Climate and other factors in the development of river and interfluve profiles in Bhutan, Eastern Himalayas

Journal of Asian Earth Sciences 22 (2004) 539–553 www.elsevier.com/locate/jseaes

Climate and other factors in the development of river and interfluve profiles in Bhutan, Eastern Himalayas I.C. Bailliea,*, Chencho Norbub a National Soil Resources Institute, Cranfield University, MK45 4DT, England, UK National Soil Services Centre, Ministry of Agriculture, P.O. Box 119, Thimphu, Bhutan

b

Received 9 July 2002; accepted 13 March 2003

Abstract The longitudinal profiles of the main N –S aligned rivers and the crests of the interfluve mountain ranges of Bhutan have been plotted against latitude. The river profiles are highly variable, even between branches of the same system. The main rivers in Eastern Bhutan are antecedent and rise in Tibet. They have irregular concave bed profiles in deep steeply sided valleys. The rivers further west rise on the southern slopes of the High Himalaya. They have stepped profiles with steep concave sections in gorges through the southern mountains and one or more concave sections upstream, separated by knickpoints. All of the N– S interfluve ranges rise steeply from the piedmont. Some then dip to major passes before again rising irregularly northwards to the High Himalaya, whilst others continue to climb northwards as irregular monoclines. The combination of various types of river and interfluve profiles creates a range of valley forms. The heterogeneity means that it is not possible to generalise about a typical Bhutanese river, interfluve or valley relief profile. There is no indication that the rivers of Bhutan have more knickpoints than those of the Central and Western Himalayas. Rainfall and runoff data, soils and natural vegetation have been examined for indications of significantly drier conditions in eastern Bhutan. The rainfall data show an eastwards decrease in the southern foothills, probably due to the rainshadow cast by the Meghalaya Plateau to the south, but mean annual totals are about or above three metres throughout, and the whole of this zone has a wet climate. There is no marked E – W climatic trend in the drier interior of Bhutan. We attribute the general topographic structure of Bhutan, and the variability of river and interfluve profiles and valley forms more to tectonic factors than to climatic variation. q 2003 Elsevier Ltd. All rights reserved.

1. Introduction The geomorphology of Bhutan has been described so far only in general terms, and there is increasing need for systematic characterisation and interpretation of the topography. Practical reasons include the growing realisation that the currently abundant water resources of Bhutan will need careful management as demand grows from the expanding hydropower sector and an urban population. This is paralleled by awareness of the scarcity and fragility of the soil resources (Norbu et al., 2003a). These concerns have prompted integrated physiographic overviews of the kingdom’s natural resources (Baillie, 2002; Norbu et al., * Corresponding author. E-mail address: [email protected] (I.C. Baillie). 1367-9120/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1367-9120(03)00092-0

2003b), and interest in integrated watershed management (Jamtsho and Hansen, 2003; Baillie et al., 2003a). There is also growing scientific interest in the landscape and climate of the eastern Himalayas, a sector of the range that has hitherto been relatively neglected. In particular, the innovative proposals of Bookhagen et al. (2001) have stimulated a re-examination of the region’s climate and its role in shaping the landscape. From their interpretation of three years of satellite microwave data, Bookhagen and his colleagues conclude that the Meghalaya (Shillong) Plateau to the south of the Himalayas casts a considerable rainshadow during the summer monsoon, and that the Eastern Bhutan section of the southern Himalayan front, which they define as land up to 3000 m a.s.l., is drier than the sectors to the east and west. They draw further support for their climatic conclusions from geomorphological

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comparisons along the Himalayan arc. In digital elevation models of 40 km wide swathes perpendicular to the range, along one river in each of the western, central and eastern sectors of the range, they find that only the most easterly, the Manas in Eastern Bhutan, appears to have significant knickpoints below 3000 m a.s.l. They attribute this to low rainfall in that section, and conclude that climatic differences are more important for topogenesis in Eastern Himalayas than has hitherto been recognised (B. Bookhagen, pers. comm., 2002). Although a rainshadow from the Meghalaya Plateau is to be expected, its effect was assumed to be limited, and the whole of Southern Bhutan has hitherto been characterised as having a wet monsoonal climate. The existence of a significantly drier zone in the Eastern Bhutanese sector of the Himalayan front was not suspected. If confirmed, it will have implications for the management of water, soil and forest resources. Some previous geomorphological summaries have treated Bhutan as an eastwards extension of the Central Himalayas, where substantial E – W strike-aligned ridges are interposed with subsequent valleys, due to differences in competence and erosion rates between the outcrops of the Lesser Himalayan sedimentary and metasedimentary formations. Extrapolation of this structure to Bhutan results in the topographic characterisation of the country as a series of altitudinal zones, which are aligned roughly E– W. In an early regional geomorphological characterisation of Bhutan, Eguchi (1987) applied Gansser’s (1983) pan-Himalayan structure, but found the zones unclear. Nonetheless, he divided Central Bhutan into four E – W zones, i.e. Southern foothills, Southern High Himalaya, Lower Midland of High Himalaya and Northern High Himalaya. Takada (1991) took Eguchi’s division as a framework for his semi-detailed geomorphological studies of Quaternary alluvial deposits and landforms in the Puna Tsang valley and of asymmetrical valleys in Eastern Bhutan. He referred to the third zone as intramontane basins, possibly to avoid confusion with the quite different Midland zone of Nepal. Eguchi (1997) followed suit in his later climatic studies, referring to the third zone as the intramontane basin. He designated the fourth zone as the Great Himalaya, to distinguish it from the High Himalaya of Nepal, where the term has geological, as well as topographic, connotations. In this paper we restrict the term ‘High Himalaya’ to the very high (. 5500 m a.s.l.) mountains, with permanent snow and ice, which are aligned E – W across the north of the country (Fig. 3). Not all of the earlier topographic summaries are unduly influenced by extrapolation from Nepal, and some recognise that the main features of the Bhutan landscape are aligned roughly N – S (e.g. Navara, 1997). The aim of this study is to use the profiles of Bhutan’s main rivers and interfluves to clarify the general structure of the landscape, and to combine them with rainfall, runoff and other environmental data to assess the relative importance of climatic variation and other factors in topogenesis. In

particular, we examine the intensity of the Meghalaya rainshadow, the occurrence of significantly drier conditions in Eastern Bhutan, and their possible contributions to landscape development.

2. Methods 2.1. Study area The kingdom of Bhutan lies between the Indian states of Sikkim and Arunachal Pradesh on the southern slopes of the Eastern Himalayas. It is bordered to the north by the Tibetan plateau, and to the south by the piedmont alluvial plain of the Brahmaputra. It covers about 46,000 km2 between latitudes 268470 N and 288260 N and longitudes 888520 E and 928030 E (Fig. 1). Its topography is steep and mountainous throughout, and altitudes range from under 200 to over 7500 m a.s.l. Over two thirds of the country is above 2000 and a quarter is above 4000 m a.s.l. The geology is shaped by the intense tectonic activity that resulted from the collision of the Indian and Eurasian continental plates, the closure of the intervening Tethys, and the uplift of the Himalayas. Bhutan is mostly underlain of thick sheets of high-grade metamorphic rocks, predominantly gneiss, with subordinate quartzite, schist and marble, which were emplaced in a series of southward and eastward thrusts. They show inverse metamorphism, with lower grade schists outcropping through windows in the overlying gneisses. The metamorphic rocks are intercalated with ultramafics and intruded by granites, which range in size from pegmatitic veins to Miocene batholiths. The metamorphic rocks are the eastwards continuation of the Crystalline Complex of the Central Himalayas (Bhargava, 1995), where it is flanked to the south by wide outcrops of Lesser Himalayan sediments and metasediments. Bhutan is more homogeneous lithologically, and the gneisses stretch southwards almost to the piedmont. They underlie more than 70% of the country, although capped with outliers of Tethyan metasediments in places (D.G.M., 2001). The Lesser Himalayan sediments and metasediments are compressed into a narrow belt along the southern border, except in the southeast (Fig. 2). Because of the wide altitudinal range, the climate ranges from subtropical, through temperate and alpine, to arctic, all within 100 km, and annual mean temperatures vary from above 20 8C in the piedmont to below zero in the High Himalaya. The winters are mostly dry and bright, due to outflows from the Tibetan high pressure system. The Eastern Himalayas are little affected by the westerlies that bring winter rain to the Western Himalayas (Mani, 1981). The rainfall occurs mainly during the summer monsoon, which makes its eastern landfall on the northern shores of the Bay of Bengal, and then progresses north-westwards. It starts earlier and finishes later in the east, and its effect

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Fig. 1. Southern slopes of Himalayas (light tone), showing the locations of Bhutan and the Meghalaya Plateau (dark tone).

Fig. 2. Generalised geology of Bhutan, showing main faults, including Main Central Thrust (MCT) and Main Boundary Fault (MBF), simplified from D.G.M. (2001).

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diminishes north-westwards, with local variations (Rao, 1981; Miehe et al., 2001). The Meghalaya (or Shillong) Plateau is a dissected and almost freestanding massif to the south of the Eastern Himalayas. It is aligned E– W, and stretches from about 908E to 938300 E (Fig. 1). It is much lower than the Himalayas, and its highest point is below 3000 m a.s.l. However, it is the first high ground encountered by the moist monsoonal air masses after they come ashore, and it receives extremely high orographic rainfall. Cherapunji is located close to the plateau’s southern edge at about 1300 m a.s.l., and has a mean annual rainfall of over 11 m and an absolute annual maximum of over 25 m, making it one of the wettest places on earth. However, the really intense orographic effect appears to be localised on the windward side. It diminishes northwards, and the mean annual rainfall at Shillong town, at an altitude of about 1500 m a.s.l. in the centre of the plateau, is only 2500 mm (Rao, 1981). This drops to 1650 mm p.a. at Gawahati, which is on the floor of the Brahmaputra Valley and in the lee of the plateau (Fig. 1). The plateau’s crosswind width of over 200 km makes it unlikely that the movements of monsoonal air masses around the sides are significant at a regional scale. 2.2. Profiles of riverbeds, interfluve crests and relief The bed profiles of the main fourth and fifth Strahler (1957) order rivers (Fig. 3) were traced from the 1:50,000 topographic maps produced by the Survey of India and the Survey of Bhutan. The contours on most of the maps are at vertical intervals of 40 m. Many of the maps are more than

30 years old, but the quality and detail are good, considering that they were produced mainly by field survey in difficult terrain and from photogrammetry of old small-scale aerial photographs. The map scale means that the riverbed profiles are slightly smoothed, and elide some meso- and microfeatures. Preliminary examination confirmed that the rivers and interfluves are aligned more or less N –S. This enables the altitudes of the riverbeds to be plotted against latitude rather than length, which facilitates comparisons between basins. It exaggerates bed gradient in reaches where the rivers do not flow N – S, but in Bhutan this is not significant. The crest profiles of the main N – S interfluve ranges (Fig. 3) were also traced from the 1:50,000 topographic sheets. Like the rivers, the crests are aligned more or less N –S and can be plotted against latitude without significant distortion. Relief within the valleys is derived from overlaying the river profiles with those of the flanking interfluves. 2.3. Rainfall and runoff The rainfall data were collected and supplied by the Ministry of Trade and Industry, Royal Government of Bhutan. None of the stations yet meet the international standard of 30 years of continuous records, and the data runs are too short and interrupted for detailed statistical analysis. The same ministry began collection of river flow data in the 1990’s, and the periods covered are also short (Land Use and Statistics Section, 2000). The longest set (16 years) is for flows at the intake of the Chhukha hydro-electricity

Fig. 3. Main N– S rivers and ranges of Bhutan, showing Inner Valleys (solid black) and Puna Tsang palaeo-lake (PTPL).

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Fig. 4. Main downstream river flow gauging stations (heavy line) and rain gauges in Bhutan. Southern foothill raingauges (diamond): 1 ¼ Chengmari; 2 ¼ Samtse; 3 ¼ Pugli, 4 ¼ Phuntsholing; 5 ¼ Gedu, 6 ¼ Darla; 7 ¼ Kalikhola; 8 ¼ Sarpang; 9 ¼ Bhur; 10 ¼ Panbhang; 11 ¼ Dechenling; 12 ¼ Nanglam; 13 ¼ Deothang; 14 ¼ Bakuli; 15 ¼ Daifam. Inner valley rain gauges (circle): 16 ¼ Namgyel; 17 ¼ Drukyel Dzong; 18 ¼ Chapcha; 19 ¼ Gidakom; 20 ¼ Thimphu; 21 ¼ Semtokha; 22 ¼ Punakha; 23 ¼ Bajo; 24 ¼ Trongsa; 25 ¼ Mangdechhu; 26 ¼ Jakar; 27 ¼ Bathpalathang; 28 ¼ Lingmethang; 29 ¼ Autsho; 30 ¼ Mongar; 31 ¼ Trashiyangtse; 32 ¼ Chazam.

scheme on the Wang in western Bhutan (Merz et al., 2003). In this study we use the data from the lowest station on each river, as these best characterise runoff from the whole catchment. The locations of the rain gauges and the flow gauging stations used are shown in Fig. 4. 2.4. Vegetation and soils The distribution of soil types is derived from soil survey reports by the National Soil Services Centre and from the overview by Baillie et al. (2003b). The distribution of vegetation types is derived from the compilations of Ohsawa (1987); Sargent et al. (1985), and from field observation during soil surveys.

3. Results 3.1. River profiles Although all of the rivers of Bhutan flow into the Brahmaputra and are working to virtually identical base levels, there are wide variations between their profiles. Fig. 5(A) shows that the Bhutan section of the Kuri has a typical ‘Ganges’ type of irregularly concave profile (Bruijnzeel and Bremer, 1989; Vorosmarty et al., 2000). The other major river with a similarly simple concave form is the Gongri-Tawang, also in eastern Bhutan. The Kuri and

the Gongri-Tawang (Fig. 3) are the only two significant antecedent rivers in Bhutan, with their headwaters on the Tibetan plateau, to the north of the High Himalaya. The Kuri crosses the High Himalaya and Bhutan’s northern border in a deep valley at 2400 m a.s.l. The Tawang enters Bhutan at only 1200 m, but this is some distance downstream from its valley through the High Himalaya. Fig. 5(A) shows that the Chamkhar branch of the Manas system has a very different, ‘Tsangpo-Brahmaputra’ type, profile (Bruijnzeel and Bremer, 1989; Vorosmarty et al., 2000), with three concave sections, which are separated by convex knickpoints. For the purposes of this study, we differentiate major and minor knickpoints in Bhutan. Upstream of major knickpoints there are concave reaches with wide and moderately dissected basin-like sections of valley, which are isolated by rugged terrain up- and downstream. These are referred to as the Inner Valleys. They have floors (thalwegs) up to 1 km wide, with suites of alluvial terraces and soils. Together with the moderately graded lower hill slopes, these make the valleys the most extensive arable areas in Bhutan. They have long been settled and cultivated, and are the demographic, economic and cultural heartland of the kingdom. The valleys of the concave reaches upstream of minor knickpoints have steep side slopes, narrow thalwegs, no extensive alluvial deposits, and do not form marked inner valleys. The upper knickpoint on Chamkhar profile at about 2500 m is major, with the wide inner valley of Bumthang upstream. The lower, at

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Fig. 5. River profiles. MCT ¼ Main Central Thrust, MBF ¼ Main Boundary Fault. (A) Comparison of concave and stepped river profiles. (B) Profiles of main branches of Wang river system. (C) Profiles of main branches of Puna Tsang river system. (D) Profiles of main branches of Manas river system.

about 900 m, is minor with no marked inner valley upstream (Fig. 5(A). The profiles of the rivers of western and central Bhutan are heterogeneous, but they all have more or less stepped forms. These rivers are all consequent, and rise on the southern slopes of the High Himalaya, which forms the true hydrological divide for an unbroken stretch of about 400 km, from the Arun in eastern Nepal, through Sikkim and western Bhutan, to the Kuri in eastern Bhutan. Fig. 3 shows the Amo (known as the Torsa in India) as rising in southern Tibet, but this is due to the southerly bulge in the international border, and nearly all of its headwaters rise on the southern slopes of the High Himalaya. Fig. 5(B) shows the profiles of the three main branches of the Wang (Raidak in India). The lower Wang has a steeply concave profile, which rises to a major knickpoint at about 1800 m. From there up to about 2100 m the system is just beginning to dissect, and these sections of the upper Wang, Paro and Thim have steep and narrow valleys. It is only above 2100 m that the Paro and Thim valleys ease in gradient and widen out to form large inner valleys. The Ha branch has developed differently, and continues to climb steeply for another 700 m upstream of its confluence, and its profile has a major knickpoint and a small inner valley at about 2600 m a.s.l. (Baillie, 2002). The profile of the Puna Tsang (Sankosh in India) in Fig. 5(C) has a major knickpoint at about 1100 m a.s.l.

The long, gently concave reach upstream forms the largest, lowest, and warmest of Bhutan’s main inner valleys. Further upstream the main Po and Mo headwater branches have similar profiles, both irregularly concave up to the High Himalaya. Fig. 5(D) shows the variations between the main headwaters of the Manas river system in eastern Bhutan (same name in India). The two eastern antecedent branches, Gongri and Kuri, have simple concave profiles. There are distinct differences between the stepped profiles of the consequent streams to the west. The Mangde has at least four knickpoints in its generally steep profile, but are all minor and this river has no distinct inner valley. As noted in Fig. 5(A), the Chamkhar has two distinct knickpoints, the lower minor and upper major. The Kholong in eastern Bhutan is an interesting hybrid, as it is a consequent stream, rising at over 5000 m on the southern slopes of the High Himalaya, but it joins the deeply incised antecedent Gongri at an altitude of less than 1000 m. This makes it one of the steepest major rivers in the country, and contributes to the localised but intense erosion in its valley (Van der Poel and Tshering, 2003). 3.2. Interfluve crest profiles The main interfluves are large and complex ranges running roughly southwards from the High Himalaya down

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Fig. 6. Interfluve crest profiles. (A) Dochula Range with low central poss. (B) Thrumsingia Range, irregularly monoclinal. (C) Other interfluve creat profiles with low central pass. (D) Other monoclinal interfluve creat profiles.

to the foothills. The ranges are named after their main passes, and their juxtapositions with the main rivers are shown in Fig. 3. Fig. 6(A) shows that the crest profile of the Dochula range rises steeply from the piedmont at about 200 m a.s.l. to reach 2000 þ m within 10 km. The profile continues to climb steeply but irregularly to altitudes of over 4500 m within 60 km of the plains. Further north the profile dips considerably, with its main central pass at just over 3000 m. From there it climbs irregularly northwards before reaching the dissected glacis flanking the High Himalaya at 5000 þ m. This configuration gives rise to an isolated block of high land in the south, i.e. the Dagala Range, with summits up to 4700 m a.s.l. Fig. 6(B) shows that the Thrumsingla profile also climbs steeply in the south to about 3000 m a.s.l. From there it climbs irregularly to over 4500 m, but it lacks a pronounced low central pass. The profile is irregularly monoclinal and does not form an isolated block of high land in the south. Low central passes and isolated southern massifs, similar to Dochula, are found on the Pelela and Yotongla profiles (Fig. 6(C)), whereas the Tegala and Korila ranges (Fig. 6(D)) have irregular monoclinal profiles like Thrumsingla.

3.3. Relief and valley shape Superimposing the profiles of the rivers and their flanking interfluves indicates the relief in the valleys. Fig. 7(A) shows the longitudinal profile of relief in the Kuri valley, which has a low concave riverbed and monoclinal interfluves. The valley is deep and relief increases rapidly upstream from the Manas confluence. It is mostly over 2000 m, and exceeds 3000 m in places on the western side. There are no sections with low relief. There are extensive stretches where the steep side slopes have distinct convexities at 300– 800 m above the present river level, giving the valley a distinct ‘sharp V incised into shallow V’ cross-section (Sinclair Knight, 1983). The valley floor is narrow, with little alluvium. The valley of the Gongri, the other antecedent river in the east, is similar in relief, form, narrow floor, and paucity of alluvium. The ‘V in V’ form occurs sporadically in the southern gorge sections of the consequent rivers further west, but is not apparent in their inner valleys (Singh and Saklani, 1978; Baillie, 2002). The relief profile of the Chamkhar valley in Fig. 7(B) shows a combination of a high inner valley and a low pass. The relief is over 3000 m in the lower reaches where

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central passes on their flanking interfluves. For instance, the relief from the inner valley of the Thim to the low central pass (Dochula) on its eastern interfluve is only 850 m. The Puna Tsang has interfluves with central passes on both banks. Fig. 7(C) shows that its valley has very high relief, exceeding 4000 m, where it cuts through the southern mountains. However, although both interfluves dip northwards by more than 1500 m from their southern massifs, the low altitude of the inner valley, at about 1100– 1200 m a.s.l., means that relief in this section is still more than 2000 m. 3.4. Rainfall The physiographic zonation of Bhutan by Norbu et al. (2003b) indicates that it is not possible to treat all of the land below 3000 m a.s.l as a single climatic unit, and that the rainfall on the southern mountains and in the N – S valleys and ranges need be considered separately. The rainfall data available for the piedmont and southern foothills are summarised in Table 1. They show that the whole of the southern front of the Bhutan sector of the Himalayas receives heavy rainfall. Annual means (. 4 m) and maxima (up to 7 m) are highest in the western stations, and generally decline to the east of longitude 918E. However, even there, all of the stations have wet climates, with annual means about or above 3000 mm. They are all wetter than Shillong town in the centre of the Meghalaya Plateau, and have almost double the rainfall of Gawahati in the Brahmaputra valley (Fig. 1). The rainfall in the Inner Valleys is summarised in Table 2. These are Himalayan dry valleys in which cloud cover and rainfall on the floors and lower slopes are suppressed by strong up-valley winds (Schweinfurth, 1956; Whiteman, 2000). The data runs are too short, and local variation too great, to permit firm conclusions about regional trends but, if anything, rainfall appears to increase towards the east. 3.5. Runoff

Fig. 7. Relief profiles. (A) Relief profile of Kurt valley. (B) Relief profile of Chamkhar valley. (C) Relief profile of Puna Tsang-Po valley.

the river runs in a gorge through the southern mountains. Upstream of the major knickpoint, the relief on the eastern side of the high altitude inner valley to the monoclinal eastern interfluve at Thrumsingla is 1100 m. The western interfluve, Yotongla, has a pronounced central pass and the relief there is even lower, at about 850 m. Similarly low relief is found wherever high inner valleys oppose low

The flow and runoff data available for the downstream gauging stations on the main rivers of Bhutan are summarised in Table 3. The flows are given as mean annual discharges and are also expressed in terms of mean specific annual runoffs per unit area, to facilitate comparison with precipitation. They indicate that the main catchments in Bhutan have specific runoffs in the range from 900 to 1600 mm p.a. Rates of actual evapotranspiration in Bhutan are estimated at 500 –800 mm p.a. for rainfed vegetation and up to 1800 mm p.a. for the limited areas of irrigated rice (Van den Brand, 2002). Specific runoffs of about or above 1000 mm therefore suggest that the catchments receive average annual precipitations averaging at least 1500 mm, probably more than 2000 mm. The data cover only a few years and are insufficient for statistical analysis of regional trends. However, they provisionally indicate that the main

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Table 1 Mean annual rainfalls in southern Bhutan Station

Number in Fig. 4

Longitude (8E)

Altitude (m a.s.l.)

Landform

n (years)

Mean annual rainfall (mm)

Chengmari Samtse Pugli Phuntsholing Gedu Darla Kalikhola Sarpang Bhur Panbhang Dechenling Nanglam Deothang Bakuli Daifam

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

898030 898060 898140 898230 898310 898340 898510 908160 908260 908580 918130 918140 918290 918420 928050

430 430 300 420 1980 1750 170 330 375 220 1000 550 800 240 280

P –H H P –H P –H H H P P –H P H H H H H P –H

6 14 6 15 17 12 17 14 7 13 14 11 6 7 10

4160 4200 4270 4940 3450 3380 4570 4480 4070 4150 3250 3280 3310 3270 2830

P ¼ Piedmont, H ¼ Foothills, Rainfall data from Ministry of Trade and Industry, Thimphu; Land Use and Statistics Section, (2000).

rivers have roughly similar flow regimes, and do not show Eastern Bhutan as being significantly drier than the rest of the country. In fact, there are geomorphological indications that some of the eastern valleys are somewhat wetter than the rest (e.g. Van der Poel and Tshering, 2003). The interior of Bhutan consists of inner valleys, northern sections of the interfluves ranges, and the High Himalaya (Norbu et al., 2003b), all of which are relatively dry. The wet southern foothills contribute disproportionately to the total runoff. Mean specific runoff from southern sub-catchments in the Wang basin in western Bhutan are estimated to be over 2000 mm, and may reach 3000 mm in wet years (Baillie, 2002). In Table 3, the discharge at Panbhang, in Eastern Bhutan,

has contributions from the Kuri, Gongri, Chamkhar and some southern tributaries. Subtraction of the discharges of the upper Kuri (Kurizampa) and Gongri (Uzorong) from the total indicate that the subcatchments of the Chamkhar and the southern tributaries combine to generate a mean annual flow of about 230 cumecs (cubic metres per second) from 3765 km2, which is equivalent to a specific annual runoff of about 1930 mm. The rainfall data in Table 2, the soils and natural vegetation all indicate that the upper Chamkhar is a typically dry inner valley. This suggests that the specific annual runoff from the southern subcatchments is substantially over 2000 mm. The similarity of this estimate to those for southern subcatchments in western

Table 2 Mean annual rainfalls in the inner valleys of Bhutan Station

Number in Fig. 4

River

Longitude (8E)

Altitude (m a.s.l.)

Landform

n (years)

Mean annual rainfall (mm)

Namgyel Drukyel Dzong Chapcha Didakom Thimphu Semtokha Punakha Bajo Trongsa Mangdechhu Jakar Bathpalathang Lingmethang Autsho Mongar Trashiyangtse Chazam

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Haa Paro Wang Thim Thim Thim Puna Tsang Puna Tsang Mangde Mangde Chamkhar Chamkhar Kuri Kuri Kuri Kholong Gongri

898 170 898 200 898 330 898 340 898 380 898 410 898 520 898 550 908 310 908 280 908 450 908 460 918 100 918 110 918 140 918 300 918 330

2620 2410 2450 2210 2375 2310 1250 1200 2120 1130 2590 2700 700 800 1600 1830 830

F F L F F F F F L L F F F F L F F

8 16 10 17 17 12 11 11 14 10 11 17 13 10 9 12 10

910 660 740 610 660 600 762 690 1321 1223 719 741 944 1157 896 1176 890

F ¼ Valley floor, L ¼ Lower slope, Rainfall data from Ministry of Trade and Industry, Thimphu; Land Use and Statistics Section, (2000).

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Table 3 Discharge and specific runoff from main catchments in Bhutan River

Main downstream gauging station

Latitude (N)

Wang Puna Tsang Mangde Kuri Gongri Manas

Chimakhoti Dubani Tingtibi Kurizampa Uzorong Panbhang

278 278 278 278 278 268

060 010 100 160 150 500

Longitude (E) 898 908 908 918 918 908

310 030 420 120 250 590

Altitude (m a.s.l.)

Catchment area (km2)

Mean annual discharge (cumecs)

Mean specific runoff (mm p.a.)

1820 300 650 550 650 200

3550 8050 3200 8600 8560 20 925

102 384 149 291 262 784

906 1504 1469 1067 965 1182

Locations shown in Fig. 4. Data from Ministry of Trade and Industry; Merz et al. (2003).

Bhutan corroborates the rainfall data in indicating that all of the southern foothills of Bhutan, including those in the east, have a wet climate. 3.6. Natural vegetation Vegetation is affected by temperature, radiation, wind, soils and land use. Forests are robust and long-lived enough to smooth out annual fluctuations in rainfall, but are sensitive enough to respond to long term climatic trends. Where forests are undisturbed or moderately exploited, as is the case for over half of Bhutan, their general physiognomies are good indicators of prevailing moisture conditions. The distribution of natural vegetation in the Himalayas is predominantly determined by altitude, but it is also possible to discern systematic variations due to moisture regimes. Two altitudinal zonations of forest types have been distinguished in Bhutan; one for mesic forest in moist sites; the other for xeric variants in drier sites (Sargent et al., 1985; Ohsawa, 1987). The xerophytic forest types occur mainly on the lower slopes of the inner valleys, and are dominated by pines, Pinus roxburghii (chir pine) in the valleys below about 1800 m, and P. wallichiana (blue pine) in the higher valleys. There are no xeric forests in the southern foothills, and these are all covered in mesic, moist, mixed evergreen and semi-deciduous, broadleaf rainforests of medium and high stature. There is no major eastwards trend towards more xerophytic types of forest, either in the southern foothills or in the interior valleys. 3.7. Soils The southern foothills of the Eastern Himalayas are seismically active, due to the proximity of the Main Boundary Fault (Fig 2). As shown in Fig. 7, local relief is often over 3000 m and slopes are long and steep. Although Bhutan is located in a seismic window and is relatively quieter than many other sectors of the Himalayas (Gahalaut and Kalpan, 2001), there are still frequent minor earthquake tremors. These combine with the high relief and the heavy and prolonged rainfalls to render many soils and regoliths unstable, so that there are large areas of recent landslip

deposits. The instability of much of the regolith means that few soils have developed mature and pedogenically horizonated profiles. However, there are limited areas of stable soils, and these are deeply and intensively weathered. They have pH values below 5, base saturations below 25%, aluminium occupies more than 50% of the exchange complexes, and some have subsoil argillans (Baillie et al., 2003b). There are no traces of pedogenic carbonates, even in sites with restricted drainage. As most of the soils are under broadleaf forest, their topsoils have of organic carbon contents in the range 2 –6% and C:N ratios below 20. This combination of features is characteristic of soils formed in moist conditions, with substantial surpluses of water available for mineral weathering, nutrient leaching, and the decomposition of organic matter. The vigorous dynamics of these landscapes makes it unlikely that the soils are relict. The soil characteristics developed in current conditions, and not in moister palaeoclimates. The data are still sparse, but there are no indications that the soils of the southern foothills in Eastern Bhutan are less weathered or leached than those further west. There are substantial areas of moderately stable regoliths on the slopes of the inner valleys. The soils have had longer to develop than those in the south, and deep weathering, to more than 5 m, is found at altitudes of up to 4000 m a.s.l. However, the moderate rainfalls mean that leaching is only moderately intense, so that the soils retain some chemical characteristics from their parent materials. The soils from granites and gneisses tend to be moderately acid (pH , 6), and have base saturations below 60% and some labile aluminium, whilst limestone, amphibolite, and calcsilicate soils are of neutral pH and are fully base saturated, with no labile aluminium (Baillie et al., 2003b).

4. Discussion The profiles of the main rivers of Bhutan are variable, ranging from single concavities to multiple knickpoints. The inner valleys of the adjacent basins of the Thim and Puna Tsang are only 20 km apart but differ in altitude by 1200 m. The headwater streams of the Manas in Eastern Bhutan show that there is considerable heterogeneity even a within

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single river system. It is therefore not possible to generalise about a Bhutanese river profile, nor is it feasible to characterise the landscape in swathes that are scores of kilometres wide and may include the catchments of more than one N – S river. Despite the heterogeneity, it is possible to separate the stepped profiles in the west and centre from the simple concave profiles of the antecedent rivers in the east. The difference does not appear to be climatogenic. The data indicate that rainfall in the southern foothills decreases eastwards but is high throughout. There does not appear to a similar trend in the interior, where rainfall is variable. River flows, soils and the natural vegetation do not show any significant overall trend to drier conditions in the east. There appears to be weak concordance of interfluves summits, at about 4000 þ m a.s.l. Motegi (1998) suggests that the summits may be the remnants of a former erosion surface, which developed and weathered at lower altitudes, and has since been uplifted and intensely dissected. The uplifted surface appears to dip slightly eastwards, to about 3500 þ m a.s.l. This could be a result of more rapid, and possibly earlier, Himalayan uplift in western and central Bhutan than in the east It could also account for the concentration of the High Himalayan peaks higher than 7000 m in the centre and west, and their absence from Eastern Bhutan. The dip may relate to the polyfurcation of the Main Central Thrust towards the east (Motegi, 1997; and, pers. comm., 2000). The suggestion of uplift moving peristaltically eastwards and slowing down as it goes, accords with the distribution of the river profile types. Slower uplift in the east would enable dissection by the rivers to keep pace with the orogeny and to cut gorges through the rising High Himalaya. Further west the rise appears to have been so rapid that former headwaters were blocked and their flows reversed northwards towards the Tsangpo or internal drainage basins in Tibet. The concave sections and knickpoints on the rivers draining the slopes of the rapidly rising range in the west are assumed to result from hiatuses in uplift, with the surfaces at high altitudes as remnants from early still-stands. The extensive deep weathering and relatively well-developed soils are consistent with the suggestion that the high inner valleys of Bhutan are remnants of relatively mature landscapes (Norbu et al., 2003b). More recent uplift episodes have lowered the base levels of the downstream sections. These work upstream as pulses of dissection, with knickpoints eating back into the higher and older sections. The slopes formed by recent downcutting are young, steep and unstable, and their soils are less developed. The division between the western consequent and the eastern antecedent river profiles thus appears to be more plausibly explained by macro-tectonic factors than by climatic differences. The more localised variability in profile form between adjacent consequent rivers is also attributed mostly to structural features at smaller scales.

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There are several sets of substantial faults in Bhutan aligned from NNW –SSE round to NNE – SSW, veering to NE – SW in the southeast of the country (Fig. 2). These may result from crustal failure during intense regional uplift (Bhargava, 1995; Kumar, 1997). Some of the crush zones appear to have been preferentially excavated by reaches of the main rivers. This may account for the stretches (Fig. 8), where the thalwegs are linear trench-like features. They are defined by apparently structurally determined parallel breaks of slope at the bases of the flanking hills. The N – S alignment of the faults allows adjacent catchments to respond separately to crustal movement and can contribute to inter-basin variations in uplift. Another possible tectonic contributor to the inter-basin variability is localised faulting across the courses of the rivers, with upthrow on the downstream side (Nakata et al., 1984; Zhang, 2001). The most prominent E –W faults are those associated with the regional thrust sheets (Fig. 8). Others have been identified within the outcrops of the Tethyan and Lesser Himalayan formations, and also at boundaries of the gneiss outcrops (D.G.M., 2001). Faults of this type may account for the concentration of interfluve crest profiles with significant central passes in the centre of the country (Fig. 3), and the isolated southern blocks of mountains may be the remnants of one or more horsts. However, non-tectonic factors also contribute to the variability in river and valley morphologies. These include: 1. The upstream migration of knickpoints may be retarded by lithologies that are markedly more competent than the country rock, such as thick bands of quartzites within the metamorphics. 2. Discharge increases downstream, as main rivers are joined by tributaries and drain increasingly large catchments. Declining gradients and flow velocities with declining bed gradients downstream may partly or wholly offset the increase in erosive power, but the net effect may be increased bed scour and downcutting. The downstream increase in discharge is particularly marked in Bhutan because the southern sub-catchments receive heavier rainfall and generate higher runoffs than those upstream. 3. Abrupt and substantial increases in discharge below the confluences with major tributaries may increase local bed dissection sufficiently to produce knickpoints. The knickpoint at about 1800 m a.s.l. on the Wang is located below the confluences of the Paro, Thim and Ha. Similarly the major knickpoint on the Chamkhar is just downstream of its confluence with the Tang, a major east bank tributary (Fig. 3). 4. The formation and breaching of ephemeral dams caused by debris flows, landslides, or fluctuations in glaciers can give rise to temporary knickpoints (Van der Poel and Tshering, 2003; Hukku et al., 1974; Pal and Mehra, 1975; Mithal et al., 1982; Hewitt, 2001). Most of these obstructions are short-lived and leave no trace (Ermini and Casagli, 2003). However, a few persist long enough to impound substantial lakes, build up lacustrine deposits, and leave remnant terraces. The clearest example in Bhutan is in

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Fig. 8. Main structural features of Bhutan, including faults (dashed lines), Main Central Thrust (MCT), Main Boundary Fault (MBF) and linear thalwegs (solid lines).

the Puna Tsang inner valley (Motegi, 2001), where some of the terrace alluvia appear to have been deposited in a substantial and relatively long-lasting palaeo-lake behind a large landslip dam above the major knickpoint (Figs. 3 and 5(C)). Our data do not support the view that Eastern Bhutan is notably drier than the rest of the country. This accords with general accounts of the Eastern Himalayan region. Specific runoff from the Brahmaputra catchment is at least twice as high as that of the Ganges and up to ten times that of the Indus. As much of the upper part of the TsangpoBrahmaputra catchment consists of Tibetan high altitude dry steppe, about three quarters of the runoff enters the system in the Brahmaputra section, downstream of the syntaxial bend (Bandyopadhyay and Gyawali, 1994). It has been estimated that about two thirds of the sediments in the Brahmaputra are derived from the southern slopes of the Eastern Himalaya (Singh and France-Lanord, 2001), and a similar partition is likely for the runoff. The valley of the Teesta is aligned approximately N –S, and is open to the monsoonal airflows. At 88 –898E, it is far enough west not to be shadowed by the Meghalaya Plateau (Fig. 1). It appears to be one of the wettest parts of the Himalayas, with mean annual rainfall of more than 5000 mm (Rudra et al., 1982), and individual storms of up to 3000 mm in three days (Bandyopadhyay and Gyawali, 1994). The regional synopsis of Eguchi (1997) and the summaries of Rao (1981); Miehe et al. (2001) show rainfall decreasing to the east of the Teesta, but still heavy right along the Himalayan front,

through Bhutan and the foothills of Arunachal Pradesh, to the syntaxial bend and beyond. There are no indications of significantly drier conditions in eastern Bhutan. Pan-Himalayan ecological overviews concur that all of the front slopes in the Eastern Himalayas are well watered (e.g. Sahni, 1981; Banerjee, 1994). They distinguish between dry-moist deciduous forests on the front slopes in the western and central Himalayas and wet evergreen forests in the east. The pattern of evergreen forest on the southern foothills and pine forests on the lower slopes in the inner valleys is the same in Arunachal Pradesh as in Bhutan (Chowdery, 1996), and there are no suggestions that the vegetation of Eastern Bhutan is significantly more xeric than elsewhere. The generally favourable moisture supply, combined with wide range of altitudes and temperature regimes, the pockets of drier conditions in the inner valleys, and the location close to the overlap between several major biogeographical zones, make the Eastern Himalayas a globally important centre of biodiversity (Whitmore, 1988; Pearce and Cribb, 2002). Thus the floristic diversity of the alpine communities of the Eastern Himalayas, with more than 7000 known species of plants, is over three times greater than that of the attitudinally equivalent but drier ecoregion in the Western Himalayas. In the Indo-Pacific zone, it is second in floristic diversity only to the hyper-rich lowland rainforest ecoregion of Borneo (Wikramanayake et al., 2002). The limited soil data available also confirm the general wetness of the Eastern Himalayas. High levels of labile

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aluminium, low pH and base saturation, podzolisation, and other features characteristic of moist and wet soil moisture regimes, similar to those in Bhutan, are found throughout Sikkim, Arunachal Pradesh, and other parts of the Eastern Himalayas (e.g. Gangopadhyay et al., 1990; Patiram, 1990; Das et al., 1996; Walia and Chamuah, 1996; Peng et al., 1997). The profiles of few other Eastern Himalayan rivers have been plotted, but Seeber and Gornitz (1983) include the Teesta (Sikkim) and Subansiri (Arunachal Pradesh) as the two most easterly profiles in their comparison of 17 major rivers along the Himalayan arc. They identify two convexities on the Subansiri and three on the Teesta, suggesting that their profiles and dissection histories are probably as varied and complex as those of the rivers of Bhutan. They do not find that river profiles in the central and western Himalayas are simpler and have fewer convexities below 3000 m than those in the east. They noted two knickpoints on the Arun and three on the Karnali with in the Central Himalayas, and two on the Sutlej in the west. More detailed analyses confirm the complexity of the profiles of the Kali Gandaki (Iwata et al., 1984) in the centre and the Spiti in the west, with six knickpoints identified on the latter (Ameta, 1979; Sah and Virdi 1997; Gupta and Virdi, 2000).

5. Conclusions Our systematic profiling confirms that the main topographic features of Bhutan are aligned more or less N –S, and that extrapolation of the E –W lineation and zonation of Nepal is not appropriate. The rivers have varied profiles, and the morphological development of each basin needs to be treated separately. It is not possible to generalise about the rivers of Bhutan, even within a single river system. The profile data do not support the idea that the rivers of Eastern Bhutan have more knickpoints than those in the rest of the country. In fact, the two most easterly rivers, Kuri and Gongri, have the least stepped of all the main N –S river profiles in Bhutan. Comparison of our data with findings elsewhere does not indicate that the rivers in Bhutan have more knickpoints at low altitudes than those in the western and central Himalayas. The rainfall data do not indicate that the Eastern Bhutan sector of the Eastern Himalayan front has a climate that is markedly drier than those in the sectors to east and west. There does appear to be a rainshadow from the Meghalaya Plateau, but the Eastern Bhutan sector of the southern foothills still receives about 3 m rainfall p.a. and is designated as perhumid in the zonation of Rao (1981). The rainshadow effect does not appear to penetrate far northwards into the mountains, and the discharge of the main rivers corroborate the rainfall data in showing that the interior of Eastern Bhutan receives as much rainfall as the rest of the country.

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Many of the topographic features accord with suggestion that Himalayan uplift was earlier and more rapid in the west than in the east of Bhutan. Some of the variation between adjacent basins can be explained by the substantial N – S faulting. The general topographic structure of Bhutan and the size, distribution and heterogeneity of knickpoints, including those in the east of the country, appear to be determined more by combinations of tectonic and other nonclimatic factors than by low rainfall.

Acknowledgements We are grateful to the Ministry of Agriculture, Royal Government of Bhutan, for permission to submit this paper, and to the Meteorology and Hydrology Units of the Ministry of Trade and Industry, and to the Chhukha Hydroelectric Power Corporation for meteorological and hydrological data. We thank our colleagues in the National Soil Services Centre for their contributions and discussions on all aspects of Bhutan’s biophysical environment; Dr Mutsumi Motegi and Piet van der Poel for access to unpublished materials and guidance on Bhutan’s geology and geomorphology; Juerg Merz for hydrological and Ms Rebecca Pradhan for ecological guidance; and Dr Bodo Bookhagen for an update on his unpublished work. We are grateful to Dr Tony Barber and the journal’s referees, Drs Adam Pain, Francis Turkelboom, Prof. Shuji Iwata, and to Professor Roy Morgan for constructive suggestions on the structure of the paper and on many points of detail.

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