12
Hydrological Aspects of Alpine and High Mountain Areas (Proceedings of the Exeter Symposium, Juiy 1982). IAHS Publ. no. 138. Natural dams and outburst floods of the Karakoram Himalaya KENNETH HEWITT Wilfrid Laurier University, Waterloo, Ontario Canada N2L 3C5 ABSTRACT Glacier dams and outburst floods ("jokulhlaups") have been reported in many glacierized mountain regions, and may create hazards for human populations. Specially large and dangerous examples occur where the rivers of extensive ice-free zones are blocked. This hydrological anomaly has been rare in modern times except for two areas: the southern Alaska-Yukon ranges and Karakoram Himalaya. In the Karakoram some 30 glaciers may form substantial dams on the Upper Indus and Yarkand river systems. Many more interfere with the flow of rivers in a potentially dangerous way. There is evidence of some 35 disastrous jokulhlaups since 1826. Rarer landslide dams have resulted in the largest dam-burst floods. The paper provides a record of known dams and related events, and identifies the glaciers involved. It indicates the role of the regional environment in the widespread potential for these glacier dams and catastrophic outbursts. Some data are given on the dimensions of past dams and the nature and impact of the flood waves. No dams were reported from the mid 1930's until 1978 when satellite imagery showed a 6 km glacier lake on the Upper Yarkand. The absence of dams in recent decades relates to a general glacier recession here. Renewed activity creates serious problems for water resource development and settlement growth that occurred in the recent, unusually favourable period. INTRODUCTION Bodies of water ponded by glaciers are common in most glaciated regions. They can range in size from small ice-marginal pools to the enormous glacier lakes dammed by certain Pleistocene ice sheets. A particularly large and dangerous dam occurs where a glacier enters and blocks a major river valley of which it is tributary. Evidently, however, these are an anomaly within the usual climatic hydrological and geomorphic progression downstream in a drainage basin. Rare and isolated cases have been reported from the southern Andes, the Canadian Arctic islands, the Caucasus, some Central Asian ranges and also the European Alps and Norway during the Little Ice Age. In one region of the world, however, the Karakoram Himalaya and neighbouring ranges, there has been a substantial number of these main valley glacier lakes in modern times. Outbursts from a series of dams on the Upper Shyok between 1926 and 1932 brought devastating floods along more than 1200 km of the Indus. Some even larger 259

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Page 1: Natural dams and outburst floods of the Karakoram …hydrologie.org/redbooks/a138/iahs_138_0259.pdf · Natural dams and outburst floods of the Karakoram Himalaya ... dams have resulted

Hydrological Aspects of Alpine and High Mountain Areas (Proceedings of the Exeter Symposium, Juiy 1982). IAHS Publ. no. 138.

Natural dams and outburst floods of the Karakoram Himalaya

KENNETH HEWITT Wilfrid Laurier University, Waterloo, Ontario Canada N2L 3C5

ABSTRACT Glacier dams and outburst floods ("jokulhlaups") have been reported in many glacierized mountain regions, and may create hazards for human populations. Specially large and dangerous examples occur where the rivers of extensive ice-free zones are blocked. This hydrological anomaly has been rare in modern times except for two areas: the southern Alaska-Yukon ranges and Karakoram Himalaya. In the Karakoram some 30 glaciers may form substantial dams on the Upper Indus and Yarkand river systems. Many more interfere with the flow of rivers in a potentially dangerous way. There is evidence of some 35 disastrous jokulhlaups since 1826. Rarer landslide dams have resulted in the largest dam-burst floods. The paper provides a record of known dams and related events, and identifies the glaciers involved. It indicates the role of the regional environment in the widespread potential for these glacier dams and catastrophic outbursts. Some data are given on the dimensions of past dams and the nature and impact of the flood waves. No dams were reported from the mid 1930's until 1978 when satellite imagery showed a 6 km glacier lake on the Upper Yarkand. The absence of dams in recent decades relates to a general glacier recession here. Renewed activity creates serious problems for water resource development and settlement growth that occurred in the recent, unusually favourable period.

INTRODUCTION

Bodies of water ponded by glaciers are common in most glaciated regions. They can range in size from small ice-marginal pools to the enormous glacier lakes dammed by certain Pleistocene ice sheets. A particularly large and dangerous dam occurs where a glacier enters and blocks a major river valley of which it is tributary. Evidently, however, these are an anomaly within the usual climatic hydrological and geomorphic progression downstream in a drainage basin. Rare and isolated cases have been reported from the southern Andes, the Canadian Arctic islands, the Caucasus, some Central Asian ranges and also the European Alps and Norway during the Little Ice Age.

In one region of the world, however, the Karakoram Himalaya and neighbouring ranges, there has been a substantial number of these main valley glacier lakes in modern times. Outbursts from a series of dams on the Upper Shyok between 1926 and 1932 brought devastating floods along more than 1200 km of the Indus. Some even larger

259

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260 Kenneth Hewitt

landslide dams and outburst floods occurred here in the nineteenth century and an exceptional concentration of surging glaciers has been found (Hewitt, 1969, 1975, unpublished). Some of the latter have formed main valley ice dams.

The only comparable region in terms of these hazardous hydrological phenomena occurs in the southern Alaska-Yukon ranges. The Karakoram is far less known than the latter. This paper provides a summary of what is known of ice dam occurrence, the glaciers involved, and the impact of outburst floods.

THE RECORD OF GLACIER DAMS AND OUTBURST FLOODS

There i s an ex tens ive i f s c a t t e r e d record of n a t u r a l damming in the Karakoram reg ion , s u f f i c i e n t t o i n d i c a t e the scope of the problem, and the p a r t i c u l a r drainage bas ins where damming recurs (Table 1 ) .

T h i r t y - f i v e d e s t r u c t i v e o u t b u r s t f loods have been recorded in the p a s t two hundred y e a r s . Th i r ty g l a c i e r s are known to have advanced across major headwater streams of the Indus and Yarkand r i v e r s . There i s unambiguous evidence of l a rge r e s e r v o i r s ponded by e ighteen of these g l a c i e r s . Meanwhile, a fu r the r t h i r t y - s e v e n g l a c i e r s i n t e r f e r e with the flow of trunk streams in a p o t e n t i a l l y dangerous way. There i s geologica l evidence of o the r dams and numerous r e p o r t s of g l a c i e r s across main r i v e r channels which they were not a c t u a l l y damming. These too may be p o t e n t i a l l y dangerous.

Geographical ly , g l a c i e r dams in main r i v e r v a l l e y s have occurred from the f a r western to the f a r e a s t e r n p a r t s of the Karakoram range , and in the Lesser Hindu Kush, Nanga Pa rba t , Haramosh, Hindu Raj , Aghi l , and far n o r t h e a s t Hindu Kush ranges ( F i g . l ) . These are a reas where we approach maximal l o c a l r e l i e f for the E a r t h ' s land

FIG.l Distribution of glacier dams and related events in the Upper Indus Basin (after Hewitt, unpublished).

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TABLE 1 Historical summary of glacier dams and outburst floods for the Karakoram Himalaya and environs. (For details and sources see Hewitt, 1968 and unpublished.)

Year Ice dam Outburst Major Glacier/River System flood disaster

1533 1780 1818-1826 1833 1833 1835 (1841 1842 1844 1848 1848 18501 1850 1852-1855 1855 (185S 1864 1865 1869-1870 1873 1879 1882 1884 1889 1891-1893 1893 1899 1901 1902-1903 1904-1905-1905 1905 1905 1906 1907 1909 (1911 1916 1924-1925-1926 1927 1927 1928 1929 1929-1930 1932 1933 1939 1953 1977 1978 1979

•1840s

' ? ;

•1858

'.-1862

•1872

•1892

•1911

•1905 •1906

1933 1927

1930

X

X

X

X

X

X

X

Massive X

X

X

X

X

X

X

Massive X

X

X

X

x(?) X

X

X

X

X

X

X

x(?) X

X

X

X

X

X

X

X

X

X

Landsli X

X

X

X

X

X

X

X

X

X

X

X

X

X

(Hunza X

X

X

X

X

(Series of barriers) X

X

X

X

landslide X

X

X

X

X

dam and outburst, X

X

X

(Series of barriers) X

X

landslide

X

X

X

X

X

X

X

X

X

X

X

X

X

X

dam and outburst,

X

X

X

X

X

X

X

de dam near Gilgit)

X

X

X

X

X

X

X

landslide <

X

X

X

X

Upper Shyok river Upper Shy ok river Upper Shyok river Upper Shyok river Upper Shyok river Yashkuk Yaz Glacier Sultan Chhussku Glacier Indus river) Upper Shyok river Ishkoman river Aktash Glacier Kichik Khumdan Glacier Chungphar Glacier Aktash Glacier Kichik Khumdan Glacier Upper Shyok ri ver Upper Shyok river Hunza river) Kichik Khumdan Glacier (?) Ishkoman ri ver Kichik Khumdan Karambar Glacier Batura Glacier Upper Shyok river Upper Shyok river Shimshall river Upper Shyok river Ishkoman ri ver Shimshall river Ishkoman ri ver Upper Shyok river Upper Shyok river (?) Kichik Khumdan Glacier Kichik Khumdan Glacier Ishkoman river Khurdopin Glacier Kichik Khumdan Glacier Karambar Glacier Khurdopin Glacier Khurdopin Glacier Khurdopin Glacier Whirgut Glacier

Ishkoman ri ver Chong Khumdan Glacier Khurdopin Glacier Chong Khumdan Glacier Kaz Yaz Glacier Khurdopin Glacier Kilik river (Hunza) Chong Khumdan Glacier Ishkoman river Kyagar Glacier Chong Khumdan Glacier Chong Khumdan Glacier Chong Khumdan Glacier Kutiah Glacier

dam related to glacier surge) Kyagar Glacier Kyagar Glacier

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262 Kenneth Hewitt

s u r f a c e . A l t i t u d i n a l d i f f e rences of some 3000-5000 m over d i s t a n c e s of 10-30 km from main r i v e r v a l l e y s are r e f l e c t e d in the f a l l of the damming g l a c i e r s (see Table 5 ) . The main r i v e r s follow extremely e longa ted , t r e l l i s drainage p a t t e r n s t h a t d iv ide up the high r anges . The l a t t e r are heav i ly g l a c i e r i z e d . The Karakoram and o the r h igh ranges have 60-70% permanent snow and i c e covers . Snowfall of the o rder of 1000-1500 mm wa te r - equ iva l en t i s i n d i c a t e d h e r e . Yet v a l l e y f l o o r s , l e s s e r ranges and the e a s t e r n p l a t e a u remnants are gene ra l l y a r i d or s emi -a r id . In a d d i t i o n , s t rong a spec tua l d i f f e r ences of c l imate g r e a t l y in f luence the g l a c i e r cover and g l a c i e r behaviour .

The da ta i n d i c a t e t h a t main va l l ey dams may have been more common in a s s o c i a t i o n with L i t t l e Ice Age advances, b u t were not confined to t h a t t ime . T r a d i t i o n and anc ien t documents from as e a r l y as the f i f t h century A.D. r e f e r to o u t b u r s t s of g l a c i a l waters as t y p i c a l for the Indus and r i v e r s d r a in ing nor th to Takla Makan.

However the re i s a conspicuous gap in the record from the 1930's to the p r e s e n t . I t seems to be a s soc i a t ed with genera l g l a c i e r r eces s ion in the r eg ion . Neve r the l e s s , in 1978 a l a rge dam formed on the nor th s lope of the Karakoram Himalaya (Fig. 2 ) . I t was

-35-45'

CONTOURS-land iC6

ACCUMULATION ZONE

MORAINE

FIRN LINE

FIG.2 The Kyagar Dam and Glacier in July 1978.

glacial and other natural hazards in the region. 2

first summer, a lake of some 6 km had developed.

identified using Landsat imagery in a continuing programme to monitor By the end of the In subsequent

years, two large flood waves coming down the Yarkand river from its Karakoram headwaters have been attributed to the bursting of the lake (personal communication from Dr Shi Yafeng). The same glacier, the

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Outburst floods of the Karakoram 263

Kyagar, dammed the Shaksgam river fifty years ago, when damming recurred over several years.

THE GLACIER DAMS

It has been possible to determine the order of magnitude of the volumes of water in three main valley reservoirs; the recent Kyagar dam, the Chong Khumdan dam of 1929, and, from field features, the Biafo Gyang dam which burst some time in the eighteenth century (Tables 2, 3 and 4). Morphological information is also given for the glaciers involved.

In addition we have information from the one large tributary valley lake that has been a serious hazard in recent times, that where the Khurdopin Glacier dams the Virjerab valley in Upper Shimshall. Before its outburst in 1907,the glacier that formed there was about 3.5 km long, 1.5 km in average width and some 88 m deep at the dam.

TABLE 2 Kyagar Dam and Glacier

Kyagar Dam (18 July 1978) Glacier

Length of lake Average width Slope of valley floor approximately Est. depth at dam Est. volume 4.6 x Min. width ice barrier 1.5 km Area of basin above barrier

2

(excluding Kyagar Basin) 366.6 km

6 .5 km 1.0 km

1 in 50 120 m 108 m3

Orientation Max. length Width, lower ablation zone Terminus altitude Highest point basin

of

NNW

21.6 km

2.5 km 4 700 m a.m. s .1

7200 m Firn line ht (1978) 5400 ma.m.s.l. Basin area 155.0 km2

Permanent snow and ice cover 112 km Accum. zone area 67.O km Ablation zone area 45.O km Slope main ice stream: Accum. zone (6 km) 1 in 3 Ablation zone (15.6 km) 1 in 22

THE GLACIERS

There is little or no basic glaciological data on movement, nourishment, thickness, or thermal characteristics for any of the glaciers that have formed major ice dams in the past hundred years, and only fair or poor topographical maps. It will be a useful beginning, however, to list locational information and such morpho­logical features as can be determined for these glaciers (Table 5).

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264 Kenneth Hewitt

TABLE 3 Chong Khumdan Dam and Glacier (see Hewitt, unpublished)

Glacier Dam (1929) Chong Khumdan Glacier

Length of lake Average width Slope of valley floor Depth at dam Volume Width of ice ice barrier Area of basin above barrier Water supply: summer discharge Chip Chap river Rates of rise of lake, August

16 km 1.6 km

1 in 130 120 m 1.5 x 10s m3

2.4 km

25,500 km2

5.1 x 106 m3/day

0.3-0.45 m/day

Orientation Max. length Width, lower ablation zone Terminus, altitude Highest point on basin Firn line Basin area

E

20 km

2.5 km 4715 m a.m.

7530 m a.m. 5250 m a.m. 140 km2

,s.

.s, ,s

.1

.1

.1

TABLE 4 Biafo Dam and Glacier (Hewitt, 1964)

Biafo Historic Dam (late eighteenth century)

Length of lake Average width Slope of valley floor approximately Est. depth at dam Min. width of ice barrier Area of basin above barrier Est. capacity of lake

15 km 4 2-3 km

1 in lOO

150-200 m

2.5 km

3500 km

3.3 x 10-

Biafo Gyang

Orientation Max. length Width, lower ablation zone Terminus altitude Highest point of basin Firn line height main stream(1961) Permanent snow and ice cover Accumulation zone main glacier Ablation zone, main glacier Slope, main ice Stream: Accumulation zone Ablation zone

SSE

62 km

3 km 3109 m a.m.s.1

7280 m a.m.s.1

4725 m

520 km2

310 km2

121 km2

1 in 10 1 in 30

High relief, steep average fall and large climatic gradient between upper and lower reaches promote vigorously active ice masses in the region. That is reflected in those few for which we have movement data. All except two of the glaciers known to have formed dams flow in northerly and easterly directions (Fig.. 3) . This

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Outburst floods of the Karakoram 265

TABLE 5 Some characteristics of damming glaciers of the Upper Indus and Yarkand drainage systems

Indus river system

Eastern Karakoram

Shyok river 1. Chong Khumdan 2, Kichik Khumdan 3. Aq Tash (Aktash) 4. Sultan Chhussku

Length (km)

21 11

8 8

Altitudes

Terminus

4715 4630 4570

-

(m)

Firn line

5250 5250 5250

-

Highest point

7530 7530 6740

-

Glacier type*

FK FK FK FK

Central Karakoram

Braldu-Shigar river 5 . Biafo Gyang 6. Khurdopin 7. Yazghil 8. Malungutti Yaz

Hi s par river 9. Barpu (Hopar) lO.Kutiah

68 47 31 23

33.8

3160 3250 3200 2900

2520

4500 4400 --

4600

7250 7760 7900 7900

7460

FS FS L L

L(Surging) L

Western Karakoram

Hunza river ll.Ghulkin 12.Batura 13 .Pas u

Chapursan river 14.Yashkuk Yaz 15 .Kaz Yaz,' (Besk-i-yeng) 16.Pekhin 17 .Wirghut IS.Chillinji 19.Chatteboi 20.Shuinj 21.Chillinji 22.Karambar

(West) (South)

(East)

19 58 23.5

24 11 13

7 11 13

9 10 23.5

2500 2460 2500

3550 3660 3050 3350 3530 3630 39 30 3500 2440

-4450 4600

4700 4600 5300 -4730 4570 --4730

7600 7708 7600

7150 6690 6420 6130 6130 5950 5800 6690 7160

L FK FK

L FK FK L L FK L L

Darkot-Thui Gol (Gilgit river) 23.West Ghamu Bar 7 3660

Tarshing river (Nanga Parbat) 24.Chungphar (Tarshing) 13 2920 25.Bazhin 11 3200

6400

6830 8129

Yarkand river system

Eastern Karakoram

Shaksgam river 26.Gasherbrum 2 7.Urdok 28 .Staghar 29 .Singhi 30. Kyagar

20 23

24 22

4350 4370 4430 4550 4700

5350 5350 5200 5400 5400

8047 8068 6460 7750 7220

FK L FK FK FK

' L = Lawinen or "Turkestan type" glacier; FK = Firnkessel or Firn Cauldron type; FS = Firnstrom or Firn Stream type; (see Hewitt, unpublished,

Chapter 11).

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266 Kenneth Hewitt

180

FIG. 3 Orientation of the main ice streams of damming glaciers of the Karakoram region. (0° is north).

sugges ts t h a t aspect i s a major f ac to r in the p o t e n t i a l for damming, as in o t h e r c h a r a c t e r i s t i c s of these h igh , s u b - t r o p i c a l g l a c i e r s .

Mason, the l a s t scholar to make an ex tens ive study of Karakoram dams (Mason, 1935) be l i eved they formed only with an "acc iden ta l " o r abnormally rap id i c e advance. He was not aware of r a t e s of flow for these g l a c i e r s , or the ro l e of win te r c o n d i t i o n s , when te rmini may advance many metres , r e a c t i v a t e dead i c e , and have l i t t l e or no r i v e r flow to contend with (Hewitt , 1967, unpubl i shed) . Never the less , some i c e dams have been a s soc i a t ed with surg ing g l a c i e r s .

Between 1975 and 1978 one can d e t e c t l i t t l e change in the f r o n t a l p o s i t i o n of the Kyagar G lac i e r . However, between 1977 and 1978 the re was a sharp change in the geometry of the r i g h t flank of the snout , as though the easternmost i ce s tream became more a c t i v e , extending across the Shaksgam Valley t o make the i c e b a r r i e r impassable .

While dams of a t l e a s t four g l a c i e r s have r epea ted ly re-formed over pe r iods as long as two decades, the lakes r a r e l y l a s t more than two summers wi thout an episode of complete d r a i n i n g .

OUTBURST FLOODS

Breaching of a dam c r e a t e s a sudden s h o r t - t e r m inc rease in d ischarge downstream. I f b reaching i s c a t a s t r o p h i c , as so of ten in the Karakoram reg ion , the impact of the f lood wave can fa r outweigh t h a t of o t h e r high flows. In p a r t , t h i s can involve a concentra t ion of flow in the upper reaches of the r i v e r s wel l i n excess of weather-produced extremes of runoff .

The 1929 o u t b u r s t f lood of Chong Khumdan Glac ie r was monitored from near the g l a c i e r over more than 1500 km downstream (Gunn, 1930; Mason et al., 1930). Along with some comparative obse rva t ions for the smal l e r 1932 o u t b u r s t , (Mason, 1932) t h i s g ives a unique record of flood-wave behaviour on the Upper Indus (Table 6 ) .

Gunn, (1930) e s t ima ted the r e s e r v o i r t o have conta ined almost 8 3 5 3

13.5 x 10 m (1.1 million acre feet) . Some 3 x lO m of ice were

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Outburst floods in the Karakoram 267

TABLE 6 Progress of the 1929 outburst-flood on the Upper Indus (after Gunn, 1930; Mason e t a l . , 1930)

Location

Sasir Khalsar Skardu Par tab P. Bunji Chi las Tarbela At took

Distance from (km)

16 217 499 719 731 803

1120 1194

dam Maximum flood rise (m)

26.0 19.2

7.6 13.7 10.6 16.1

7.0 8.1

Rise to peak (h)

4.0 2.0 3.5

8.0 5.9

4.0 12.0 17.0

Duration of wave (h)

40 10 28

60 50 ) 40 ) 50 ) 70

Rate of travel (km h-1.

8.3

20.0 22.0 13.2

18.8

5.9

a l so c a r r i e d with the f lood and s t randed on l a rge blocks in the v a l l e y below the dam. I f l o s s to channel s to rage and seepage i s somewhat g r e a t e r than gains from inflows below the dam, the complete d ra in ing in 48 h sugges ts an average discharge between S a s i r Brangsa and Khalsar in the region of 7100 m 3 s _ 1 (250000 c f s ) . In the s t e ep ly r i s i n g and f a l l i n g main flood peak however water d i scharges in excess of 22 650m 3 s _ 1 (800000 cfs) are i n d i c a t e d . That equals the l a r g e s t d i scharges measured for the e n t i r e Upper Indus a t Attock. The Upper Syok d ra in s l e s s than two per cent of the b a s i n , and most of i t s a rea i s a r i d .

The mode of dam f a i l u r e i s c r i t i c a l to the s i ze and shape of flood wave. All we know of the 1926, 1929 and 1932 Khumdan o u t b u r s t s i s t h a t breaching began through subg l ac i a l t u n n e l s , bu t then c a r r i e d away the e n t i r e t h i cknes s of i c e above. Gunn (1930) desc r ibes the breach of 1929 as having: " . . . b u r s t along a curving l i n e from the near r i g h t - h a n d bank of the lake on the nor thern s ide through the highest p o r t i o n of the dam, nea r ly to the l e f t bank of the r i v e r on the south s i d e . The cu t . . . was about 400 f t £l20 m] wide (and 500 f t + [lSO m +] deep) and the i ce stood v e r t i c a l l y on e i t h e r s i d e . The lowes t -por t ion of the dam along the c l i f f s was unaf fec ted . "

This c o n t r a s t s with most examples d iscussed in the l i t e r a t u r e , in which the sudden c lo s ing of the o u t l e t when the lake i s p a r t i a l l y emptied i s s a i d to produce another c h a r a c t e r i s t i c f ea tu re of " jokulh laups" (see Maag, 1969; Young, 1980).

The flood peak was h i g h e s t a t S a s i r Brangsa, b u t the 1929 wave showed remarkable r ecupe ra t i ve power in the Indus gorges , below Skardu. The e f f e c t i s the same as in the well-known Johnstown, Pennsylvania flood d i s a s t e r of 1889. As the flood waters ga the r i n , then leave intermontane bas ins such as a t Skardu, they r e - e n a c t a pseudo-dam break .

SOME FLOOD-WAVE IMPACTS

The s i g n i f i c a n c e of these floods l i e s e s p e c i a l l y in the excep t iona l r i s k to human communities or i n s t a l l a t i o n s , and a l so in t h e i r r o l e in

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268 Kenneth Hewitt

erosion and sedimentation. Over much of their course in the mountains, the recorded floods reach heights well above peak discharges from summer melting. Their dynamic character greatly magnifies their erosional competence and capacity. These two matters are of singular importance in the erosional context of the Karakoram valleys, and sediment transport into downstream reservoirs.

Sediment yield from the Upper Indus Basin represents the highest known rate of regional erosion over such an area, of about one metre per thousand years (Hewitt, unpublished). Data for the tributaries where the dams occur suggest rates in excess of 1.8 m per thousand years. Nevertheless, average sediment concentration upon which rating curves are based may be several orders of magnitude lower than the highest concentrations for given discharges. These exceptional concentrations generally occur in association with flood waves (Fig.4). What is reflected here is a highly constrained sediment availability in the fluvial zone. The humid, glacial areas not only provide most of the water, but also most of the seasonal debris transported.

0) a) 9 0 -

<= 1 o =•

c £ CD °-

E 5

INDUS RIVER at DARBAND Lat. 34 2 4 Long. 72 48'

Summer 1962

i i i I i i i i I — i — i — i — I — r 20 21 22 23 24 25 20 27 28 29 30 1 2 3

June 1962 July 1962

•15 u

Q

FIG.4 An example of exceptional sediment concentration (broken line) on the Upper Indus, related to the passage of a flood wave (solid line) (from data supplied by Water and Power Development Authority, Pakistan). (c.f.s. = 2.83 x 10~2m3s~1).

The great height and erosional energy of dam-burst flood waves especially allows them to reach and cut into the abundant lag deposits in or stranded upon, arid river terraces. Huge numbers of landslides have been reported on terraces and valley sides after the passage of damburst floods. Substantial channel widening, deepening and even changes of course have been reported.

If one extrapolates the existing sediment rating curve for Darband - or Attock before the Tarbela Dam was built - the 1929 flood curve would have carried the equivalent of one average year's sediment yield. With greater concentrations of sediment and

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Outburst floods in the Karakoram 269

enhanced bedload this would be much higher. Moreover, the mobilization and sluicing of transportable sediment into stream channels would tend to increase total yields for some months or years after a major outburst.

In the event of a phase of recurrent damming such as occurred prior to 1940, these erosional events could increase the rate of sedimentation in artificial dams on these rivers, and reduce their economic lifetimes.

REFERENCES

Gunn, J.P. (1930) Report on the Khumdan Dam and Shyok Flood of 1929. Government of Punjab Publication, Lahore.

Hewitt, K. (1964) A Karakoram ice dam. Indus: J. Water and Power Devel. Ruth. (Pakistan) 5, 18-30.

Hewitt, K. (1967) Ice-front sedimentation and the seasonal effect: a Himalayan example. Trans. Inst. Brit. Geogrs (42), 93-106.

Hewitt, K. (1968) Records of natural damming and related events. Indus; J. Water and Power Devel. Auth. (Pakistan) 10(4).

Hewitt, K. (unpublished) Studies in the Geomorphology of the Upper Indus Basin. 2 vols. PhD dissertation, University of London.

Hewitt, K. (1969) Glacier surges in the Karakoram Himalaya (Central Asia). Can. J. Earth Sci. 6(4), 1009--1018.

Hewitt, K. (1975) Perspective on disasters and natural hazards in the Northern Areas. Dawn Magazine. Karachi, Pakistan. January 1-12.

Maag, H.V. (1969) Icedammed lakes and marginal giacjal drainage on Axel Heiberg Island. Axel Heiberg Research Report. McGill Univ. Montreal, 14 7 p.

Mason, K. (1932) The Chong Khumdan Glacier, 1932. Himalayan J. 5, 128-130.

Mason, K. (1935) The study of threatening glaciers. Geogr. J. 85(1) , 24-35.

Mason, K., Gunn, J.P. and Todd, H.J. (1930) The Shyok flood in 1929. Himalayan J. 2, 35-47.

Young, G.J. (1980) Monitoring glacier outburst floods. Nordic Hydrology. 11, 285-300.

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