Journal of Hydrology 464–465 (2012) 447–458

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The role of mega dams in reducing sediment fluxes: A case study of large Asian rivers Harish Gupta a,⇑, Shuh-Ji Kao a,b, Minhan Dai a a b

State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China Research Center for Environmental Changes, Academia Sinica, Nankang, Taipei 115, Taiwan

a r t i c l e

i n f o

Article history: Received 31 January 2012 Received in revised form 10 July 2012 Accepted 21 July 2012 Available online 31 July 2012 This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of Ellen Wohl, Associate Editor Keywords: Suspended sediment Indian peninsular rivers Large Asian rivers Mega dams Major events

s u m m a r y In order to sustain the ever growing population and to meet water and energy requirements of the rapidly growing economies, most of the large rivers draining through East, Southern and Southeast (ESSE) Asian region have been regulated all along their courses, over the past few decades. For instance, ESSE Asian countries (China, Taiwan, Vietnam, Myanmar, Thailand, India, Pakistan and Bangladesh) host about 250 mega dams and several tens of thousands of large and small reservoirs. The present study provides a revised estimate on annual suspended sediment fluxes of the large rivers draining through ESSE region, including the latest data of the Indian peninsula rivers. In the last 50 years, the combined annual sediment flux of the large Chinese rivers has been reduced from 1800 million tons (Mt) to about 370 Mt. We estimate that at present the Indian peninsular rivers collectively transport about 83 Mt of sediment annually. The Ganga–Brahmaputra and the Indus, contribute 850 and 13 Mt of sediments, respectively to the oceans. Our revised estimates suggest that at present the large rivers of ESSE region, collectively delivering 2150 Mt of sediment annually to the oceans. We show that at decadal scale, decline in sediment fluxes of the large Asian rivers are proportional to the number of mega dams present in the respective catchments. We also demonstrate that storage of sediment-laden water of major flood events (majorevent), led to huge sediment trapping behind mega dams. Thus, ongoing and planned dam constructions activities across ESSE Asia may further reduce the annual sediment fluxes. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction It is evident that geological processes relentlessly change the face of the Earth, but we pay less heed to a second type of process of change, extremely recent if considered at geological time scale, consists of the permanent activities by man (Van Loon, 2001). At the beginning of the Holocene, as a result of the transition from food appropriation to food production, the human interaction and influence on the environment has increased and intensified (Ter-Stepanian, 1988). One of the major consequences of increased agriculture production was, a large-scale conversion of forested areas to agriculture lands, leading to increased soil erosion. Rivers in Asia have been centers of ancient civilizations and it is likely that the human influence on soil erosion, dates back to as early as 9000 years ago with deforestation and spread of agriculture from the ‘‘Fertile Crescent’’ (Heun et al., 1997). It was followed by wild rice cultivation about 7500 years ago in ESSE Asian region (Glover and Higham, 1996); whereas, irrigated rice farming become popular 5000 years ago (Roberts, 1998). Wilkinson and McElroy (2007) calculated that present farmland denudation is proceeding ⇑ Corresponding author. E-mail address: [email protected] (H. Gupta). 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.07.038

at a rate of 600 m/million year (75 Gt yr 1) and is largely confined to the lower elevations of the Earth’s land surface. Increased soil erosion led to increased sediment fluxes in most of the rivers across the globe, which prompted Milliman and Syvitski (1992) to propose that due to growing human activities, natural processes of soil/sediment erosion have been accelerated, perhaps by a factor of two on a global scale. Over the past three centuries, the human population increased 10-fold to 6 billion, growing by a factor of four during the past century alone (McNeill, 2000). Man’s intervention has reached its peak during the 20th century with energy consumption, water use, irrigated land and crop area increasing by factors of 16, 9, 5 and 2 respectively; while deforested land increased by 20% (McNeill, 2000). In this modern era of rapid human-caused changes (also known as the Anthropocene), driven by a need to cope with the growing demand of food and energy, both for agriculture and industrial needs, tens of thousands of dams have already been constructed and several thousand more are planned across the globe. It has been shown that dams trap a significant proportion of the global sediment fluxes (Syvitski et al., 2005; Vorosmarty et al., 2003) that would otherwise be delivered to the oceans and this number appears to be steadily increasing (Liquete et al., 2004). It is obvious that human actions persistently change the trends of the suspended loads in the

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world’s rivers (Meybeck, 2003; Syvitski et al., 2005; Walling and Fang, 2003) and with the growing human activities, much has changed in terms of sediment delivery with variances in both directions (Dearing and Jones, 2003; Walling and Fang, 2003). On a global scale, Asian rivers have been recognized as the largest sediment supplier to the world’s oceans (Milliman and Meade, 1983; Milliman and Syvitski, 1992; Walling and Webb, 1983). Milliman and Meade (1983) estimated that the large rivers flowing through ESSE Asia, contribute approximately 6300 Mt of sediment annually to the coastal seas and about 1800 Mt of it comes from the large Chinese rivers. A compilation of sediment flux data from some of the key studies in the last century (Abbas and Subramanian, 1984; Holeman, 1968; Milliman and Meade, 1983; Milliman and Syvitski, 1992; Narayana and Babu, 1983; Vaithiyanathan et al., 1988) suggest that the rivers flowing through Indian subcontinent transported about 2500 Mt of suspended sediments annually; thus, accounting for 15–20% of the global sediment flux. Although the large Himalayan rivers, such as the Ganga–Brahmaputra (1235 Mt; (Abbas and Subramanian, 1984) and the Indus (481 Mt; (Holeman, 1968) together contributed approximately 70% of it, the rest come from the peninsular rivers. Recently, Syvitski et al. (2005) estimated that the Asian rivers (except Indonesian) carry approximately 4.74 ± 0.8 Gt of sediments annually and pointed out that humans have increased the inland sediment transport by the global rivers through soil erosion by 2.3 ± 0.6 Gt yr 1. According to them despite this increase, the annual flux of sediment reaching to the world’s coasts, has been reduced by 1.4 ± 0.3 Gt, because of the retention within reservoirs; thus resulting in a 10% lower modern global sediment flux

(12.6 Gt) compared with the pre-Anthropocene load (14 ± 0.3 Gt). Vorosmarty et al. (2003) estimated that large reservoirs (>0.5 km3 maximum storage capacity) and small reservoirs in regulated basins, trap 30% and 23% of the sediment flux at basin scale, respectively. According to Vorosmarty et al. (2003) all registered reservoirs (45,000) collectively trap 4–5 Gt or 25–30% of the total sediment annually; while an additional impact of smaller unregistered impoundments (80,000) yet remains unknown. Syvitski et al. (2005) stressed that in a modern world without reservoirs, the global annual sediment flux would be about 16.2 Gt. Presently, around 70% of the world’s rivers are intercepted by large reservoirs (Kummu and Varis, 2007), thus compelling Walling and Fang (2003) to mention that reservoir construction currently represents the most important influence on land–ocean sediment export. Based on data from the International Commission on Large Dams (ICOLD), Farnsworth and Milliman (2003) mentioned that as of year 1999, the number of large dams (defined as being higher than 15 m) under construction in China and India were 330 and 650, respectively. Among the 47,425 dams listed by ICOLD, China accounted for more than half, a remarkably high number considering that in 1949 China only had three large dams. According to the Chinese National Committee on large Dams (Chin-COLD, 2011) by the end of 2008 there were over 80,000 large and small-scale reservoirs in China, of which 5340 dams (completed or under construction) were higher than 30 m. Among these large dams, about 115 are mega dams (with height 100 m and above and/or with storage capacity of >1 km3 (Fig. 1). Farnsworth and Milliman (2003) pointed out that continuing dam construction throughout Africa and Southern Asia may significantly affect water and

Fig. 1. Spatial distribution of mega dams and changes in annual sediment flux from East, South and Southeast Asian large rivers draining into the Bohai sea, East China Sea, South China Sea, Andaman Sea, Bay of Bengal and the Arabian Sea. Thickness of arrow-bars denotes the Historical and Current amount annual sediment load (Mt).

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sediment deliveries to the global oceans. Recent studies on the large Asian rivers (i.e. Yangtze, Yellow, Pearl, Indus and Red) demonstrated that water discharge and sediment fluxes of these rivers have been drastically altered, either directly due to trapping behind dams and/or protective measures to prevent soil erosion (Chu et al., 2009; Inam et al., 2007; Le et al., 2007). Most of the Indian rivers (including Ganga) have been regulated in the last few decades and declines in downstream sediment loads have already been reported for some of the large rivers (Biksham and Subramanian, 1980, 1988; Chakrapani and Subramanian, 1990, 1993; Gupta and Chakrapani, 2005, 2007; Ramesh and Subramanian, 1986, 1988; Vaithiyanathan et al., 1992). Since the beginning of the 21st century, several studies have been devoted to understand the dynamics of reduced sediment fluxes of the large Chinese rivers (Chen et al., 2001; Chu et al., 2009; Dai et al., 2009; Gao et al., 2010; Hu et al., 2009; Kong et al., 2009; Li et al., 2011; Liu et al., 2007; Miao et al., 2010; Wang et al., 2007; Xu and Milliman, 2009; Yang et al., 2002, 2007, 2011; Zhang et al., 2008, 2009; Zhu et al., 2008). However, except Gupta and Chakrapani (2005, 2007) and Panda et al. (2011), not many recent studies documented the changes in sediment fluxes of the Indian peninsular rivers. Additionally, no further attempt has been made to quantify the overall impact of construction of dams, on annual sediment fluxes at regional scale and to revise the annual sediment exports from the rivers draining through Indian subcontinent. Recent study by Panda et al. (2011) relates the sharp decline in annual sediment loads of the most peninsular rivers with climatic factors, which seems to be only partly true. According to India’s National Register of Large Dams-2009 (In-NROLD, 2009) there are about 4711 completed large dams (as per ICOLD definitions) and another 390 are under construction. Among these large dams, 78 are mega dams (61 completed and 17 under construction; Fig. 1) and are considered of national importance. Therefore, besides modifications in sediment fluxes due to change in rainfall (Panda et al., 2011) the overall impact of these dams cannot be ignored and warrant a thorough investigation. Note that in case of the peninsular rivers, regular sediment load measurement started by the year 1965 (IHDB, 2009). However, as of 1965, there were already about 13 mega and 900 large dams on the Indian rivers (In-NROLD, 2009) and thus, making it difficult to estimate pre-dam sediment loads. In the wake of present development, our study provides a new set of data for the large peninsular rivers and shows the impacts of dams on annual sediment delivery. Here we use annual sediment load data of 10 large rivers, spanning a considerable length of time (Appendix A); thus providing reliable, precise and updated information on sediment fluxes of the Indian peninsular rivers. Given the unique status of the Asian rivers, in terms of densely populated catchments with relatively greater annual sediment fluxes to the coastal seas, construction of dams and other anthropogenic activities will affect the overall sediment transportation by these rivers. Therefore, based on the present knowledge and recently published data, we also provide a revised estimate on sediment fluxes of the large rivers draining ESSE Asian region.

2. Study area and methods Ten large peninsular rivers with catchment areas of >20,000 km2 (Appendix A), were studied to estimate present day suspended sediment fluxes to the coastal seas and to examine the modification in annual sediment loads during the recent decades (Fig. 1). Among them, rivers such as Godavari, Krishna, Mahanadi, Cauvery, Pennar and Brahmani discharge into the Bay of Bengal, whereas Narmada, Tapti, Mahi and Sabarmati are flowing into the Arabian Sea. The catchments of these 10 large rivers together constitute an area of 1.11  106 km2, which is more or less comparable to the

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total catchment of the Ganga–Brahmaputra system in India (1.06  106 km2) (Jain et al., 2007). Himalayan rivers, Ganga–Brahmaputra and Indus, are characterized by high relief, deep valleys and originate from glaciers. In contrast, the peninsular rivers run through hard bedrock with gentle slope and mostly depend on seasonal monsoon rains for the flow. Although the peninsular rivers are not as huge as the Himalayan rivers, they constitute an important water domain. The drainage areas of the peninsular rivers are highly urbanized and support about 50% (600 million) of the total Indian population. Details of hydrological characteristics of these rivers, name and geographical location of downstream gauge stations and the time period of data used in this study, are provided in Appendix A. Multi-annual non-classified sediment load data, recently made available by Central Water Commission (CWC) government of India (http://www.cwc.nic.in) in the public domain, were used to evaluate the present day sediment load of the large peninsular rivers. However, being transnational rivers, data accessibility remains a major obstacle for estimating recent annual sediment loads of the Ganga and the Brahmaputra rivers. The coastal sediment flux estimate for the Ganga at Hoogly (India) and the Ganga–Brahmaputra at Mawa gauge station (Bangladesh) are revised using data of Abbas and Subramanian (1984) and Islam et al. (1999), respectively. The latest data for the Indus river, the large Chinese rivers and the Southeast Asian rivers, were compiled from recent publications (Inam et al., 2007; Tanabe et al., 2003; Winterwerp et al., 2005; Le et al., 2007; Robinson et al., 2007; Wang et al., 2007; Liu et al., 2008; Walling, 2008; Zhang et al., 2008; Dai et al., 2009; Furuichi et al., 2009; Hu et al., 2009; Xue et al., 2011). While choosing recent fluxes estimates, preference was given to the studies involving long-term data of annual sediment loads at extreme downstream gauge stations/sampling locations. Annual rainfall data of the peninsular basins were taken from Ranade et al. (2007). The National Register of Large Dams, published in 2009 (In-NROLD, 2009) provides information of mega dams in India. We extracted information of mega dams in China and Southeast Asian countries, from published literature and Wikipedia.

3. Results and discussion 3.1. Sediment flux of Indian subcontinent: past and present 3.1.1. Peninsular rivers Table 1 summarizes average annual sediment loads of the 10 large peninsular rivers. For comparison, sediment loads of these rivers are separated in three different columns (Table 1). Data provided by all of the previous studies are termed as Historical. The column with title Long-term refers to the average annual sediment loads, calculated from the data available with us (Appendix A) for a particular river system. Average annual sediment loads observed during the 10 most recent years is termed as Current. The Current (i.e.10 year’s average) annual sediment loads (82.9 Mt; Table 1) of the peninsular rivers, are remarkably lower than the historical estimates (Ramesh and Subramanian, 1993; Vaithiyanathan et al., 1988). However, in comparison to the previously estimated water volume of 281 km3 yr 1 (Ramesh and Subramanian, 1993), at present the large peninsular rivers are discharging 238 km3 yr 1 (Table 1). It is interesting that in contrary to 15% decline in water discharge, overall sediment flux reduced by two-third. According to Panda et al. (2011) a non-significant decreasing trends in monsoon rainfall and frequent drought years, could be responsible for the observed decline in sediment fluxes. They opinioned that a small change in rainfall towards the deficit side, leads to a significant reduction in sediment load. Fig. 2, however suggests either a constant rainfall (Godavari, Tapti and Narmada) or shows an

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Table 1 Large peninsular rivers: hydrological parameters of basin; present day water and sediment flux and comparison with previous estimates. River and place of origin

Area (km2)

Length (km)

Elevation (m)

Rainfall (mm)

Water flux (km3 yr Historicala

1

Long termb

)

Suspended sediment flux (Mt yr 1) Currentc

Historical

Long termb

Change in annual sediment load (%)A Currentc

Rivers flowing into the Bay of Bengal Godavari (Sahyadri Range) 312,812

1465

1067

1042

92.25

84.3

76.7

95.51 1702 1703 38.84

Krishna (Mahadev Range)

258,948

1401

1337

784

32.4

16.8

12.5

4.115 642 4.116 0.324

Mahanadi (Maikala Range)

141,589

851

442

1417

54.4

48.2

49.9

68.07 15.78 30.66 13.24

Cauvery (Sahyadri Range)

81,155

800

1341

1092

11.5

7.56

6.99

32.09 1.5910 1.46 0.474

0.37

0.32

80

Pennar (Nandi Hills)

55,213

597

762

700

5.2

1.99

2.5

7.011 6.96 0.264

1.60

1.62

77

Brahmani (Ranchi Plateau)

39,033

799

600

1305

16.3

17.04

14.9

20.36 13.34

7.12

5.10

75

46.7

28.1

19.5

69.79 61.06 44.44 28.512

20.2

3.23

95

19.5

45.4

1.09

17.6

44.2

0.52

10.0

74

87

67

Rivers flowing into the Arabian Sea Narmada (Maikala Range)

98,796

1312

1057

1180

Tapi (Satpura Range)

65,145

724

752

830

Mahi (Aravalli Range)

34,842

583

500

700

Sabarmati (Aravalli Range)

21,674

371

762

800

Total

9.71

10.8

1.45 280.8

7.83

6.53

1009 24.76 10.54

4.51

4.49

22.09 9.76 5.884

2.73

3.13

68

0.54

0.42

4.66 0.0184

0.163

0.163

96

220.1

238.0

341.96

117.3

14.6

82.93

41

75.7

1

Biksham and Subramanian (1980)d; 2Milliman and Syvitski (1992); 3Biksham and Subramanian (1988); 4Chandramohan et al. (2001); 5Ramesh and Subramanian (1988)e; 6Ramesh and Subramanian (1993); 7Holeman (1968); 8Chakrapani and Subramanian (1990)f; 10Narayana and Babu (1983); 11Vaithiyananathan et al. (1988)g; 12Gupta and Chakrapani (2005)

a b c d e f g A

From Ramesh and Subramanian (1993). For the maximum period as shown in Appendix A. Average of 10 recent years (Appendix A). Five years between 1969 and 1974. Five years between 1984 and 1989. Five years between 1980 and 1981, 1985 and 1986 (except 1984 and 1985). For 9 years between 1971 and 1981 (except 1976 and 1977). To calculate percentage decline in annual sediment load current estimates were compared with the historical estimates shown as bold.

insignificant increase (Mahandi and Krishna) in different large basins. Only the Cauvery river shows an insignificant declining trend in annual rainfall (Fig. 2d), however it’s sediment load remains constant. The annual water discharge of most of the large peninsular rivers (except Krishna) also remains largely unchanged. Thus, changes in rainfall and water discharge could not explain, the notable decline in the annual sediment fluxes of the large peninsular rivers. In a study of the 292 large global river systems, Nilsson et al. (2005) showed that in comparison to the Himalayan rivers (e.g. Ganga–Brahmaputra) the large peninsular rivers are strongly impacted due to river channel fragmentation and water flow regulation by dams. It seems to be true as more than half of Indian

mega dams (41 already completed and 6 under construction) and about 75% of large dams (about 3800) are located in the peninsular region. Thus, the increasing number of dams across the peninsula could be obvious reasons for the decline in the annual sediment supply of the large rivers. Out of 41 mega dams, the Krishna and the Godavari basins each host 9, followed by the Narmada (4), Cauvery (3), Mahanadi (2) and the Mahi (2), whereas the Pennar, Tapti, and the Brahmani each having one such dams (In-NROLD, 2009). After construction of nine mega dams, the annual sediment load of the Krishna river has been reduced from 64 Mt to less than 1 Mt (Table 1). We estimated that between 1978 and 2003, two mega dams on the Narmada mainstream, namely Bargi (upstream) and Sardar Sarovar (downstream) together retained about 500 Mt

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451

Fig. 2. Annual variations and trends in water discharge (km3) and sediment flux (Mt) at the terminal gauge stations of six large peninsular rivers. The annual rainfall (cm) data (basin average) have also been plotted for reference. Vertical dashed lines represent the number of mega dams in basin with the year of closure. Note that in the recent past no mega dam was constructed in the Cauvery and the Tapti basins.

of sediments. However, in absence of annual time series data, quantifying the amount of sediment trapped by other three-mega dams (one mainstream and two other on tributaries) in the Narmada basin remains difficulty. Time-series plots of six largest peninsular rivers (Fig. 2) show the impact of construction of mega dams on annual water discharge and sediment fluxes. Among these six rivers, the Cauvery and the Tapti show insignificant trends in annual sediment delivery. Interestingly, in recent years no additional mega dams were constructed in these two basins (Fig. 2d and e). In comparison to Historical sediment loads, all six rivers showed a marked decrease in the Current annual sediment fluxes. The overall decline is about 67%, 74%, 87%, 80%, 41% and 95% for the Mahanadi, Godavari, Krishna, Cauvery, Tapti and the Narmada rivers, respectively (Fig. 2). The rest of the large peninsular rivers also registered remarkable decreases in the annual sediment fluxes (Table 1). 3.1.2. Extra peninsular rivers (Ganga–Brahmaputra and Indus) Milliman and Meade (1983) estimated that the Ganga– Brahmaputra and the Indus transport about 1670 and 100 Mt of sediment annually into the Bay of Bengal and the Arabian Sea, respectively. Abbas and Subramanian (1984) estimated that the annual sediment load of the Ganga river at Farakka (10 km upstream to India–Bangladesh border) is about 729 Mt. Before

entering Bangladesh, the Ganga bifurcates into two distributaries, one called the Hoogly (or Hooghly) river (the first distributary of the Ganga) flows in India and the other is known as the Padma river after entering into Bangladesh. A significant portion of sediment transported by the Ganga at Farakka is diverted into the Hooghly distributary via a feeder canal. The annual sediment load transported by the Hoogly distributary into the Bay of Bengal accounts for 328 Mt (Abbas and Subramanian, 1984). According to Islam et al. (1999) between 1979 and 1995, the Ganga at Hardinge Bridge (Bangladesh) annually transported 316 Mt of suspended sediments. Thus, annual flux numbers for the Ganga river at Harding bridge are lower than the previous estimates, such as Coleman (1969; 485 Mt), Milliman and Meade (1983; 680 Mt) and Hossain (1991; 350–600 Mt). In addition to differences in methods/periods of measurements in these studies, the observed decrease in sediment load could also be due to construction of several mega dams in the Ganga basin, closure of Farakka barrage (1974) and diversion of sediments laden water into the Hoogly distributary. The Brahmaputra river flows through Tibet in China (1600 km), eastern India (900 km) and Bangladesh (400 km) and discharges into the Bay of Bengal. Number of attempts have been made to estimate the annual sediment load of the Brahmaputra river, including Holeman (1968; 800 Mt), Coleman (1969; 617 Mt), Milliman and Meade (1983; 1157 Mt) and Hossain (1991;

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650 Mt). Recently, Islam et al. (1999) estimated that between 1989 and 1994, the Brahmaputra river (at Bahadurabad, Bangladesh) annually transported 721 Mt of suspended sediments. Based on 13 year long measurements (between 1979 and 1995) at Mawa gauge station, Islam et al. (1999) provided, combined sediment load of the Ganga–Brahmaputra in Bangladesh. The Mawa gauge station (located near to the mouth of Ganga–Brahmaputra system) is about 160 km and 235 km downstream of Hardinge Bridge and Bahadurabad gauge stations, respectively. According to Islam et al. (1999), the combined sediment load (525 Mt) of the Ganga–Brahmaputra at Mawa gauge (Fig. 1) station represents the mean annual sediment input into the Bay of Bengal. Thus, the Hoogly distributary (328 Mt) and the Ganga–Brahmaputra (525 Mt) collectively transport about 853 Mt of sediments annually into the Bay of Bengal. Consequently, the revised estimates of the Ganga–Brahmaputra are lower than the previous estimates provided by Coleman (1969; 1130 Mt), Milliman and Meade (1983; 1670 Mt), Milliman and Syvitski (1992; 1060 Mt). According to Islam et al. (1999) deposition of sediments upstream of Mawa gauge station could be one of the main reasons.

Holeman (1968) estimated that the Indus river transports 440 Mt of sediment annually and later Milliman and Meade (1983) revised it to 100 Mt yr 1. However, recent estimates by Inam et al. (2007) tell a different story and explain how human interventions, particularly construction of dams have modify natural processes of sediment transportation. Inam et al. (2007) presented annual sediment loads of the Indus river at Kotri Barrage (270 km upstream from river mouth) during the last 73 years and showed that the annual sediment load of the Indus river reduced drastically from 193 Mt (between 1931 and 1954) to 13 Mt (between 1993 and 2003). According to them, construction of three large dams on the Indus river, namely Kotri Barrage, Mangla and Terbela led to this situation causing annual water discharge to reduce from 110 km3 to 37 km3. However, the possible influences of four additional mega dams, Bhakra (1963; on Sutlej river), Pong (1974; on Beas river), Salal (1986; on Chenab rver) and Chamera (1994; on Ravi river) constructed in the upstream watershed (in India) on sediment transport were not considered. According to Inam et al. (2007) out of sixteen major creeks forming the Indus delta, only one outlet (Khobar Creek)

Table 2 Historical and Current water discharge and sediment flux in of the large Asian rivers draining through Indian subcontinent, China and Southeast Asia. River/System

Area (103 km2)

Water discharge (km3 yr

1

237.22

341.91 (Table 1) 16703 10444 12355 (1981-spot samples) 1003 2504 1937 (1931–1954) 4783 4804 4798 (1954–1963) 2549 (1956–2002) 10803 1204 133010 (1950–1980) 11878 (1954–1963) 693 804 67.28 (1954–1963) 813 814 0.058 (1960–1969) 143 22.48 (1954–1963) 413 414 38.78 (1954–1963) 7.58 (1954–1963) 28 (1977–1986) 1603 1604 14514 (1962–2003) 165.813 (1960–1962) 2653 2604 36415 (1877–1878) 1004 18815 (NA) 114 2518 (1960–1972) 1303 1234 10819 (1960–1969) 4435

832 (Table 1) 8506 Ganga (1979–1995) Brahmaputra (1989–1994)

73.9 31

137 (1993–2003)

93

2798 (1996–2005) 1569 (2003–2008)

67

16010 1938 (1996–2005)

84

5411 52.78 (1996–2005)

22

0.0078 (1996–2005)

86

5.18 (1996–2005)

77

960

1077

NA

107 9003 1807 899.48 Yellow (Huangho) 752

493 48.8510

Zhujiang (Pearl)

453

3023

Haihe

245

23

318

285.7 0.98 NA 28.88 220

6

3

38 Minjiang Qiantangjiang Mekong

Irrawaddy (Ayeyarwady)

61 41.5 795

52.58 16.48

NA 4703 384.313

38813

4283 42215

37916

272 160

21115 2317

21115 30

120

1233

106.919

414

Salween (Thanlwin) Chao Phraya Red (Hungho)

Total 1

Change in annual sediment load (%)A

Current (period)

Indus

Liaohe

)

Historical (period)

280.81 9713

270

1

Current

1110 1480

Huaihe

Annual sediment load (Mt yr

Historical

Indian peninsular rivers Ganga–Brahmaputra

Yangtze

)

8

2.7 (1996–2005)

93

2.48 (1996–2005) 1.6012 (1996–2005) 16813 (1997–2002)

69 20 –

32516 (1966–1996)

–

18815 (NA)

–

5

18

(1973–1993)

36.319 (2000–2008)

83 75.8

2240

Ramesh and Subramanian (1993); 2 Present study; 3 Milliman and Meade (1983); 4 Hovius (1998); 5 Abbas and Subramanian (1984); 6 Islam et al. (1999); 7 Inam et al. (2007); 8 Dai et al. (2009); 9 Hu et al. (2009); 10 Wang et al. (2007); 11 Zhang et al. (2008); 12 Liu et al. (2008); 13 Walling (2008); 14 Xue et al. (2011); 15 Robinson et al. (2007); 16 Furuichi et al. (2009); 17 Tanabe et al. (2003); 18 Winterwerp et al. (2005); 19 Dang et al. (2010). A To calculate percentage decline in annual sediment load current estimates were compared with the historical estimates shown as bold.

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now receives the Indus water downstream to Kotri Barrage, thus leading to an ecological disaster and increased coastal erosion. Our estimate suggests that the present day annual sediment supply to the coastal seas from the Indian subcontinent is approximately 950 Mt, which is significantly lower than previous estimates. Though, there is no dam on the Brahmaputra presently, however at least two mega dams, Subansiri on the Subansari river (India; to be completed by 2012) and Zangmu dam on the Brahmaputra mainstream (China; to be completed by 2015) are under construction. In contrast, the Ganga basin already has 13 mega dams and five more are under construction. With impoundment of the Ganga mainstream by Tehri dam (2005) and completion of other mega dams in the Ganga basin, sediment flux is bound to decrease in the near future. 3.2. Latest estimates of large East and Southeast Asian rivers 3.2.1. East Asia (China mainland) Table 2 compares Historical and Current annual sediment fluxes of the large Asian rivers. In a regional scale study, Dai et al. (2009) estimated that over the past 50 years (between 1954 and 2005), nine large Chinese rivers collectively transported 1360 km3 of water and 1313 Mt of sediments annually. However, by giving decadal-scale data, they further elaborated that annual sediment transport by these rivers reduced by 70% during the last half century, from 1809 Mt (1954–1963) to 540 Mt (1996–2005). Recent studies by Zhang et al. (2008), Hu et al. (2009) and Kong et al. (2009) provided revised sediment fluxes of the Zhujiang (Pearl), Yangtze and the Yellow rivers, respectively (Table 2) and suggested that the Current combined sediment flux of these three rivers is about 360 Mt yr 1. Among the large Asian rivers, the Yellow river provides an excellent example of the reduction in annual sediment loads, caused by a combination of human activities and recent climate change. Before the commissioning of large dams, the annual sediment input of the Yellow river (between 1919 and 1960) was about 1200 Mt (Chien and Zhou, 1965). However, between the year 2000 and 2005, Yellow river delivered only 150 Mt of sediment annually into the Bohai sea (Kong et al., 2009). Thus, the annual sediment loads of the large Chinese rivers reduced from 1800 Mt (Milliman, 1995; Milliman and Meade, 1983) to about 415 Mt in the recent years. Note that present day collective sediment flux of all the large Chinese rivers (draining > 4  106 km2 area) is just double of the annual sediment flux from Taiwan island (195 Mt; Kao and Milliman, 2008). Chu et al. (2009) estimated the amounts of sediment trapped in the basins of the Yellow (17.5 Gt), Yangtze (4.54 Gt), Haihe (1.74 Gt) and Liaohe (1.26 Gt), and projected that for the nine large Chinese rivers, the total amount of sediment trapped by the dams and reservoirs (>85,000) between 1959 and 2007, is about 28 Gt. 3.2.2. Southeast Asia Irrawaddy–Salween (into Andaman Sea), Mekong, Red and Chao Phraya (into South China Sea) are the large Southeast Asian rivers. Similar to the large Chinese and Indian rivers, the Red river also registered a drastic decline (70%) in annual sediment flux, which reduced from 130 Mt (Milliman and Meade, 1983) to 36 Mt (Dang et al., 2010). Due to construction of Bhumipol (1965) and Sirikit (1972) dams, annual sediment flux of the Chao Phraya river in Thailand reduced by >80%, from 25 Mt (before 1965) to 5 Mt by the 1990s (Winterwerp et al., 2005). Annual sediment flux of the Mekong river remains almost unchanged from 160 Mt (Milliman and Meade, 1983) to 168 Mt (Walling, 2008). Robinson et al. (2007) revised the annual sediment budget for the Irrawaddy– Salween system from 360 Mt (Hovius, 1998) to 600 Mt. Thus, despite a decline in the annual sediment load of the Red and the Chao Phraya rivers, due to revised sediment flux of Irrawaddy–

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Salween system, the collective sediment load from Southeast Asia increased from 572 Mt (Hovius, 1998; Milliman and Meade, 1983) to 810 Mt (Table 2). 3.3. Revised sediment budget of East-Southeast and South Asia At present the collective annual sediment flux of 27 large rivers draining through ESSE Asian region, is about 2150 Mt. In addition, coastal rivers (70%) behind dams in most of the large Asian rivers basins. 3.4. Reduced sediment load: causes and implications In recent years, studies on riverine sediment fluxes to the oceans and the sediment transportation processes have received more attention as annual variations in sediment fluxes have become an ideal index for measuring the effects of climate change and human activities in the river basins (Wang et al., 2007). Fig. 1, provides an exclusive illustration of the spatial distribution (along with ground elevation) of mega dams located across ESSE Asian region. Note that before 1960, there were only 25 mega dams in ESSE Asia, whereas in last 50 years (1961–2010) about 180 mega dams have been commissioned and 40 mega dams are under construction (to be completed by 2020). Construction of mega dams at elevated regions (>1000 m) could be one of the main reasons for reduced sediment loads (Fig. 1), since headwater regions serve as the main source of sediments in most of these rivers. In order to understand the impact of these mega dams on sediment transport, we compared the decadal variations in sediment loads and the number of the mega dams present in the catchments of large rivers (Fig. 3). Apparently, there is a significant inverse correlation (r2 > 0.9) between the number of dams and decadal sediment fluxes, suggesting a strong control of mega dams on the sediment transport regime across the region. It also implies that the amount of sediment trapped by mega dams is far greater than that of the cumulative amount trapped by several hundred large and tens of thousands of smaller reservoirs. Our observations are broadly in agreement with Chu et al. (2009) who pointed out that between 1959 and 2007, China’s 52 largest reservoirs together trapped 25 Gt of sediments. According to their estimates, the overall amount of trapped sediment is about 28 Gt; implying that the rest of the large and smaller dams (>80,000), together accounted for only a 12% decline in historical sediment fluxes. It could be due to the relatively smaller storage capacity of small dams, as they are mainly constructed for local irrigation purposes. Once filled with water during early rains, they allow excess water to flow into the larger ones. Even the storing capacity of large dams (>0.5 to < 1.0 km3) is normally not enough to accommodate huge influx during big flood-events (major-events) and thus allowing sediment

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laden excess water to flow downstream. If there is a mega dam downstream, it will store this large amount of flood-water for a longer time, consequently allowing suspended sediment to be deposited in the reservoir. In order to understand the role of mega dams in retaining sediments, we examined 13 years (1987–1999) long daily sediment load data of the monsoon seasons (June to November) at upstream (Rajghat) and downstream (Garudeshwar) gauge stations of a mega dam (Sardar Sarovar) in the Narmada basin. Sardar Sarovar dam is among the largest dams in India, with maximum water storage capacity of 9.5 km3. Sardar Sarovar dam’s construction was started in 1979 and completed 2006 (Jain et al., 2007). Fig. 4, demonstrates time-series of daily sediment flux at Rajghat and Garudeshwar gauge stations. It is evident that in contrast to Rajghat, daily sediment flux from Garudeshwar shows declining trend. Based on daily sediment load data, we calculated that between 1987 and 1999, the Sardar Sarovar dam retained about 225 Mt of sediment. To understand the transportation–retention mechanism behind the mega dam, we compared changes in the daily sediment loads between these two-gauge stations. In order to facilitate comparison, we classified the daily sediment fluxes in three different transport regimes (Fig. 5), specifically 70%) to many of the large Asian deltas such as, Chao Phraya, Krishna, Pearl, Yangtze and the Yellow are causing them to sink at rates many times faster than global sea level rise and classified them as deltas in greater peril. Syvitski et al. (2009) placed the world’s largest delta of the Ganga river with Irrawaddy and Mekong in deltas in peril category, defined as ‘reduction in aggradation plus accelerated compaction overwhelming rates of global sea-level rise’. Many other Asian deltas, such as Brahmani, Godavari, Indus and Mahanadi also come among deltas at greater risk category, defined as reduction in aggradation, where rates no longer exceed relative sea-level rise (Syvitski et al., 2009). It is estimated that global rivers contribute about 95% of present day sediment fluxes entering into the oceans (Syvitski, 2003). Riverine sediment transport accounts for more than 90% of the total river-borne fluxes of the immobile elements, such as P, Ni, Mn, Cr, Pb, Fe and Al (Martin and Meybeck, 1979) and about 45% of riverine organic carbon from the land to the oceans in particulate (POC) form (Ludwig et al., 1996). This implies that reduction in annual sediment fluxes to coastal seas will have a direct influence on oceanic primary productivity, since many of these elements are used by phytoplankton as micronutrients. Denudation of continental rocks, transfers via rivers to the marine environment and subsequent sedimentation are important processes of the rock cycle that operate at geological time scales. Therefore, sediments retained by dams/reservoirs may not have a considerable impact on either rock cycle or geochemical cycle of different elements at this time scale, due to small active age of reservoirs (mostly few decades) and subsequent dismantling of these structures will open the way to release deposited sediments to finally be transported to the oceans. However, at relatively shorter time scale (i.e. decadal, centennial and millennial), any large-scale modification in riverine sediment fluxes to coastal environment would strongly influence a variety of natural pro-

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Fig. 3. Decline in sediment flux of large Asian rivers coincides with an increasing number of mega dams in respective catchments at a decadal scale. A similar inverse correlation exists for China, with growing numbers of mega dams, the sediment load reduced notably.

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Fig. 4. Time series of daily sediment flux during monsoon seasons (June to November) of 13 years at Rajghat and Garudeshwar gauge stations on Narmada mainstream.

Fig. 5. Variations in collective sediment flux during non-event (75%), however this happened mostly in the last 3–4 decades. Between 1930 and 2003, the sediment load of the Indus river dramatically reduced by >90%. Among the Southeast Asian rivers, the annual sediment load of the Chao Pharya and the Red river reduced by 83% and 75%, respectively. Up until now, construction of several tens of thousands of dams across the ESSE Asian region and particularly in China and India has been considered as main reason for the declining delivery of fluvial sediments. It seems to be true, as in the last five-six decades, China alone built 5340 large dams (higher than 30 m) whereas in India, the number of large dams (higher than 15 m) erected in same period is 3118. However, we have found that it is not the several tens of thousands dams, but this is basically due to about 250 mega dams, which restrict downstream transport of 70–90% of sediment load. This finding is evident from the strong inverse correlation between the number of mega dams and sediment loads at a decadal-scale, across the region. While using daily sediment load data of two monitoring stations, upstream and downstream of a mega dam, we exemplify and suggest a possible mechanism to explain, why the mega dams trap huge amount of sediments. We found that storage of event-driven (major-events) flood-water, results in the sequestering of huge amounts of sediments behind the mega dams. The present day fluvial sediment fluxes may further decline with the closure of several additional under-construction mega dams in the ESSE Asian region. Despite, we demonstrate mega dams as the main cause of decline in sediment fluxes, yet it is also important to remember that with ever-growing population and increasing need of power and water for domestic, industrial and agricultural necessities at present mega dams seems to be the only option to serve multiple-purposes. Acknowledgments We are thankful to Narmada Basin Organization, Bhopal, Central Water Commission India, for proving annual water discharge and sediment load data of the Narmada river. Besides, this study benefited from availability of the annual water discharge and sediment load data released by CWC, India in public domain. This work was supported by National Science Foundation of China (NSFC Grant Nos. 41176059 and 41121091). We appreciate J.D. Milliman and K. Selvaraj for giving valuable suggestions to improve the manuscript. We are grateful to AE Ellen Wohl and Des Walling for their thoughtful reviews and comments. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhydrol.2012. 07.038. References Abbas, N., Subramanian, V., 1984. Erosion and sediment transport in the Ganges river basin (India). J. Hydrol. 69 (1–4), 173–182. Biksham, G., Subramanian, V., 1980. Chemical and sediment mass-transfer in the Godavari River Basin in India. J. Hydrol. 46 (3–4), 331–342. Biksham, G., Subramanian, V., 1988. Sediment transport of the Godavari River Basin and its controlling factors. J. Hydrol. 101 (1–4), 275–290. Chakrapani, G.J., Subramanian, V., 1990. Factors controlling sediment discharge in the Mahanadi River Basin, India. J. Hydrol. 117 (1–4), 169–185. Chakrapani, G.J., Subramanian, V., 1993. Rates of erosion and sedimentation in the Mahanadi river basin, India. J. Hydrol. 149 (1–4), 39–48.

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