present address: Centre for Earth Sciences, Indianlore, India.

0277-3791/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.quascirev.2013.07.022

a b s t r a c t

Determining the frequency, magnitude and causes of large floods over long periods in the flood-proneHimalaya is important for estimating the likelihood of future floods. A thousand year record (withsome information from 2600 years ago) of the frequency and some estimates of velocities and dischargesof large floods has been reconstructed in the Upper Ganga catchment, India, using written reports, litho-stratigraphy and sedimentology, and dated by optical and radiocarbon methods. In the Upper Gangacatchment rainfall triggers large landslides that dam rivers and release large amounts of water whenthey burst, thereby amplifying the effects of rainfall. The large floods in the catchment may be the resultof landslide dam bursts rather than glacial lake bursts, and these are likely to continue and possiblyworsen as the monsoon intensifies over the next century. However preliminary information suggeststhat the recent devastating flood of June 2013 was the result of heavy rainfall not landslide dam bursts.The frequency record is non-random and shows a high frequency between AD 1000 and AD 1300(omitting uncertainties), then a low frequency until a cluster of floods occurred about 200 years ago, thenincreased frequency. This temporal pattern is like but not identical with that in Peninsular India, andboth appear to be the result of variations in the monsoon.

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1. Introduction

According to The World Bank (2011) the economic impact andloss of life from floods has been greatest in the Asia-Pacific regionduring the last 30 years, human vulnerability to floods has beenincreasing, and there is some evidence that small to medium sizefloods have been increasing in cities worldwide. Changes in largefloods are much harder to discern, and some of the change in smalland medium size floods may be the result of increased reporting.Munich Re (2010) however has no doubt that increasing flooddamage is a result of climate change. The most recent report onweather and climate extremes by the IPCC (2012) concludes thatwhile there is a statistically significant trend in the number ofheavy precipitation events, with more increases than decreases

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globally, there is only limited to medium strength evidence ofchanges in the frequency and magnitude of floods at regional scale.This conclusion is the result of data limitations and the confoundingeffects of land use and engineering (cf. Milly et al., 2005).

The IPCC makes the important but obvious point that ‘Extremeevents are rare, which means there are few data available to makeassessments regarding changes in their frequency or intensity’(p.6). Moreover the probability of detecting trends in rare events intypical instrumental records is low (Frei and Schär, 2001). The onlyway to extend the gauged record of floods, upon which the IPCCdepends entirely, is to use sedimentary records that provide ausually lower temporal resolution but longer history than gaugedrecords (Baker, 2008). These centennial tomillennial records can beused to coax history to conduct experiments (paraphrasing theevocativewords of Deevey,1969) to complement the gauged recordin the important task of identifying changes in flood frequency andmagnitude in relation to both past and present climate change.

While it is clear that gauged records have limitations, both intime and space, hampering assessments of flood trends, there arealso insufficient studies of palaeofloods; that is, those floods

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reconstructed from sedimentary archives, which might overlap intime with gauged records. The notable exception in the AsianMonsoon region, a place of large and devastating floods, is thenumber and quality of palaeoflood studies in Peninsular India (for areview see Kale and Baker, 2006). However, the Himalaya has yet toreceive the same attention, despite the occurrence of often-devastating floods and the existence of studies of individualfloods and some flood histories. In an important document con-cerned with adaptation to climate change, particularly in the face ofboth too little and too much water, Pradhan et al. (2012) note thatthe Hindu Kush Himalaya Region (HKH) is a ‘white spot’ withinsufficient scientific information to make forecasts and designintervention and adaptation plans.

Accordingly, the aim of this paper is to provide new palaeoflooddata from a 1000-year record (with some information from 2600years ago) in the flood-prone Upper Ganga catchment, IndianHimalaya in the state of Uttarakhand, set in the context of previousstudies of floods and climate change in the HKH. These data arethen compared with the results of studies from Peninsular India, aclimatic explanation offered, and thoughts provided about thefuture of large floods as climate changes. The emphasis here is onreconstructing flood frequency, although some estimates of veloc-ity and discharge will be provided ahead of planned hydraulicmodelling to provide a more complete picture. Also brief and pre-liminary mention is made of the large June 2013 flood.

1.1. Previous studies of palaeofloods in the HKH

Sedimentary records of large floods extend over periods as longas 104 years in the HKH. Burbank (1983) documented in thePeshawar Basin of Pakistan at least 40 sedimentary units each ofwhich fines upward from sand at the base to mud at the top. TheIndus River deposited each of these units over perhaps 600 � 103

years. Cornwell (1998) concluded that these deposits are at least130 � 103 years old and were the result of glacier lake outburstfloods (GLOFs) from water bodies as large as 32 km3 or perhaps128 km3. However, Burbank (1983) notes that the devastating floodof AD 1841 in the Indus was caused by a landslide dam outburstflood (LLOF) near Gilgit (see Butler et al., 1988 for more detail).Cornwell and Hamidullah (1992) view the flood record in thePeshawar Basin as the result of both GLOFs and LLOFs, and citeprevious estimates of peak discharge for the AD 1841 flood of57 � 103 m sec�1 and 509 � 103 m sec�1.

Also in the Indus catchment Seong et al. (2009) used the size ofboulders deposited from LLOFs to calculate mean entrainment ve-locities (using the methods of Mears, 1979; Costa, 1983) of 9e14 m sec�1 and mean discharges of the order of 104 m3 sec�1. Thesefloods occurred 6.5 � 103 and 1 �103 years ago, and are consistentwith GLOF magnitudes throughout the Himalaya (Richardson andReynolds, 2000). In the Lahul Himalaya Coxon et al. (1996) esti-mated GLOF discharges between 21 and 27 � 103 m sec�1 at 37e43 � 103 years ago.

Montgomery et al. (2004) estimated peak discharges from twoGLOFs near the upper gorge of the Tsangpo River in Tibet:5 � 106 m3 sec�1 at 10.5 � 103 cal BP and 1 � 103 m3 sec�1 at1.4 � 103 cal BP and suggest that such events may be frequent. Theyalso suggest that with calculated unit stream powers of 2 � 105e1 �106 W m�2 for the younger GLOF and 1e5 � 106 W m�2 for theolder GLOF, such events may play a significant role in carving thedeep valleys of the Himalaya. Further evidence of this erosional rolecomes from Pangong Tso on the IndianeTibetan border where abedrock dam, created tectonically, was overtopped by a flow ofsome 110� 103m3 sec�1 about 11�103 years ago during a period ofintensified monsoon activity, increasing valley incision and movingvery large boulders and other sediment (Dortch et al., 2011).

Other studies of GLOFs and LLOFs in the Himalaya (Kuenza et al.,2004; Dunning et al., 2006; Weidinger, 2006; Bajracharya et al.,2007; Worni et al., 2012) provide valuable information, but forthe purposes of this paper comparisons of GLOF and LLOF dis-charges with ‘normal’ monsoon discharges are most valuable.Cenderelli and Wohl (2001) showed that seasonal high flow floods(SHFFs; ‘normal’ climatic floods) in the Mount Everest region were7e60 times less than GLOF discharges, with lower differencesdownstream from breached moraines because of attenuation ofGLOFs and an increase of SHFFs as catchment area increases. Fortet al. (2010) demonstrated a smaller difference of 7 times be-tween LLOFs and monsoonal flows in the Kali Gandaki valley ofNepal where boulders moved into channels by landslides are notmoved by monsoonal flows.

2. Regional setting and palaeoflood deposits

Most of the studies just reviewed are based on calculations ofLLOFs and GLOFs rather than from flood deposits. The exceptionsare the vertically stacked deposits in the Peshawar Basin and theboulders used to calculate velocities and discharges. In contrast thepalaeoflood reconstructions in Peninsular India are based on classicslackwater deposits; that is, deposits forming benches on rivermargins or in tributarymouths. Such deposits provide amuchmorecomplete history of large floods than can be extracted from indi-vidual GLOFs or LLOFs.

The flood deposits and a floodplain sequence, reported here, arein the Upper Ganga catchment in the State of Uttarakhand, India(Fig. 1). This catchment has a total area of 21,730 km2 and is drainedby two major rivers: the Alaknanda and the Bhagirathi. Thetopography and geology are as follows (after Wasson et al., 2008):the Siwalik Hills that have relief of about 1 km consist of sedi-mentary rocks including sandstone and conglomerate, borderingthe Ganga Alluvial Plain; the Lesser Himalaya (LH), upstream of theSiwaliks, consists of metasedimentary rocks and basic volcanics(Ahmad et al., 2000) with mean relief of 1.2 � 0.2 km (Bookhagenand Burbank, 2006); the High Crystalline Himalaya (HCH) withmean relief of 2.1 � 0.2 km and peaks >7 km in altitude consistingof metamorphic rocks with steep slopes, glaciers and glacial lakes,and many landslides; and the Tethyan Sedimentary terrain on theTibetan border. The catchment has a mean annual precipitation of1.4 m, with up to 4 m on the outer mountains and peaks and thehighest rainfall intensity at the LH and HCH fronts. The meanannual river discharge is 1.1 � 103 m3 sec�1 of which 22% is fromsnowmelt, and 75% of the rainfall contribution occurs during theSouthwest Monsoon, the remainder coming from Westerly Dis-turbances during winter (Bookhagen and Burbank, 2010).

At Bhainswara (Figs. 1 and 2) a slackwater deposit lies on the leftbank in a slightly wider part of a bedrock gorge. At Jakhni, 9 kmupstream of Bhainswara, another deposit occurs on the right bankwhere floods are backed-up and reverse flow has been observed(Holland, 1894, p.23) as the river enters the gorge. In addition, afloodplain at Raiwala, between Rishikesh and Haridwar (Fig. 1) and102 km downstream of Bhainswara, provides further information.Finally the results of dating by Srivastava et al. (2008) of palae-oflood deposits near Devprayag (Fig. 1), 33 km downstream ofBhainswara, are combined with the new data to provide a 1000-year history of floods in the Upper Ganga River. The Jakhni,Bhainswara and Devprayag deposits are in the LH, and the Raiwaladeposit is at the foot of the Siwaliks (Fig. 1).

3. Material and methods

Eleven flood units based on physical and sedimentary charac-teristics were identified at Bhainswara. The flood units usually have

Fig. 1. Map of the Upper Ganga catchment showing the major geologic-topographic zones (based on Wasson et al., 2008).

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sharp upper and lower boundaries often with either fine sandy orfine sandy clay in the upper few centimetres (the result of waningflood flows) overlying parallel laminated (upper flow regime),ripple laminated (lower flow regime), or massive fine to mediumsand with some coarse sand (rapid deposition). There are also someintervals within flood units of alternating sand and sandy claysuggesting fluctuations of flow. The sediments are mainly fine sand(Mz ranging from 2.7 to 3.5), well to moderately sorted (sI rangingfrom 0.8 to 0.4), and mostly negatively skewed (SkI ranging

Fig. 2. Topographic map showing the slackwa

from �0.5 to 0.04). The flood units vary between 30 cm and 170 cmin thickness with no thinning trend upward.

Seven of the flood units of Bhainswara have been dated by OSL(Fig. 3, Table 1), following themethod given by Aitken (1998), at thePhysical Research Laboratory, Ahmadabad, India. The samples werecollected in light tightmetal pipes that were opened in subdued redlight in the laboratory and the central part of the sample in the pipeused for dating. Sediment samples were sequentially pretreated toremove carbonates and organic carbon by using 1N HCl and 30%

ter deposits at Bhainswara and Bagwan.

Fig. 3. Litho-stratigraphic and age correlation diagram for the Bhainswara, Jakhni and Devprayag (from Srivastava et al., 2008) deposits. Note that only two palaeoflood deposits arereported from Devprayag which are comparable with units 4 and 8 in the Bhainswara flood deposit.

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H2O2 respectively. Oven dried samples were sieved and the 90e250 mm size was used for the extraction of pure quartz using aFrantz magnetic separator (Model LB1). The outer alpha irradiatedskin was removed using 40% HF for 80 min followed by 12N HCltreatment for 30 min to remove any soluble fluorides formedduring etching. Quartz and feldspar minerals were separated usingthe heavy liquid Sodium Polytungstate. The extracted quartz ismounted as a monolayer on stainless steel discs and the purity of

Table 1Table showing the palaeodose, dose rate and ages obtained on quartz extract from flood

Sample number Depth fromtop (cm)

U (ppm) Th (ppm) K (%)

RaiwalaRW 11 (P3) 20 4.4 � 0.8 16.8 � 2.8 1.7 � 0.1RW 8 (P2) 0 3.9 � 1.0 20.3 � 7.8 2.4 � 0.2RW 6 (P2) 42 2.2 � 0.6 19 � 6 2.5 � 0.2RW 4 (P2) 118 5.4 � 0.5 26.1 � 5.6 2.2 � 0.1RW 3 (P1) 20 3.9 � 0.9 15.1 � 3.1 1.7 � 0.1RW 2 (P1) 95 5.0 � 0.8 12.1 � 2.8 1.3 � 0.1RW1 (P1) 165 3.6 � 0.7 14 � 2 1.9 � 0.1BhainswaraUnit 1 30 6.8 � 0.4 14.5 � 0.7 2.6 � 0.3Unit 2 80 5.3 � 0.3 15.5 � 1.2 2.3 � 0.1Unit 4 150 4.2 � 0.2 14.0 � 0.7 2.4 � 0.1Unit 6 350 5.3 � 0.2 10.8 � 0.5 2.3 � 0.1Unit 7 420 3.1 � 0.1 14.3 � 0.7 2.2 � 0.1Unit 10 590 10.0 � 0.6 24.9 � 9.0 2.5 � 0.3Unit 11 650 7.6 � 0.4 27.4 � 1.4 3.4 � 0.1

the quartz checked with Infrared Stimulation (IR). No IR signal wasdetected showing that the samples are pure quartz free from anysignificant contamination from feldspar.

The single aliquot regenerative dose (SAR) procedure of Murrayand Wintle (2000) was used to obtain equivalent doses using theRisø TL-DA-15 instrument (Boetter-Jensen et al., 2003). Opticalstimulationwas achieved with photons of wavelength 470� 30 nmusing blue LEDs and the stimulated luminescence emission photons

deposits at Raiwala and Bhainswara.

Least Palaeodose (Gy) Dose rate (Gy/ka) Age (in ka for Raiwala andin years for Bhainswara)

2.5 � 0.4 3.2 � 0.3 0.8 � 0.13.5 � 0.8 3.9 � 0.6 0.9 � 0.22.5 � 0.3 3.6 � 0.4 0.7 � 0.111 � 2 4.3 � 0.5 2.6 � 0.53 � 1 3.2 � 0.3 1.0 � 0.4

2.7 � 0.5 2.7 � 0.3 1.0 � 0.28.2 � 1.6 3.1 � 0.3 2.6 � 0.6

0.13 � 0.09 4.6 � 0.2 28 � 200.40 � 0.27 4.3 � 0.2 114 � 280.92 � 0.32 4.2 � 0.2 217 � 760.9 � 0.3 3.9 � 0.2 232 � 781.5 � 0.3 6.4 � 0.7 237 � 573.4 � 0.6 6.9 � 0.3 490 � 883.0 � 0.3 5.0 � 0.2 596 � 65

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were detected using a PMT (EMI 9235QA) and a U-340 optical filter.A preheat temperature of 180 �C for 60 s was optimized from thepreheat plateau plot. Good recycling ratios and low recuperationwere found. In order to ascertain the dose response, a few sampleswere subjected to a dose recovery test and the ratio of the given torecovered dose was near unity (1.00 � 0.14). The optimization ofequivalent dose was achieved using the minimum age model(MAM3) except for Unit 10 which was analysed using the centralage model (CAM) (Galbraith et al., 1999). The U, Th and K concen-trations for dose rate estimation were measured in a High PurityGermanium detector (HPGe). The cosmic ray component of thedose rate was estimated by using the method of Prescott andHutton (1994). Table 1 gives the details of the optical ages obtainedon different flood units.

In addition, small hard fragments of charcoal from two unitshave been radiocarbon dated by AMS at the Australian NationalUniversity using the method described by Fallon et al. (2010). Fromunit 3 both OSL and 14C dates are available for comparison (Fig. 3).The average relative uncertainty for the OSL dates is 31% but themost probable calendar ages (AD) are provided without un-certainties for ease of presentation. The oldest unit (11) has an OSLage of 596 � 65 years (a most probable calendar age of AD 1413)and the youngest unit (1) 28 � 19 years (AD 1980 with a rangebetween AD 1961 and AD 1999). The youngest unit is confidentlyascribed to the flood of AD 1970 on the basis of its sedimentarycharacteristics (see Wasson et al., 2008) and the clear recollectionsof the local villagers. There have not been any floods since 1970 andbefore 2013 that have overtopped the 1970 deposit at Bhainswara.Data provided by the Department of Irrigation shows that the peakstage at Srinagar of the 1970 flood was 538 m ASL on 20th July. Thepeak stages of the largest floods in 1979, 1995 and 2010 were be-tween 2 and 3 m lower than the 1970 peak. These floods appear tohave eroded the deposit at Bhainswara leaving no sedimentaryrecord. However, the flood of 17th June 2013 had a peak stage of541 m ASL, 3 m higher than the 1970 peak at Srinagar and 4 mhigher at Bhainswara. The 2013 flood has deposited several metresof sediment on the 1970 deposit at Bhainswara. Unit 2 is ascribed tothe flood of AD 1894 on the basis of its most probable OSL date (AD1896) and a lack of evidence in local records of any large floodsbetween the well-documented event of 1894 and the 1970 flood.Where either OSL or 14C ages are not available, the ages of units 3,5,and 8 have been estimated by assuming that they occurred midwaybetween the dated units above and below.

To infer the likely triggering mechanisms of the floods theprovenance of some of the flood sediments has been approximated.If all floods came from the HCH then GLOFs could have createdthem all. Wasson et al. (2008) determined the rare earth tracer3Nd(0) at Bagwan, a site close to Bhainswara (Fig. 2), for the AD1970 deposit. Three pulses of sediment appear to have occurred atBagwan with sediment from the HCH varying from 45 to 73%; theremainder came from a LLOF in the LH in the Birehi Ganga sub-catchment. Unit 4 has the same age as a deposit at Srinagar (Fig. 1)with a HCH content of 67% suggesting a source higher in themountains than in unit 1. Even though geochemical data are notavailable for unit 2 it is thought that most of this sediment alsocame from the LLOF in the Birehi Ganga subcatchment (Holland,1894).

4. Results

4.1. Bhainswara

The top of the deposit is at an elevation of 511 m ASL, and 12 mabove and 60 m distant from low water in the Alaknanda River,forming a bench that slopes downstream and which has been

eroded on its river-facing margin by flood flows that apparentlyoccurred between 1970 and 2013. There are 11 clearly identifiableflood units resting on cobble gravel with a sandy matrix that ex-tends beneath the flood deposits (Fig. 3). Large boulders ofquartzite and phyllite with intermediate axes ranging from 0.74 mto 1.7 m lie on the exposed part of the alluvial gravels near the edgeof the low water channel. These boulders have been transporteddownstream by floods, evidenced by crescentic percussion markson the surfaces of the quartzite boulders, their sub-rounded form,and because quartzite is not a rock found in the gorge walls butcomes from much further upstream.

Thin units (30e35 cm) occur at both the bottom and near thetop. This suggests that while censoring of small floods probablyoccurred as the stack of flood deposits accumulated, it is plausiblethat large floods are all represented. Some units dip toward theriver at w10� suggesting that at times the deposit was a slopingmound.

4.2. Jakhni

The exposed part of this deposit is 9 km upstream of Bhains-wara, at most 4.6 m thick and rests on a phyllite bench that is onaverage 10 m above low water in the Alaknanda River. The base ofthe section could not be exposed but it is likely that there is at leastone more metre of flood sediments below the units documented inFig. 3. The sediments and sedimentary structures at Jakhni andBhainswara are sufficiently similar to allow correlation (Fig. 3).Given that there appears to be one metre unexposed at Jakhnibelow unit 8 and there is a little over one metre of flood sedimentsbelow unit 8 at Bhainswara, it is likely that all 11 units occur at bothsites. Sub-rounded quartzite boulders also occur at Jakhni on thebench 10m above lowwater, with intermediate diameters between0.75 and 0.99 m, and are buried by the slackwater deposits andwere deposited before the oldest unit that is dated to AD 1413 atBhainswara. Phyllite and quartzite boulders with intermediate axesbetween 1.43 and 1.9 m lie on a bedrock bench about 2m above lowwater mark. All quartzite boulders have crescentic percussion scars,showing that they have been transported, a conclusion supportedby noting that the local bedrock is phyllite.

4.3. Raiwala

About 100 km downstream of Bhainswara on the right bank ofthe Ganga River, partially protected by a remnant of a gravel-richalluvial fan, is a small floodplain that abuts the fan. The flood-plain is about 250 mwide and a transect was measured across it atright angles to the direction of flow in the river and three pitsexcavated (Fig. 4). The highest point on the floodplain surface isabout 3 m above lowwater in the river. The basal sediments consistof pebble gravel with a medium to coarse sand matrix in the twopits closest to the river, and medium sand in the distal pit, all ofalluvial origin. OSL dates were obtained using the method appliedat Bhainswara and in the same laboratory. Two identical OSL datesfrom this unit show it to be 2.6� 0.5 ka. Above this unit in the distalpit are three flood units between 80 and 110 cm thick, grading frommedium/coarse sand (with faint laminations) at the base to siltyfine sand at the top. OSL dates show that these units are all about1000 years old. Stratigraphically above these units are three sets offlood units, with three to five individual flood units in each set, thatrange in age from about 700 to 900 years old. Each flood unit gradesfrom medium/coarse sand at the base to silty fine sand at the top.All units above the gravel layer are fining upward in the way ex-pected in flood deposits. About 700 years ago deposition stoppedeven though the local people tell of floodwaters infrequentlyovertopping the floodplain that may have only added small

Fig. 4. Litho-stratigraphic and age correlation diagram for the Raiwala deposits.

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amounts of fine sediment to the surface. Major deposition probablyshifted to other sites on the river but this explanation needs to betested by examining other deposits in this reach. With current in-formation the absence of young deposits cannot be taken as evi-dence of cessation of flooding.

The two uppermost flood units dated to about AD 1000 at Rai-wala have 3Nd(0) values between �15.41 � 0.14 and �17.14 � 1.62or 49e57% HCH sediment (see Wasson et al., 2008 for methods).These values are similar to the mid range in the AD 1970 deposit.

4.4. Devprayag

Srivastava et al. (2008) used OSL dating and the Chemical Indexof Alteration to estimate the age and provenance respectively oftwo sets of palaeoflood deposits that lie unconformably on alluvialterraces near Devprayag, downstream of the junction of the Ala-knanda and Bhagirathi rivers that forms the Ganga River. The olderset, which lies about 18 m above lowwater, is 1200 � 200 years oldand consists of 40% local phyllite near the base and 60% phyllitenear the top of the deposit. The younger set, which lies about 9 mabove the river, is 320 � 60 years old at the base with 70e85% HCHcontent and 209� 44 years old at the top with 40% HCH content. AtSrinagar, upstream of Devprayag (Fig. 1), the 200 year old deposithas 67% HCH content (Wasson et al., 2008). These deposits (andthose at Raiwala) are different from those at Bhainswara and Jakhnias they consist of many small flood couplets and therefore can onlybe compared with the other data as phases of flooding rather thanas single flood events.

5. Discussion

5.1. Chronology of flooding

The data from Bhainswara, and therefore Jakhni by correlationand Devprayag are compiled in Fig. 3. The AD 1970 flood (unit 1) issecurely identified by OSL and sediment characteristics, as is the AD1894 flood (unit 2) based on OSL. The age of the AD 1844 flood (unit3) is interpolated and, given the dating uncertainties, it may havebeen the result of the AD 1868 LLOF reported by Glass (1896). TheAD 1792 flood (unit 4) is based on OSL and appears to correlate withflooding at Devprayag, andmay have been the result of the AD 1803

LLOF reported by Glass (1896). The estimated age of the AD 1784flood (unit 5) is interpolated and the AD 1777 flood (unit 6) is basedon OSL with support from a 14C age. The AD 1772 flood (unit 7) isalso based on OSL and the AD 1725 flood (unit 8 at Bhainswara) isbased on a 14C age that appears to correlate with flooding at Dev-prayag that is dated by OSL to AD 1686. The OSL age is likely to bemore accurate than the 14C age given that charcoal can take sometime to be entrained and deposited in flood sediment and thereforewill be older than the mineral sediment, as seems to be the case inunit 8. The age of unit 9 (AD 1622) is interpolated while the ages ofthe AD 1519 (unit 10) and AD 1413 (unit 11) floods are based on OSL.The most uncertain age is for the AD 1622 flood, given that it isinterpolated between dates 206 years apart, but this does not havea serious impact on the interpretation of flood frequency. The nextmost uncertain age is for the AD 1844 flood, being interpolatedbetween ages 52 years apart. The only other interpolated age, AD1784, is between ages only 15 years apart. The age of AD 1725 maybe too old for the reasons discussed above, and so the age of AD1686 is adopted.

The older flood units at Devprayag and Raiwala cluster between700 and 900 years and around 1000 years ago, based on OSL. Theremay be a gap in ages between about 600 years (unit 11) and 700years but this will only be certain if a more complete stack of flooddeposits is found. It is noteworthy that the three floods (units 11,10and 9) that occurred after both the cluster of floods ending about700 years ago and the putative gap, and before a major change offrequency occurred about 200 years ago, have a mean age spacingof 104 � 1 years which is the same as the so-called gap. The gaptherefore may not be real.

5.2. Provenance of the flood deposits

The provenance estimates for the Bhainswara and Devprayagdeposits are provided in Fig. 3. Unit 8 is most likely from the HCH,with 15e30% LH sediment entrained as the flood moved down-stream. Unit 4 is likely to have mostly come from the HCH given itsgeochemistry at Srinagar, and, if so, the flood entrained a further27% of LH sediment before it reached Devprayag. The AD 1970deposit is known to have mostly come from the LH, suggesting thatthe high value of 73% HCH material is the result of entrainment ofpreviously deposited HCH material as the flood moved

Fig. 5. Velocity and water surface elevations for the 1894AD flood in the Alaknanda-Ganga River (based on Holland, 1894).

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downstream. The provenance of the 1000-year-old flood deposit atRaiwala is the same as the modern river sediment (Wasson et al.,2008), that is, about half HCH and half LH.

It is therefore likely that some of the flood deposits and thewater thatmoved them came from the HCH and others from the LH.As shown byWasson et al. (2008) sediment from the narrow belt ofthe Siwalik foothills has not added detectable sediment to themodern river and therefore is unlikely to have added to the Raiwaladeposits. While not entirely secure, this inference suggests that allof the floods recorded at Raiwala came fromwell upstream and notfrom near the mountain front.

It is recognized that these results are uncertain because ofmixing of sediments by entrainment of sediments of different ori-gins deposited along the valley. A more detailed analysis is requiredbased on a more thorough sampling of both the main channel andmajor tributaries. The data and interpretations presented here aretherefore preliminary.

5.3. Velocity and discharge estimates

As noted earlier the research reported here is focused on floodfrequency rather than magnitude, which must await hydraulicmodelling. Nonetheless some velocity and discharge estimates canbe made in advance of the modelling. Movement of the bouldersdescribed at Bhainswara and Jakhni requires high streamvelocities.Using the method of Costa (1983) a mean flood velocity of5.8 � 0.4 m sec�1 at Bhainswara is calculated from the mean in-termediate axes of boulders for a time before deposition of unit 11(i.e. >600 years ago) given that similar boulders do not occur in thesandy flood deposits. The average velocity needed to overturnboulders at Bhainswara is 5.1 � 0.3 m sec�1 using the method ofMears (1979). To move the boulders on the bench 10 m above lowwater at Jakhni the average velocity is 4.8 � 0.16 m sec�1 and theaverage overturning velocity is 5.2 � 0.3 m sec�1, occurring beforethe deposition of the sandy flood deposits, as at Bhainswara. On thebench 2 m above low water at Jakhni the required mean velocity is6.6 � 0.8 m sec�1 and the overturning velocity is 5.6 � 0.8 m sec�1.

High velocities therefore occur in this river and observationsduring the AD 1894 flood supporting observations made during the1894 flood. Holland (1894) reported maximum water height at 10locations along the Alaknanda-Ganga River above the ‘normalflood’ level, although sometimes he refers to height above theriverbed or above an arbitrary datum. The reports following theflood of AD 1894 focus on a cause in a LLOF at Gohna Tal (lake) in theBirehi Ganga subcatchment (Fig. 1), an event that deposited unit 2at Bhainswara and Jakhni. Although there are inconsistencies in thereport by Holland (1894), it appears that the lake discharged283 � 106 m3 in 4.5 h at an average rate of 17.5 � 103 m3 sec�1. Theheights and velocities are shown in Fig. 5. Just below the lake theflood rose 85 m with a mean velocity of 12 m sec�1 and these twoquantities fell downstream and varied presumably as the channeland valley widened or narrowed. At Chamoli the valley widens andthe height and velocity of the flood fell. At Srinagar the valleywidens more and both water height and velocity fell. Just down-stream of Srinagar, in the vicinity of the Bhainswara and Jakhnislackwater deposits, the water was about 8 m above ‘normal flood’level and the velocity was about 7 m sec�1, based on interpolationof Fig. 5. But this estimated water height is too low by comparisonwith the height of unit 2 at 11 m above low water at Bhainswaraand 14 m at Jakhni.

There are several possible reasons for this apparent discrepancy.First, the elevation of the ‘normal flood’ or riverbed is not reportedby Holland (1894) and if the height difference was that betweenindicators of stage and the bed of the river, once the flood hadreceded, the reported water heights may have been affected by

sedimentation in the channel during the AD 1894 flood. Second,linear interpolation between measured heights may be invalidgiven the topographic and hydraulic complexity of the transitionfrom thewide valley at Srinagar to the backwater area at Jakhni andthe gorge at Bhainswara. Even so there is a strong relationshipbetween the reported heights and velocities in the form:Velocity ¼ 2.44 þ 3.66 (log10Height), r2 ¼ 0.72. This relationshipsuggests that the differences between the estimated heights of theflood and the elevations of unit 2 are the result of uncertainty in thedatum used for the height estimates.

One other discharge estimate is available. Between Gohna Taland Chamoli the peak discharge was 53,000 m sec�1 with anaverage velocity of 8 m sec�1 based on information provided byHolland (1894) and repeated by Glass (1896). The velocity is thesame as the value of 7.8 � 0.3 m sec�1 calculated from boulders inthe flood deposit of AD 1970 at the mouth of the Birehi Ganga.

It is not possible at this stage to assign discharges to each of theslackwater flood deposits. But what is clear is that each of the de-posits was the result of a large flood, given that at Jakhni the waterneeds to reach >10 m above low water to deposit sediment. It istherefore unlikely that any of the deposits at Jakhni or Bhainswarawas the result of ‘normal’ monsoon flood flows, a conclusion thatprobably also applies to the deposits at Devprayag despite thecautions of Srivastava et al. (2008).

5.4. Frequency and causes of the floods

The best estimates of the ages of floods recorded by sediments atBhainswara (and therefore Jakhni), Devprayag, and Raiwala havebeen plotted cumulatively in Fig. 6 along with phases of alluviationon the Peninsular and on the Ganga Plain (Kale, 2007). The Raiwaladata are shown for the period between 700 and 900 years ago andfor 1000 years ago as two groups because the temporal resolution isinsufficient to determine the age of individual floods. The cumu-lative curve does not increase uniformly but shows two rapid in-creases: at about 200 and 700 years ago. Because of the differingtemporal resolutions only the floods less than 600 years can betested statistically for randomness. The one sample runs test(Siegel, 1956) has been applied to the occurrence of floods in 50-year periods, beginning in AD 1970 when the latest deposit

Fig. 6. Cumulative plot of palaeofloods in the Upper Ganga catchment, with the timingof alluviation in Peninsular India and the Ganga Plain (from Kale, 2007), and showingthe period of few palaeofloods during the LIA on the Peninsular (after Kale and Baker,2006).

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occurred, ignoring the 2013 flood until further information isavailable. The null hypothesis that the series is random is rejectedat p ¼ 0.025. While the runs test is sensitive to the order of objects,it is not sensitive to the frequency of occurrence. Therefore a one-sample c2 test was used. Because of the small number of valuesin each 50-year category it was necessary to group data and reducethe number of categories to giving only one degree of freedom. Thenull hypothesis that the frequency of floods is equal through time isrejected at p ¼ 0.05. This test is probably less robust than the runstest given the need for grouping of data, but both tests show thatthe changes visible in the cumulative plot of flood numbers are realand therefore deserve explanation.

To explain the patterns in Fig. 6 it is necessary to understand thecause(s) of the floods. It has been shown that the flood of AD 1894was the result of a LLOF, as was the flood of AD 1970 (Wasson et al.,2008), in the Birehi Ganga subcatchment (aided by deforestation)along with LLOFs further up the Alaknanda valley in the tributaryvalleys of the Patal Ganga and Dhauli Ganga in particular. It is alsopossible that the floods of AD 1803 and AD 1868 were the result ofLLOFs in the same subcatchment. It is little wonder that BirehiGanga is known as the valley of sorrows! Noting the conclusionreached above that all of the floods younger than 600 years musthave been large, and that only GLOFs or LLOFs seem to have thecapacity to produce such events (see the review of relevant litera-ture above), it is possible that all of the<600 year old flood depositsare the result of natural dam bursts. While the same conclusion canbe extrapolated to the deposits >600 years old it is less securebecause there are no estimates of the magnitude of floods at Rai-wala. Also the 2013 flood was not a result of a LLOf or GLOF, so theproffered cause of the floods younger than 600 years is not secure.

The origins of the landslides are also of interest. Holland (1894)attributed the Gohna landslide of AD 1893 to heavy and sustainedmonsoon rainfall on a geologically unstable hillslope undercut bythe river. The landslides that led to LLOFs in AD 1970 were also theresult of monsoon rainfall (Gupta,1974;Wasson et al., 2008), as wasthe major landslide in the Mandakini subcatchment of the UpperGanga in 1998 (Gupta and Chaisgaoankar, 1999). While none ofthese reports mention earthquakes as causes of landslide dams,ground motion could be a cause. There have been 7 earthquakes ofMW > 6.4 in the area since AD 1500, the largest of which wasMW ¼ 8.1 in AD 1803 (Bilham and Ambraseys, 2005) at the same

time as a LLOF in the Birehi Ganga. However, the temporal patternof earthquakes in or near the catchment does not mirror the historyof floods.

Based on the provenance estimates only the flood of AD 1725could have been the result of a GLOF given that 70e85% of thesediment in this event came from the HCH. Others for which noprovenance estimates are available (units 2, 5, 6, 7, 10, and 11) couldhave come from GLOFs but unit 2 did not. Along with unit 2, units 1and 4 and the 1000 year old floods at Raiwala could have been theresult of LLOFs in the Lesser Himalaya, an idea that requires furthertesting.

The average interval between floods is 55 years during the past600 years and only 8 years between AD 1772 and AD 1803. Korupet al. (2010) have estimated the life spans of natural dams (bothglacial and landslide) in the valleys of the Indus and Tsangpo Rivers.For a dam of the size of that at Gohna Tal (300 m high), but in acatchment of low relief, life spans are between <10 and 200 years.In areas of high relief, such as the Birehi Ganga subcatchment, thelife span is <1 year. These life span estimates are consistent withthe frequency of large floods over the past 600 years in theAlaknanda-Ganga River.

5.5. History of flood frequency and climate change, and the future

It has been argued above that only natural dam breaks canproduce the very large floods recorded by the slackwater depositseither from LLOFs or GLOFs. Landslide dams could be the result ofeither rainfall or earthquakes, although the evidence providedabove suggests that earthquakes are not a common cause, whileglacial dams are the result of glacial retreat. If it can be shown thatflood frequency changes with climate then landslide dams are alsosimilarly controlled, with a small delay between their formationand destruction. If GLOFs have been sources of large floods thenclimate variations could also play a role. The conclusions about arole for LLOFs must now be considered preliminary as a result ofinformation about the 2013 flood for which there is no evidence ofa role for LLOFs.

The Upper Ganga record is now compared with that of theIndian Peninsula where there is a clear climate signal in the fre-quency of palaeofloods. Then a comparison is made with inde-pendent palaeoclimate and glacial records. Kale and Baker (2006)showed that large floods in Peninsular India were common beforethe Little Ice Age (LIA) at the time of the Medieval Warm Period(better described as the Medieval Climate Anomaly, MCA; Diazet al., 2011) along with widespread alluviation both on thePeninsula and the Ganga Plain (Kale, 2007), and after the LIA.During the LIA large floods either did not occur or were very rare.An average large flood frequency of 34 years in the Upper GangaRiver has occurred since the end of the LIA, during the LIA it was57 years, and before the LIA 21 years. This shows a temporalpattern like that of the Peninsula as can be seen in the cumulativeplot of flood numbers (Fig. 6). The long-term pattern therefore hasa climatic explanation that also appears to be partially reflected inthe glacial record. An advance at w1000 years, the Gangotri Stageof Barnard et al. (2004) which these authors suggest is the resultof increased penetration of the monsoon into the mountainsduring the MCA, coincides with major floods at Raiwala. But GLOFsdo not occur during advances so this cannot be the explanation ofthese floods. By contrast, the floods between 1000 and 700 yearsago could be the result of GLOFs as the glaciers retreated. A furtheradvance, the Bhujbas Stage, is correlated with the LIA (also seeNainwal et al., 2007). The timing of the LIA advance is uncertain,but may have occurred before the cluster of floods between AD1772 and AD 1803. If so GLOFs may have occurred followingretreat. But there is no evidence of former large glacial lakes in this

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catchment; therefore GLOFs seem the least likely source of thefloods. Interestingly there are currently 86 small glacial lakes witha total area of only 1.81 km2 in the Upper Ganga catchment andnone is considered ‘dangerous’ (Ives et al., 2010). These conclu-sions are supported by the results of Worni et al. (2012) althoughthey conclude that about half of the small number of glacial lakescould become critical.

Several proxy records of rainfall and the water balance areavailable. Tree rings from Uttarakhand (and Himachal Pradesh)show that spring rainfall was low when floods were frequentduring the latter part of the LIA (Singh et al., 2006, 2009), but largefloods are likely during the monsoon rather than in spring. Theflood cluster occurs during a transition from cool/moist conditionsto warm/wet conditions recorded by diatoms and pollen in peat inthe Pinder Valley (Rühland et al., 2006). The increased moistureduring the last w100 years is not reflected at the nearest meteo-rological station and the change in the peat is therefore attributedto runoff from melting of local glaciers and snow in a warmer at-mosphere. But precipitation is highly spatially and temporallyvariable in this complex terrain as shown by the w20 year fluctu-ations of spring rainfall since AD 1560 (Singh et al., 2006),increasing monsoon and annual precipitation from AD 1902 to AD1964, then a decline to AD 1980 with a few high altitude stations(e.g. Joshimath in the upper Alaknanda catchment) showing anincrease throughout the period of record in Uttarakhand (Basisthaet al., 2008), and spatially variable increases and decreases in 1-day,5-day, and 30-day rainfall events in the northwest Himalaya (Royand Balling, 2004). Currently, the palaeoprecipitation record inthe Upper Ganga catchment is inadequate for comparison with theflood record.

Departing from the use of proxy records in or close to the UpperGanga catchment, a southwest monsoonwindspeed proxy from theArabian Sea, that is suggested correlates with rainfall, has a peak atthe time of the clustered floods in the Upper Ganga (Anderson et al.,2002) which is most likely to have occurred between AD w1715and AD w1868 (Fig. 3). This explanation provides a plausible ifpartial account of the cluster as a result of higher monsoon rainfall.Proxy-based windspeeds continued to increase after a brief declineAD w1800. Nonetheless there is a general correlation with a floodevery 34 years on average since the end of the LIAwhenwindspeedand rainfall were on average high, and every 57 years during the LIAwhenwindspeed and rainfall were on average low. The pre-LIA partof the windspeed record shows higher values from AD w1150falling to a low at AD w1600. Other proxies (Thamban et al., 2007)provide evidence of substantial monsoon enhancement at ADw1000. It is therefore concluded that the variations of large floodfrequency in the Upper Ganga catchment are controlled by varia-tions of monsoon precipitation, as they appear to be in thePeninsula.

Rupa Kumar et al. (2006) and Rajendran and Kitoh (2008) haveused high-resolution modelling to project increased monsoonprecipitation, by up to 40%, and increased rainfall intensity in theUpper Ganga catchment toward the end of the twenty-first century.But Turner and Annamalai (2012) raise serious doubts about theskill of current models of the South Asian monsoon. Also, climaticprojections may be complicated by aerosols, particularly blackcarbon (Lau et al., 2006; Meehl et al., 2008), but the effect iscurrently uncertain. LLOFs are nonetheless likely to continue even ifthe climate does not change and possibly increase in the comingcentury. The likelihood of GLOFs in the future is harder to deter-mine. Glaciers in the Himalaya are showing complex patterns ofbehaviour, with some retreating, others stagnant, and someadvancing (Scherler et al., 2011). Glacial futures and therefore theformation of GLOFs will depend upon the balance between tem-perature and precipitation changes, topography, and also the

amount of insulating debris on the ice. The apparent lack of a rolefor GLOFs as the cause of large floods during the past millenniumand the current absence of ‘dangerous’ glacial lakes in the UpperGanga catchment does not necessarily mean that they will notoccur in the future. As a consequence of the 2013 flood heavy andsustained rainfall, without either LLOFs or GLOFs, as a cause of largefloods must be considered seriously.

5.6. Sediment transport

The large floods generated by GLOFs and LLOFS also carry largeamounts of sediment that may dominate the sediment yield inthe Himalaya (Brunsden and Jones, 1984). The AD 1970 floodtransported about 15.9 � 106 t of sediment (Kumar and Shone,1970) in about one day. Chakrapani and Saini (2009) estimateda mean annual suspended sediment load at Rishikesh of9.1 � 106 t that may be 70% higher if bedload is included (Wulfet al., 2010). The flood of AD 1970 therefore may have moved themean annual load. It seems that large floods move a large fractionof the annual sediment load in this area, a result consistent withthe conclusions of Korup (2012) that yields from LLOFs are secondonly to post volcanic eruption yields but are greater than yieldsfrom GLOFs.

6. Conclusions

A 1000 year history of floods has been reconstructed fromslackwater and floodplain deposits in the Upper Ganga catch-ment. All of the floods were large, with one estimate for the AD1894 flood of 53 � 103 m sec�1. One process capable of generatingsuch large floods is the bursting of natural dams caused by eitherlandslides or glaciers. There is no convincing evidence for a rolefor glacial outburst floods (GLOFs), and the history of earthquakesdoes not match the history of floods. It is therefore concludedthat rainfall triggered landslide dams, and their rupture aslandslide dam outburst floods (LLOFs), played a significant role inflood generation. Also sustained and large amounts of rainfallwere the apparent trigger for the 2013 flood and may havetriggered other floods recorded in the slackwater and floodplainsediments.

Of the 25 large floods that occurred over the past 1000 years(at an average interval of 40 years, omitting uncertainties), 14occurred between AD 1000 and AD 1300 (with an average intervalof 21 years, during the Medieval Climate Anomaly), 8 between AD1300 and AD 1800 (at an interval of 63 years during the Little IceAge), and 3 between AD 1800 and AD 1970 (every 57 years onaverage, and 53 years when the 2013 flood is included). During theLIA there was a cluster of large floods between AD 1772 and AD1803.

Comparison between the flood history and the record of largefloods in Peninsular India, and with independent palaeoclimaterecords, shows that changes of flood frequency were caused byvariations of monsoon rainfall. The cluster of floods w200 yearsago occurred at a time when the monsoon appears to have beenat a peak, but the windspeed proxy from which the monsoonpeak is inferred increased even more after this time without acommensurate increase in flood frequency. Therefore the clustercannot be fully explained. Projections of the climate for the endof the current century indicate possible enhancement ofmonsoon rainfall and rainfall intensity in the Upper Gangacatchment, although the basis for the projection is not strong.Nonetheless it is likely that large floods generated by LLOFs, andheavy rainfall as seen in 2013, will continue, and may increase infrequency.

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Acknowledgements

Professor M. McCulloch, University of Western Australia, pro-vided the 3Nd(0) data for Raiwala. Lee Li Kheng drew the figures.RJW thanks Charles Darwin University, Australia, for financialsupport. YPS and SC are thankful to DST for financial support videgrant no. SR/S4/ES-139/2005. NJ, MJ and PM thank the PhysicalResearch Laboratory, Ahmadabad for financial support. We alsothank an anonymous referee for helpful comments.

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