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Asia – Monsoon Asia
Asia – Monsoon Asia M Meybeck, Universite´ Pierre et Marie Curie, Paris, France ã 2009 Elsevier Inc. All rights reserved.
General Features of South and Southeast Asian Drainages The relief, climate, and lithological features of this region are summarized in Table 1 for 17 aggregated coastal catchments that link river networks to the coasts of the Indian Ocean and of the South China Sea (Figure 1). The northern hydrographic limits of this drainage are the anti-Taurus (3500 m) in Turkey, the Zagros Range in Iran (4070 m), the Belutschistan mountains and Hindu Kush range in North Pakistan (7690 m), the Karakorum (8600 m) and Himalaya (8840 m) ranges, the Tibetan Plateau (6000 m), and for Southeast Asia the Yunnan mountains in Southwest China. As a whole, these rivers are therefore characterized by headwaters of very high elevation with extended snow cover and permanent ice in areas exceeding 5000 m in the Hindu Kush and Karakorum (Indus basin), in the Himalayans (Ganges and Brahmaputra basins), and in some parts of the Tibet Plateau. These headwaters snow and ice covers provide most of the water resources from Anatolia to the Indus Valley and function as water towers in providing water to lower elevations. The lithology of this region, a major controlling factor of river chemistry, is characterized by mixed sedimentary and crystalline rocks folded in the alpine ranges. An important volcanic plateau in Central India (the Deccan Traps) must be mentioned; the rest of Deccan and Sri Lanka have crystalline rocks originating from the former Gondwana continent. Sedimentary detrital rocks are found in all lowlands as in the Shatt el Arab, Indus, Ganges, Irrawaddy, and Mekong plains. Climates of South and Southeast Asia are greatly dependent on elevation, which controls both temperature and precipitation, and by the presence of the Asian monsoon, which affects the central and eastern parts of this region from the Indus Valley to the South China Sea basin. The monsoon is characterized by winter–spring dryness and summer rain (June and August) that accounts for 50–80% of the annual rain. In India, the monsoon starts abruptly in June in the Western Ghats highlands, south of Mumbai, and progresses eastward during summer. The period of maximum precipitation depends on station location, ranging from July (e.g., Mumbai) to November (Chennai). The annual rainfall varies between 660 (Delhi) and 1700 mm year1 (Hanoi) with a local maximum at 3000 mm year1 in the Western Ghats.
318
World extreme rainfall is recorded in the Khasi Hills (10 800 mm year1), north of Bangladesh in the upper Meghna basin, a tributary of the Ganges/Brahmaputra common estuary (also termed Padma). Another precipitation maximum (4800 mm year1) is observed in the southeastern Salween basin (Myanmar). In the southern part of the South China Sea basin (Malaysia, Indonesia, Philippines), near the equator, the rainfall is nearly equally distributed throughout the year and reaches 3200 mm in North Borneo. In the Middle East from the Indus Valley to the Persian Gulf Basin, annual precipitation reaches 500–800 mm year1 in the mountains but barely exceeds 200 mm year1 in the Shatt El Arab Valley (160 mm year1 in Baghdad) and in the Indus plain (180 mm year1). In this region, river temperature, a major control of aquatic species distribution, is very variable, including along the greatest basins that cross several climatic zones (Table 1). In high mountains, from the Anatolian Plateau to Tibet, rivers are commonly frozen during winter, while in equatorial catchments, from Sri Lanka to Philippines, thermal variations are limited, and monthly temperatures may exceed 22 C. The greatest catchments, such as Shatt el Arab, Indus, Ganges, Brahmaputra, Irrawaddy, Salween, and Mekong, are characterized by the greatest spatial and temporal thermal variability.
Drainage Network and River Discharge Regimes Because of the predominance of mountainous regions and of active tectonics, the drainage network of South Asia has not generated very large river basins compared with those found on other continents. The largest river drainage area does not exceed 1.05 million km2 (Ganges), while on islands (Sri Lanka, Indonesia, Philippines), peninsulas (Malaysia), and narrow coasts (Iran, Oman, W. Deccan, Annam), river basins do not generally exceed 20 000 km2. In the Arabian Peninsula and on the Iranian coast, medium-sized river networks are found, but they are mostly dry or flow only occasionally (desert wadis). The major river systems from East to West are: the Shatt el Arab, draining the Anatolian Plateau to the Persian Gulf; the Indus, which reaches the Arabian Sea through one of the world’s largest deltas; the Ganges (Ganga), which drains the southern side of the Himalayas; the Brahmaputra (named Tsang Po in
Table 1 General characteristics of South and Southeast Asian coastal catchments Sea basin name
Code (a)
28
West South China Sea
19
Sunda Strait
30
SuluCelebes Sea Banda Sea
31
South Timor Coast Java Trench
33
Andaman Sea
35
Bengal Gulf
36
East Deccan Coast
37
Laccadive Basin
38
West Deccan Coast Indus Delta Coast Oman Gulf
39
32
34
40 41
Persian Gulf
42
South Arabian Coast East Red Sea
43
44
Sea basin area (Mkm2)
Runoff (mm year1)
Population density/ sea basin (people km2)
Sediment yield (t km2 year1)
Relief
Kapuas, Rajang, Aguson Mekong, Chao Phraya, Song Koi Barito, Mahakam Mindanao
0.35
1824
115
512
21.7
1.49
1038
94
280
0.55
1220
122
0.36
1194
No important rivers No important rivers Cinamuk, Citanduy Irrawaddy, Salween, Kelantan, Musi Ganges, Damodar, Pennar Godavari, Krishna, Mahanadi, Cauweri, Brahmani Ponnani, Payaswani, Kalinadi Narmada, Tapti, Mahi, Rabarmati Indus
0.19
% low
Climate % mid
% high
% polar
77.3
1.0
40.7
52.2
278
49.0
107
605
800
105
0.01
560
0.17
Geology % dry < 3 mm (b)
% dry 3 mm (b)
% tropical <680 mm (c)
% tropical 680 mm (c)
% plutonic metam.
% volcanic
% carbon.
% other rock type
% cold
% temperate
0.0
0.0
10.3
0.0
0.0
0.0
89.7
8.3
15.5
4.5
24.8
7.1
6.0
0.8
21.6
0.0
0.0
36.5
35.2
11.2
7.8
37.2
21.5
51.0
0.0
0.0
0.0
6.3
0.0
0.0
8.3
85.4
4.8
27.2
5.4
26.9
14.8
85.2
0.0
0.0
0.0
1.3
0.0
0.0
9.1
89.6
4.2
34.6
16.9
9.6
326
4.8
95.2
0.0
0.0
0.0
3.1
0.0
0.0
34.3
62.6
20.0
43.0
14.0
8.0
214
66
0.0
100.0
0.0
0.0
0.0
0.0
0.0
0.0
100.0
0.0
0.0
25.0
49.9
0.0
1306
128
557
2.3
97.7
0.0
0.0
0.0
4.5
0.0
0.0
17.7
77.9
0.0
48.9
30.2
9.3
0.96
1106
79
652
18.7
64.1
17.1
16.1
1.5
37.6
2.5
2.1
10.9
29.3
12.7
9.7
35.0
14.6
1.82
820
261
612
48.7
23.6
27.6
22.2
2.2
60.9
3.3
3.4
5.0
3.0
24.7
3.1
17.7
40.7
1.12
273
315
355
45.8
54.2
0.0
0.0
0.0
11.2
17.0
12.0
55.5
4.3
53.9
23.0
8.4
10.6
0.12
695
377
732
32.5
67.5
0.0
0.0
0.0
0.0
0.0
6.0
45.5
48.5
83.8
0.0
0.0
13.5
0.34
498
297
441
47.3
52.7
0.0
0.0
0.0
16.6
6.2
33.6
26.4
17.2
21.4
51.3
6.1
19.4
1.39
111
191
195
47.2
23.0
29.8
12.4
7.4
10.5
58.4
9.7
1.6
0.0
22.7
4.5
11.9
44.6
No important rivers Shatt el Arab, Dawasir Muqshin
0.26
2
45
10
9.7
90.3
0.0
0.0
0.0
0.0
87.9
12.1
0.0
0.0
6.6
5.5
19.9
6.7
2.47
48
28
122
45.1
52.3
2.7
0.0
6.4
10.0
79.4
4.2
0.0
0.0
13.2
3.4
50.6
29.6
0.80
2
18
1
34.5
64.0
1.5
0.0
0.0
0.0
97.6
2.4
0.0
0.0
9.2
2.5
59.1
27.1
No important rivers
0.44
0
36
1
0.7
97.1
2.2
0.0
0.0
0.0
100.0
0.0
0.0
0.0
52.9
23.3
4.0
8.8
(a) See location in Figure 1. (b) Annual runoff. (c) Annual precipitation. Meybeck M, Du¨rr HH, and Vo¨ro¨smarty CJ (2006) Global coastal segmentation and its river catchment contributors: a new look at land-ocean linkage. Global Biogeochem. Cycles, 20, GB IS 90, doi 10.1029/2005 GB 002540.
Rivers and Streams _ Asia – Monsoon Asia 319
East South China Sea
Principal basins
320 Rivers and Streams _ Asia – Monsoon Asia
Figure 1 Major rivers South Asia and South-East Asia and regional coastal catchments as defined by Maybeck et al. (2006).
Tibet, Jamuna in Bangladesh), which drains the northern side of the Himalayas and forces its way eastward across this range near the Namtcha Barwa Peak (7758 m) between Tibet and Assam, and finally flows south and shares the Bengal Delta with the Ganges, forming the Padma River or estuary; the Meghna, a relatively small basin in Bangladesh with an enormous discharge (3500 m3 s1 for only 80 000 km2); the Irrawaddy (Ayerarwady in Myanmar); and the Salween (Thanlwin in Myanmar), which originates from the Tibet Plateau at 6000 m, where it is named Nag Chu then Lukiang, both of which have a very narrow and elongated basin caused by tectonic forcing; the Mekong (Dza Chu in China), also originating from the Tibetan Plateau with an extremely narrow upper course down to Vientiane (Laos) then a wider lower and middle course; the Song Koi or red river (Yunnan, China, and Vietnam). Other middlesized basins include the Godavari, Krishna, Mahanadi, Narmada, Tapti in Deccan, and the Chao Phraya in Thailand. In deserts (Arabian Desert, Rajasthan Desert between Pakistan and India), ephemeral or occasional rivers (wadis) flow only during rare rain events. Internal regions in Iran and Afghanistan also have very limited river networks, excepted for the Helmand River (Afghanistan).
The Mekong river system has a unique hydrological feature, the Tonle Sap, or Great Lake, in Cambodia. This important lake is fed by the Mekong during its high water stage (June to November). At this period the lake progressively deepens and extends to a maximum size of 15 000 km2. Then the connection between the Tonle Sap with the Mekong is reversed and the lake recedes to 3000 km2 (annual depth variation: 9 m). A very diverse and abundant aquatic life is in equilibrium with this pulsing system, which also provides essential food and fibre resources to the riparian populations well adapted to this changing environment (e.g., floating villages). Other examples of such seasonal wetlands can also be observed on other continents as for the Niger, Nile, Senegal, Okavango in Africa and upper Parana in South America, although the Tonle Sap is the greatest pulsing lake. Natural hydrologic regimes, as described by monthly long-term specific runoff (l s1 km2), reflect climatic control factors. They are presented in Table 2 on the basis of river discharges prior to damming according to the earliest records and from west to east. The natural Shatt el Arab regime, here reconstituted from Tigris plus Euphrates discharges, is controlled by winter rains and snowmelt from the
Table 2 Flow regimes of South Asian rivers before damming presented from west to east Station
Country
Euphrates Narmada Damodar Mahamadi Godavari Krishna Pennar Kelani Chao Phraya Mekong Se Ban Kelantan Agusan
DS Baghdad Garudeshwar Rhondia Kaimundi Dowlaishwaram Vijayawada Nellore Nagalagam Nakohn S.
Iraq India India India India India India Sri Lanka Thailand
Kratie Ban Komphun Guillemard Poblacion
Cambodia Cambodia Malaysia Philippines
Jan (l s1 km2)
Feb (l s1 km2)
3.5 1.75 1.8 1.07 0.81 0.44 0.47 51.8 2.1
4.8 1.3 1.8 0.80 0.65 0.27 0.24 48.0 1.4
4.45 9.9 77.5 139
3.25 6.8 44.3 173
Mar (l s1 km2) 6.95 0.90 1.4 0.76 0.47 0.17 0.28 51.3 1.0 2.5 5.2 33.0 105.9
Apr (l s1 km2)
May (l s1 km2)
Jun (l s1 km2)
Jul (l s1 km2)
Aug (l s1 km2)
Sep (l s1 km2)
Oct (l s1 km2)
Nov (l s1 km2)
10.35 0.66 1.1 0.78 0.38 0.15 0.17 63.3 0.86
11.1 0.41 1.75 0.57 0.24 0.77 0.82 123.7 1.61
6.1 2.2 10.7 1.7 3.1 2.38 0.28 129.4 4.3
2.65 24.0 39 31.3 26.6 21.9 0.88 88.7 5.94
1.5 52.5 59.8 58.3 39.8 26.1 1.74 77.2 11.8
1.12 62 49.8 43.1 35.5 17.3 3.2 76.7 19.6
1.18 16.8 22.0 12.1 14.2 10.9 4.4 127.5 26.5
1.6 4.9 4.8 4.5 3.7 4.0 5.2 109.3 15.3
2.4 5.6 27.5 60.7
4.5 14.0 36.8 53.7
14.1 25.3 33.4 57.7
38.5 58.5 28.1 54.0
58.7 83.4 29.2 60.6
64.8 74.7 39.5 62.1
40.0 55.2 55.9 66.2
19.8 25.7 69.6 65.0
UNESCO (1969) Discharges of selected rivers of the world. Studies and reports in Hydrology, no. 5, p. 70. Paris: UNESCO. UNESCO (1996) Global river discharge database (Riv Dis), Technical Documents in Hydrology, p. 41. Paris: UNESCO. A: River basin in area.
Dec (l s1 km2) 1.6 2.3 2.5 1.4 1.35 1.0 3.5 70.5 4.75 9.7 17.2 87.4 125
Year (l s1 km2)
A (103 km2)
Period
4.42 14.1 16.5 12.9 10.6 6.9 1.8 84.9 7.9
398 89.3 19.9 132.1 299 251 53.3 2.085 111.4
1932/66 1949/62 1934/61 1947/61 1901/60 1961/60 1934/47 1924/60 1905/66
22.0 31.7 47.0 81.9
646 48.2 11.9 7.39
1933/53 1961/66 1949/64 1955/63
Rivers and Streams _ Asia – Monsoon Asia 321
River
322 Rivers and Streams _ Asia – Monsoon Asia
Anatolian Plateau (maximum discharge in April and May, low flows from July to December). The upper Indus and its tributaries (Jhellum, Ravi, Sutlej, and Chenab, which altogether represent 40% of the drainage basin, are mainly fed by the Hindu Kush and Karakorum snow and ice melt. After their confluence, the Indus River crosses a very dry region and does not receive any other important tributary; it is a typical ‘allogenic’ river in which the mountain water tower allows the river to make its way through desert, as is also true for the Nile River. The upper Indus basin now has several major dams in Kashmir and Pakistan used for hydropower, irrigation, and water supply of downstream users. The Narmada River, south of Mumbai, is characteristic of the West Deccan monsoon regime. From June to August the average specific discharge increases from 2.2 to 53.8 l s1 km2 and 82% of the annual flow is discharged from July to September. East Deccan regimes, from the Damodar River in the North Bengal Gulf to the Pennar in the south (Table 2), are much dryer but also very seasonal. It must be noted that the average specific discharge in monsoon-fed rivers can be quite variable: 1.8 l s1 km2 for the Pennar compared with 31.7 l s1 km2 for the Se Ban (a lower Mekong tributary) and much more in some parts of the Meghna and in the lower Salween basins. Ganges, Brahmaputra, Salween, and Mekong headwaters are fed by late spring snowmelt and by ice melt for their higher tributaries, after which their middle and lower basins receive the monsoon rains (July–September). The result is a mixed regime with a well-marked summer maximum (‘monsoon period’) and an extended low flow period from December to May (‘pre-monsoon period’), as for the Mekong (Table 2). River basins near the equator, in western Sri Lanka (e.g., Kelani), Malaysia (e.g., Kelantan), the Philippines (e.g., Agusan), and Indonesia are characterized by minor seasonal discharge variations and by very high annual specific runoff (84.9, 47, and 81.9 l s1 km2, respectively, Table 2), which is among the world’s highest. Regional variability can be important: there is a 50-fold difference between the dry Pennar basin of Southeast Deccan and the Kelani basin, and a 250-fold difference between their minimum monthly runoff (0.17 and 48 l s1 km2, respectively) over a distance of only 600 km.
Suspended Loads and Water Chemistry South Asian rivers are much more turbid and erosive than world average because their headwaters are located in areas of very high relief, and because monsoon and snowmelt regimes are very seasonal, generating peak runoff with high velocities. Also, in some
regions basin rock types are very erodible (e.g., volcanic ashes in Java). The average concentrations of suspended particulate matter (SPM), calculated as the ratio of annual sediment loads over annual water volumes at the river mouth, are here presented for a selection of large and medium rivers, prior to damming (Table 3). All documented rivers have SPM concentrations higher than the world median of about 200 mg l1. SPM concentrations are much higher in headwaters, as in the upper Mekong, the upper Brahmaputra, and upper Indus, or in the Himalayan tributaries of the Ganges River, such as the Kosi River. Small coastal rivers in tectonically active regions (e.g., Indonesia and the Philippines) also are very turbid and erosive (e.g., Cimanuk, Citanduy, and Citarum in Java, Table 3). As a result, the major part of the global sediment fluxes to oceans is generated by the South and East Asian rivers. Dissolved solids in South and Southeast Asian rivers are generally close to or slightly higher than the world median, with a dominance of calcium and bicarbonate ions, as in most world rivers. Because of the weathering of the volcanic Deccan Traps and the high evapotranspiration rate, Deccan rivers have a high ionic content (2000–5400 meq l1 for the cation sum (TZþ)), as in the Damodar, Godavari, Krishna, Narmada, and Tapti rivers (Table 3). Rivers draining the Himalayan and other alpine ranges (Indus, Ganges, Mekong, Brahmaputra, Irrawaddy) are less mineralized, with TZþ from 1180 (Mekong) to 2350 meq l1 (Irrawaddy). The Salween is an exception, possibly linked to the presence of carbonate rocks. In very wet regions, where the effect of evaporation on ionic content is quite limited, water chemistry is directly controlled by the presence or absence of easily weathered minerals. In the Mahakam River, which drains only crystalline rocks in Western Borneo, TZþ can be as low as 392 meq l1, while it ranges from 800 to 1700 meq l1 for rivers of Java. In central Thailand, the Chi and Mun rivers, which drain evaporitic rocks, are naturally very high in Naþ and Cl.
Human Effects on South Asian Rivers South Asian rivers, with the exception of the Irrawaddy, Salween, Brahmaputra, and Middle and Lower Mekong, are now extensively dammed. This greatly fragments the river courses, modifies the flow regimes, and decreases the downstream fluxes of particulate matter. In addition, reservoirs are generally associated with extensive irrigated areas where the water is lost by evaporation. As a result, the discharge
Table 3 Water chemistry and suspended sediments before damming for South Asian rivers Station Country
L (km)
A (103 km2)
Q (km3 year1)
SPM (mg l1)
SiO2 (mg l1)
Ca2þ (mg l1)
Mg2þ (mg l1)
Naþ (mg l1)
Kþ (mg l1)
Cl (mg l1)
SO2 4 (mg l1)
HCO 3 (mg l1)
TZþ (meq l1)
Brahmaputra Cauweri Chao Phraya Cimanuk Citanduy Citarum Damodar Ganges Godavari Indus Irrawaddy Krishna Mahakam Mahanadi Mekong Musi Narmada Salween Shatt el Arab Tapti
Bangladesh (a) India Thailand Java (Indonesia) Java (Indonesia) Java (Indonesia) India India/Bangladesh India Pakistan Myanmar India Borneo (Indonesia) India Cambodia/Vietnam (b) Sumatra (Indonesia) India Myanmar (c) Turkey/Syria/Iraq India
3000 800 1200
580 88 111.4 3.7 3.6 5.9 20 1050 313 916 410 259 65.3 141.6 795 57 99 325 541 65
510 20.9 27.8 4.45 5.3 4.9 10 493 105 90 486 30 87 66 467 80 40.7 211 45.7 18
1058
7.8 19 15.8 19.2 13.3 30
14 28 22 16 6.7 11 22 23.2 30.2 26 10 27.5 3.2 10.4 14.2 3.2 25 46 52 49.6
3.8 24 6.3 6.2 3.2 3.9 8.2 6.5 2.4 6 6 13.5 1.0 9.5 3.2 1.1 19.2 16 22 30.4
2.1 60
3.9 5.5
9.0 4.3 8 15 9.6 8.1 9 30 42.5 2.9 10.2 3.6 4.6 35 10 31 8.3
2.0 1.1 2 3 2.6 2.2 2 2 3.0 0.95 1.5 2.0 1.1
1.1 50 4.0 4 3.3 5 9.7 5.0 14.1 7 18 37 1.0 30.9 5.3 4 17 20 32 44
10 32 8.4 19 4.7 13 45 8 10 26 5 63 2.8 15 3.8 5.2 5 1 73 39.4
58 177 76 81 36 52 62 119 105 90 120 125 18.0 60.9 57.9 15.5 175 212 180 242
1200 6120 1615 1726 812 1267 2500 2177 2113 2221 2350 4409 392 1783 1180 470 4350 4070 5830 5415
2525 1500 3180 2300 1290 858 4850 1300 2820 2760 724
395 5600 1800 2800 1055 1619 535 2130
321
11.7 21.1 10 5 11.8 9.0 8.9 24.5
3071 6.9
1 3 3
Meybeck M and Ragu A (1996) River discharges to the oceans. An assessment of suspended solids, major ions and nutrients. Environnement Information and assessment Rpt., 250 p. Nairobi: UNEP (loadable from Gems Water:http://www.gemsstat.org/descstats.aspx) A, basin area; Q, mean annual discharge; SPM, discharge weighted suspended particulate matter; TZþ, sum of cations; L, river length. (a) With headwaters in Tibet and India. (b) With headwaters in China and Laos. (c) With headwaters in China.
Rivers and Streams _ Asia – Monsoon Asia 323
River
324 Rivers and Streams _ Asia – Monsoon Asia
of many rivers in this region has decreased, as in the Shatt el Arab, the Indus, and the East Deccan rivers discharging to the Bay of Bengal. Impoundments are still created in India and in the upper Mekong Basin. The Shatt el Arab headwaters (Tigris and Euphrates) also are extensively impounded in Turkey, the regional water tower of the Middle East (e.g., Attatu¨rk Dam, one of the world’s largest), and again in Syria and Iraq. The lower reaches of the Shatt el Arab and the Karun River in Iran form an extensive wetland that has been greatly affected by conflicts, oil drilling, and drainage. The restoration of this world-class wetland remains uncertain. The upper Indus and its tributaries are impounded, and the Kotri dam in the lower reach also takes water for irrigation. As a result, the Indus water discharge to the Arabian Sea has been decreased by 80%, and its sediment load has been reduced even more. The upper Mekong, in China also, is now dammed in five places, as are some of its Thai tributaries. In the lower Mekong, a major tributary in Laos is being impounded. The water quality evolution influenced by irrigation is not much documented, but some increase of salt content is likely to occur in Pakistan and Iraq. In South and Southeast Asia, the degradation of river quality is particularly caused by faecal contamination and by organic pollution, which depends on population density and on the degree of urban and industrial sewage collection and treatment. The pressure on water quality is therefore very high during the pre-monsoon period in densely populated regions where urban development is growing faster than sewage collection and treatment. Such a situation is found in most of the Indian subcontinent (Himalayan headwaters excepted), in Sumatra and Java, and in populated lowlands of Myanmar, Thailand, and Viet Nam, and is likely to occur in the lower Shatt El Arab. Some of the largest and fastest growing cities are located in South Asia. Many of them are located inland on relatively small river courses with limited dilution or self-purification capacity, particularly during the pre-monsoon period, as is the case for Hyderabad (Pakistan) on the Indus; Lahore on the Ravi River; Hyderabad (India) on the Krishna River headwaters; Bangalore on Ponnayar River headwaters; New Delhi, Allahabad, and Varanasi on the Yamuna River, a Ganges tributary; Nagpur on the Godavari headwaters; Poona on the Bima headwaters, a branch of the Krishna River; Calcutta on the Hoogly, a branch of the Ganges Delta; Ahmedabad on the Sabarmati River; Dacca in the Bengal Delta; Baghdad on the Tigris; Ho Chi Minh City on the Saigon River; and Hanoi on the Song Koi (Red River).
Industrial activities are developing rapidly in the Indian subcontinent and in Southeast Asia. They may result in hotspots of pollution involving heavy metals already evidenced on the Yamuna River from Delhi to Varanasi, and on the Gomati River and on the Loni River at Lucknow, both Ganges tributaries, on Saigon River and delta branches of Song Koi River (Viet Nam). Other possible threats are associated with conflicts (e.g., defoliant use in Viet Nam, leakage from destroyed industrial facilities in Iraq). However, the levels of toxic contaminants remain largely undocumented (see GEMS Water, a global program on river quality). The effect of rapid deforestation in Southeast Asia (e.g., Malaysia, Indonesia, Thailand) on water quality and aquatic species biodiversity should also be assessed.
Perspectives River basins and water resources of South and Southeast Asia region are changing very rapidly in response to increasing water demand. From Turkey to Pakistan, shared basins, already sensitive to climate variations, are exposed to regional water management conflicts between water-rich upstream countries and water-poor downstream users. In this region, most of the river water is already being used for irrigation and water supply for large cities. The upper river courses are heavily fragmented by large dams and seasonal drought can now be observed in the lower courses, such as the lower Indus. Although human influences on river quality are not yet adequately documented with regard to the water use and demand, the fast growth of urbanization, intensive agriculture, and industrialization could cause important problems in the future, particularly in densely populated floodplains (Ganges, Meghna, Irrawaddy, Chao Phraya, Mekong, Song Koi), and where water resources have already been much used (Shatt El Arab, Indus).
Glossary Allogenic river – Any river the main stem of which is sustained almost entirely by waters derived from the uppermost regions of the drainage basin, because the upper part of the drainage basin is well watered, whereas the lower part of the basin is arid. Water tower – In a hydrologic context, a water tower is a large source of water that feeds the lower reaches of a river, often because the upper reaches are mountainous and receive abundant precipitation.
Rivers and Streams _ Asia – Monsoon Asia 325
Suspended particulate matter – Suspended particulate matter (SPM), which is also referred to as total suspended solids (TSS), consists of all material carried by water that can be separated from the water by use of a filter. SPM consists of both inorganic and organic particles. TZþ – Sum of cations dissolved in water, expressed as microequivalents per liter (meq l1). TZþ is an index of total salinity or total dissolved solids. See also: Africa; Asia – Eastern Asia; Flood Plains; Fluvial Export; South America.
Further Reading Ansari AA and Singh B (2000) Importance of geomorphology and sediment processes for the metal dispersion in sediments and soils of the Ganga Plain. Chemical Geology 162: 245–266. Carbonnel JP and Guiscafre´ (1965) Grand Lac du Cambodge, Sedimentologie et Hydrologie, 1962–1963, 401pp. Paris: Rapport de Mission , Ministe`re des Affaires Etrange`res. Jennerjahn T, et al. (2004) Biogeochemistry of a tropical river affected by human activities in its catchment: Brantas River estuary and coastal waters of Madura Strait, Java, Indonesia. Est. Coastal Shelf Science 60: 503–514.
Korzun VI (ed.) (1978) World Water Balance and Water Resources of the World. Studies and Reports. Hydrology vol. 25, 663 pp. Paris: UNESCO (þatlas). Le Thi Phuong Q, Garnier J, Billen G, Thery S, and ChanVan M (2007) Hydrological regime and suspended load of the Red River (Viet Nam), observation and modelling. Journal of Hydrology 334: 199–214. Meybeck M, Du¨rr HH, and Vo¨ro¨smarty CJ (2006) Global coastal segmentation and its river catchment contributors: A new look at land-ocean linkage. Global Biogeochemical Cycles 20, GB IS 90, doi 10.1029/2005 GB 002540. Milliman JD and Syvitski JPM (1992) Geomorphic/tectonic control of sediment discharge to the oceans, the importance of small mountains. Journal of Geology 95: 751–762. Salomons W, Kremer HH, and Turner RK (2006) The catchment to coast continuum. In: Crossland CJ, et al. (eds.) Coastal Fluxes in the Anthropocene, pp. 145–200. Springer. UNESCO (1969) Discharges of selected rivers of the world. Studies and reports in Hydrology, no. 5, 70pp. Paris: UNESCO. UNESCO (1996) Global river discharge database (Riv Dis), Technical Documents in Hydrology, 41pp. Paris: UNESCO.
Relevant Website http://www.gemsstat.org/descstats.aspx – Gems Water program UNEP, global river water quality data.
General Features of South and Southeast Asian Drainages The relief, climate, and lithological features of this region are summarized in Table 1 for 17 aggregated coastal catchments that link river networks to the coasts of the Indian Ocean and of the South China Sea (Figure 1). The northern hydrographic limits of this drainage are the anti-Taurus (3500 m) in Turkey, the Zagros Range in Iran (4070 m), the Belutschistan mountains and Hindu Kush range in North Pakistan (7690 m), the Karakorum (8600 m) and Himalaya (8840 m) ranges, the Tibetan Plateau (6000 m), and for Southeast Asia the Yunnan mountains in Southwest China. As a whole, these rivers are therefore characterized by headwaters of very high elevation with extended snow cover and permanent ice in areas exceeding 5000 m in the Hindu Kush and Karakorum (Indus basin), in the Himalayans (Ganges and Brahmaputra basins), and in some parts of the Tibet Plateau. These headwaters snow and ice covers provide most of the water resources from Anatolia to the Indus Valley and function as water towers in providing water to lower elevations. The lithology of this region, a major controlling factor of river chemistry, is characterized by mixed sedimentary and crystalline rocks folded in the alpine ranges. An important volcanic plateau in Central India (the Deccan Traps) must be mentioned; the rest of Deccan and Sri Lanka have crystalline rocks originating from the former Gondwana continent. Sedimentary detrital rocks are found in all lowlands as in the Shatt el Arab, Indus, Ganges, Irrawaddy, and Mekong plains. Climates of South and Southeast Asia are greatly dependent on elevation, which controls both temperature and precipitation, and by the presence of the Asian monsoon, which affects the central and eastern parts of this region from the Indus Valley to the South China Sea basin. The monsoon is characterized by winter–spring dryness and summer rain (June and August) that accounts for 50–80% of the annual rain. In India, the monsoon starts abruptly in June in the Western Ghats highlands, south of Mumbai, and progresses eastward during summer. The period of maximum precipitation depends on station location, ranging from July (e.g., Mumbai) to November (Chennai). The annual rainfall varies between 660 (Delhi) and 1700 mm year1 (Hanoi) with a local maximum at 3000 mm year1 in the Western Ghats.
318
World extreme rainfall is recorded in the Khasi Hills (10 800 mm year1), north of Bangladesh in the upper Meghna basin, a tributary of the Ganges/Brahmaputra common estuary (also termed Padma). Another precipitation maximum (4800 mm year1) is observed in the southeastern Salween basin (Myanmar). In the southern part of the South China Sea basin (Malaysia, Indonesia, Philippines), near the equator, the rainfall is nearly equally distributed throughout the year and reaches 3200 mm in North Borneo. In the Middle East from the Indus Valley to the Persian Gulf Basin, annual precipitation reaches 500–800 mm year1 in the mountains but barely exceeds 200 mm year1 in the Shatt El Arab Valley (160 mm year1 in Baghdad) and in the Indus plain (180 mm year1). In this region, river temperature, a major control of aquatic species distribution, is very variable, including along the greatest basins that cross several climatic zones (Table 1). In high mountains, from the Anatolian Plateau to Tibet, rivers are commonly frozen during winter, while in equatorial catchments, from Sri Lanka to Philippines, thermal variations are limited, and monthly temperatures may exceed 22 C. The greatest catchments, such as Shatt el Arab, Indus, Ganges, Brahmaputra, Irrawaddy, Salween, and Mekong, are characterized by the greatest spatial and temporal thermal variability.
Drainage Network and River Discharge Regimes Because of the predominance of mountainous regions and of active tectonics, the drainage network of South Asia has not generated very large river basins compared with those found on other continents. The largest river drainage area does not exceed 1.05 million km2 (Ganges), while on islands (Sri Lanka, Indonesia, Philippines), peninsulas (Malaysia), and narrow coasts (Iran, Oman, W. Deccan, Annam), river basins do not generally exceed 20 000 km2. In the Arabian Peninsula and on the Iranian coast, medium-sized river networks are found, but they are mostly dry or flow only occasionally (desert wadis). The major river systems from East to West are: the Shatt el Arab, draining the Anatolian Plateau to the Persian Gulf; the Indus, which reaches the Arabian Sea through one of the world’s largest deltas; the Ganges (Ganga), which drains the southern side of the Himalayas; the Brahmaputra (named Tsang Po in
Table 1 General characteristics of South and Southeast Asian coastal catchments Sea basin name
Code (a)
28
West South China Sea
19
Sunda Strait
30
SuluCelebes Sea Banda Sea
31
South Timor Coast Java Trench
33
Andaman Sea
35
Bengal Gulf
36
East Deccan Coast
37
Laccadive Basin
38
West Deccan Coast Indus Delta Coast Oman Gulf
39
32
34
40 41
Persian Gulf
42
South Arabian Coast East Red Sea
43
44
Sea basin area (Mkm2)
Runoff (mm year1)
Population density/ sea basin (people km2)
Sediment yield (t km2 year1)
Relief
Kapuas, Rajang, Aguson Mekong, Chao Phraya, Song Koi Barito, Mahakam Mindanao
0.35
1824
115
512
21.7
1.49
1038
94
280
0.55
1220
122
0.36
1194
No important rivers No important rivers Cinamuk, Citanduy Irrawaddy, Salween, Kelantan, Musi Ganges, Damodar, Pennar Godavari, Krishna, Mahanadi, Cauweri, Brahmani Ponnani, Payaswani, Kalinadi Narmada, Tapti, Mahi, Rabarmati Indus
0.19
% low
Climate % mid
% high
% polar
77.3
1.0
40.7
52.2
278
49.0
107
605
800
105
0.01
560
0.17
Geology % dry < 3 mm (b)
% dry 3 mm (b)
% tropical <680 mm (c)
% tropical 680 mm (c)
% plutonic metam.
% volcanic
% carbon.
% other rock type
% cold
% temperate
0.0
0.0
10.3
0.0
0.0
0.0
89.7
8.3
15.5
4.5
24.8
7.1
6.0
0.8
21.6
0.0
0.0
36.5
35.2
11.2
7.8
37.2
21.5
51.0
0.0
0.0
0.0
6.3
0.0
0.0
8.3
85.4
4.8
27.2
5.4
26.9
14.8
85.2
0.0
0.0
0.0
1.3
0.0
0.0
9.1
89.6
4.2
34.6
16.9
9.6
326
4.8
95.2
0.0
0.0
0.0
3.1
0.0
0.0
34.3
62.6
20.0
43.0
14.0
8.0
214
66
0.0
100.0
0.0
0.0
0.0
0.0
0.0
0.0
100.0
0.0
0.0
25.0
49.9
0.0
1306
128
557
2.3
97.7
0.0
0.0
0.0
4.5
0.0
0.0
17.7
77.9
0.0
48.9
30.2
9.3
0.96
1106
79
652
18.7
64.1
17.1
16.1
1.5
37.6
2.5
2.1
10.9
29.3
12.7
9.7
35.0
14.6
1.82
820
261
612
48.7
23.6
27.6
22.2
2.2
60.9
3.3
3.4
5.0
3.0
24.7
3.1
17.7
40.7
1.12
273
315
355
45.8
54.2
0.0
0.0
0.0
11.2
17.0
12.0
55.5
4.3
53.9
23.0
8.4
10.6
0.12
695
377
732
32.5
67.5
0.0
0.0
0.0
0.0
0.0
6.0
45.5
48.5
83.8
0.0
0.0
13.5
0.34
498
297
441
47.3
52.7
0.0
0.0
0.0
16.6
6.2
33.6
26.4
17.2
21.4
51.3
6.1
19.4
1.39
111
191
195
47.2
23.0
29.8
12.4
7.4
10.5
58.4
9.7
1.6
0.0
22.7
4.5
11.9
44.6
No important rivers Shatt el Arab, Dawasir Muqshin
0.26
2
45
10
9.7
90.3
0.0
0.0
0.0
0.0
87.9
12.1
0.0
0.0
6.6
5.5
19.9
6.7
2.47
48
28
122
45.1
52.3
2.7
0.0
6.4
10.0
79.4
4.2
0.0
0.0
13.2
3.4
50.6
29.6
0.80
2
18
1
34.5
64.0
1.5
0.0
0.0
0.0
97.6
2.4
0.0
0.0
9.2
2.5
59.1
27.1
No important rivers
0.44
0
36
1
0.7
97.1
2.2
0.0
0.0
0.0
100.0
0.0
0.0
0.0
52.9
23.3
4.0
8.8
(a) See location in Figure 1. (b) Annual runoff. (c) Annual precipitation. Meybeck M, Du¨rr HH, and Vo¨ro¨smarty CJ (2006) Global coastal segmentation and its river catchment contributors: a new look at land-ocean linkage. Global Biogeochem. Cycles, 20, GB IS 90, doi 10.1029/2005 GB 002540.
Rivers and Streams _ Asia – Monsoon Asia 319
East South China Sea
Principal basins
320 Rivers and Streams _ Asia – Monsoon Asia
Figure 1 Major rivers South Asia and South-East Asia and regional coastal catchments as defined by Maybeck et al. (2006).
Tibet, Jamuna in Bangladesh), which drains the northern side of the Himalayas and forces its way eastward across this range near the Namtcha Barwa Peak (7758 m) between Tibet and Assam, and finally flows south and shares the Bengal Delta with the Ganges, forming the Padma River or estuary; the Meghna, a relatively small basin in Bangladesh with an enormous discharge (3500 m3 s1 for only 80 000 km2); the Irrawaddy (Ayerarwady in Myanmar); and the Salween (Thanlwin in Myanmar), which originates from the Tibet Plateau at 6000 m, where it is named Nag Chu then Lukiang, both of which have a very narrow and elongated basin caused by tectonic forcing; the Mekong (Dza Chu in China), also originating from the Tibetan Plateau with an extremely narrow upper course down to Vientiane (Laos) then a wider lower and middle course; the Song Koi or red river (Yunnan, China, and Vietnam). Other middlesized basins include the Godavari, Krishna, Mahanadi, Narmada, Tapti in Deccan, and the Chao Phraya in Thailand. In deserts (Arabian Desert, Rajasthan Desert between Pakistan and India), ephemeral or occasional rivers (wadis) flow only during rare rain events. Internal regions in Iran and Afghanistan also have very limited river networks, excepted for the Helmand River (Afghanistan).
The Mekong river system has a unique hydrological feature, the Tonle Sap, or Great Lake, in Cambodia. This important lake is fed by the Mekong during its high water stage (June to November). At this period the lake progressively deepens and extends to a maximum size of 15 000 km2. Then the connection between the Tonle Sap with the Mekong is reversed and the lake recedes to 3000 km2 (annual depth variation: 9 m). A very diverse and abundant aquatic life is in equilibrium with this pulsing system, which also provides essential food and fibre resources to the riparian populations well adapted to this changing environment (e.g., floating villages). Other examples of such seasonal wetlands can also be observed on other continents as for the Niger, Nile, Senegal, Okavango in Africa and upper Parana in South America, although the Tonle Sap is the greatest pulsing lake. Natural hydrologic regimes, as described by monthly long-term specific runoff (l s1 km2), reflect climatic control factors. They are presented in Table 2 on the basis of river discharges prior to damming according to the earliest records and from west to east. The natural Shatt el Arab regime, here reconstituted from Tigris plus Euphrates discharges, is controlled by winter rains and snowmelt from the
Table 2 Flow regimes of South Asian rivers before damming presented from west to east Station
Country
Euphrates Narmada Damodar Mahamadi Godavari Krishna Pennar Kelani Chao Phraya Mekong Se Ban Kelantan Agusan
DS Baghdad Garudeshwar Rhondia Kaimundi Dowlaishwaram Vijayawada Nellore Nagalagam Nakohn S.
Iraq India India India India India India Sri Lanka Thailand
Kratie Ban Komphun Guillemard Poblacion
Cambodia Cambodia Malaysia Philippines
Jan (l s1 km2)
Feb (l s1 km2)
3.5 1.75 1.8 1.07 0.81 0.44 0.47 51.8 2.1
4.8 1.3 1.8 0.80 0.65 0.27 0.24 48.0 1.4
4.45 9.9 77.5 139
3.25 6.8 44.3 173
Mar (l s1 km2) 6.95 0.90 1.4 0.76 0.47 0.17 0.28 51.3 1.0 2.5 5.2 33.0 105.9
Apr (l s1 km2)
May (l s1 km2)
Jun (l s1 km2)
Jul (l s1 km2)
Aug (l s1 km2)
Sep (l s1 km2)
Oct (l s1 km2)
Nov (l s1 km2)
10.35 0.66 1.1 0.78 0.38 0.15 0.17 63.3 0.86
11.1 0.41 1.75 0.57 0.24 0.77 0.82 123.7 1.61
6.1 2.2 10.7 1.7 3.1 2.38 0.28 129.4 4.3
2.65 24.0 39 31.3 26.6 21.9 0.88 88.7 5.94
1.5 52.5 59.8 58.3 39.8 26.1 1.74 77.2 11.8
1.12 62 49.8 43.1 35.5 17.3 3.2 76.7 19.6
1.18 16.8 22.0 12.1 14.2 10.9 4.4 127.5 26.5
1.6 4.9 4.8 4.5 3.7 4.0 5.2 109.3 15.3
2.4 5.6 27.5 60.7
4.5 14.0 36.8 53.7
14.1 25.3 33.4 57.7
38.5 58.5 28.1 54.0
58.7 83.4 29.2 60.6
64.8 74.7 39.5 62.1
40.0 55.2 55.9 66.2
19.8 25.7 69.6 65.0
UNESCO (1969) Discharges of selected rivers of the world. Studies and reports in Hydrology, no. 5, p. 70. Paris: UNESCO. UNESCO (1996) Global river discharge database (Riv Dis), Technical Documents in Hydrology, p. 41. Paris: UNESCO. A: River basin in area.
Dec (l s1 km2) 1.6 2.3 2.5 1.4 1.35 1.0 3.5 70.5 4.75 9.7 17.2 87.4 125
Year (l s1 km2)
A (103 km2)
Period
4.42 14.1 16.5 12.9 10.6 6.9 1.8 84.9 7.9
398 89.3 19.9 132.1 299 251 53.3 2.085 111.4
1932/66 1949/62 1934/61 1947/61 1901/60 1961/60 1934/47 1924/60 1905/66
22.0 31.7 47.0 81.9
646 48.2 11.9 7.39
1933/53 1961/66 1949/64 1955/63
Rivers and Streams _ Asia – Monsoon Asia 321
River
322 Rivers and Streams _ Asia – Monsoon Asia
Anatolian Plateau (maximum discharge in April and May, low flows from July to December). The upper Indus and its tributaries (Jhellum, Ravi, Sutlej, and Chenab, which altogether represent 40% of the drainage basin, are mainly fed by the Hindu Kush and Karakorum snow and ice melt. After their confluence, the Indus River crosses a very dry region and does not receive any other important tributary; it is a typical ‘allogenic’ river in which the mountain water tower allows the river to make its way through desert, as is also true for the Nile River. The upper Indus basin now has several major dams in Kashmir and Pakistan used for hydropower, irrigation, and water supply of downstream users. The Narmada River, south of Mumbai, is characteristic of the West Deccan monsoon regime. From June to August the average specific discharge increases from 2.2 to 53.8 l s1 km2 and 82% of the annual flow is discharged from July to September. East Deccan regimes, from the Damodar River in the North Bengal Gulf to the Pennar in the south (Table 2), are much dryer but also very seasonal. It must be noted that the average specific discharge in monsoon-fed rivers can be quite variable: 1.8 l s1 km2 for the Pennar compared with 31.7 l s1 km2 for the Se Ban (a lower Mekong tributary) and much more in some parts of the Meghna and in the lower Salween basins. Ganges, Brahmaputra, Salween, and Mekong headwaters are fed by late spring snowmelt and by ice melt for their higher tributaries, after which their middle and lower basins receive the monsoon rains (July–September). The result is a mixed regime with a well-marked summer maximum (‘monsoon period’) and an extended low flow period from December to May (‘pre-monsoon period’), as for the Mekong (Table 2). River basins near the equator, in western Sri Lanka (e.g., Kelani), Malaysia (e.g., Kelantan), the Philippines (e.g., Agusan), and Indonesia are characterized by minor seasonal discharge variations and by very high annual specific runoff (84.9, 47, and 81.9 l s1 km2, respectively, Table 2), which is among the world’s highest. Regional variability can be important: there is a 50-fold difference between the dry Pennar basin of Southeast Deccan and the Kelani basin, and a 250-fold difference between their minimum monthly runoff (0.17 and 48 l s1 km2, respectively) over a distance of only 600 km.
Suspended Loads and Water Chemistry South Asian rivers are much more turbid and erosive than world average because their headwaters are located in areas of very high relief, and because monsoon and snowmelt regimes are very seasonal, generating peak runoff with high velocities. Also, in some
regions basin rock types are very erodible (e.g., volcanic ashes in Java). The average concentrations of suspended particulate matter (SPM), calculated as the ratio of annual sediment loads over annual water volumes at the river mouth, are here presented for a selection of large and medium rivers, prior to damming (Table 3). All documented rivers have SPM concentrations higher than the world median of about 200 mg l1. SPM concentrations are much higher in headwaters, as in the upper Mekong, the upper Brahmaputra, and upper Indus, or in the Himalayan tributaries of the Ganges River, such as the Kosi River. Small coastal rivers in tectonically active regions (e.g., Indonesia and the Philippines) also are very turbid and erosive (e.g., Cimanuk, Citanduy, and Citarum in Java, Table 3). As a result, the major part of the global sediment fluxes to oceans is generated by the South and East Asian rivers. Dissolved solids in South and Southeast Asian rivers are generally close to or slightly higher than the world median, with a dominance of calcium and bicarbonate ions, as in most world rivers. Because of the weathering of the volcanic Deccan Traps and the high evapotranspiration rate, Deccan rivers have a high ionic content (2000–5400 meq l1 for the cation sum (TZþ)), as in the Damodar, Godavari, Krishna, Narmada, and Tapti rivers (Table 3). Rivers draining the Himalayan and other alpine ranges (Indus, Ganges, Mekong, Brahmaputra, Irrawaddy) are less mineralized, with TZþ from 1180 (Mekong) to 2350 meq l1 (Irrawaddy). The Salween is an exception, possibly linked to the presence of carbonate rocks. In very wet regions, where the effect of evaporation on ionic content is quite limited, water chemistry is directly controlled by the presence or absence of easily weathered minerals. In the Mahakam River, which drains only crystalline rocks in Western Borneo, TZþ can be as low as 392 meq l1, while it ranges from 800 to 1700 meq l1 for rivers of Java. In central Thailand, the Chi and Mun rivers, which drain evaporitic rocks, are naturally very high in Naþ and Cl.
Human Effects on South Asian Rivers South Asian rivers, with the exception of the Irrawaddy, Salween, Brahmaputra, and Middle and Lower Mekong, are now extensively dammed. This greatly fragments the river courses, modifies the flow regimes, and decreases the downstream fluxes of particulate matter. In addition, reservoirs are generally associated with extensive irrigated areas where the water is lost by evaporation. As a result, the discharge
Table 3 Water chemistry and suspended sediments before damming for South Asian rivers Station Country
L (km)
A (103 km2)
Q (km3 year1)
SPM (mg l1)
SiO2 (mg l1)
Ca2þ (mg l1)
Mg2þ (mg l1)
Naþ (mg l1)
Kþ (mg l1)
Cl (mg l1)
SO2 4 (mg l1)
HCO 3 (mg l1)
TZþ (meq l1)
Brahmaputra Cauweri Chao Phraya Cimanuk Citanduy Citarum Damodar Ganges Godavari Indus Irrawaddy Krishna Mahakam Mahanadi Mekong Musi Narmada Salween Shatt el Arab Tapti
Bangladesh (a) India Thailand Java (Indonesia) Java (Indonesia) Java (Indonesia) India India/Bangladesh India Pakistan Myanmar India Borneo (Indonesia) India Cambodia/Vietnam (b) Sumatra (Indonesia) India Myanmar (c) Turkey/Syria/Iraq India
3000 800 1200
580 88 111.4 3.7 3.6 5.9 20 1050 313 916 410 259 65.3 141.6 795 57 99 325 541 65
510 20.9 27.8 4.45 5.3 4.9 10 493 105 90 486 30 87 66 467 80 40.7 211 45.7 18
1058
7.8 19 15.8 19.2 13.3 30
14 28 22 16 6.7 11 22 23.2 30.2 26 10 27.5 3.2 10.4 14.2 3.2 25 46 52 49.6
3.8 24 6.3 6.2 3.2 3.9 8.2 6.5 2.4 6 6 13.5 1.0 9.5 3.2 1.1 19.2 16 22 30.4
2.1 60
3.9 5.5
9.0 4.3 8 15 9.6 8.1 9 30 42.5 2.9 10.2 3.6 4.6 35 10 31 8.3
2.0 1.1 2 3 2.6 2.2 2 2 3.0 0.95 1.5 2.0 1.1
1.1 50 4.0 4 3.3 5 9.7 5.0 14.1 7 18 37 1.0 30.9 5.3 4 17 20 32 44
10 32 8.4 19 4.7 13 45 8 10 26 5 63 2.8 15 3.8 5.2 5 1 73 39.4
58 177 76 81 36 52 62 119 105 90 120 125 18.0 60.9 57.9 15.5 175 212 180 242
1200 6120 1615 1726 812 1267 2500 2177 2113 2221 2350 4409 392 1783 1180 470 4350 4070 5830 5415
2525 1500 3180 2300 1290 858 4850 1300 2820 2760 724
395 5600 1800 2800 1055 1619 535 2130
321
11.7 21.1 10 5 11.8 9.0 8.9 24.5
3071 6.9
1 3 3
Meybeck M and Ragu A (1996) River discharges to the oceans. An assessment of suspended solids, major ions and nutrients. Environnement Information and assessment Rpt., 250 p. Nairobi: UNEP (loadable from Gems Water:http://www.gemsstat.org/descstats.aspx) A, basin area; Q, mean annual discharge; SPM, discharge weighted suspended particulate matter; TZþ, sum of cations; L, river length. (a) With headwaters in Tibet and India. (b) With headwaters in China and Laos. (c) With headwaters in China.
Rivers and Streams _ Asia – Monsoon Asia 323
River
324 Rivers and Streams _ Asia – Monsoon Asia
of many rivers in this region has decreased, as in the Shatt el Arab, the Indus, and the East Deccan rivers discharging to the Bay of Bengal. Impoundments are still created in India and in the upper Mekong Basin. The Shatt el Arab headwaters (Tigris and Euphrates) also are extensively impounded in Turkey, the regional water tower of the Middle East (e.g., Attatu¨rk Dam, one of the world’s largest), and again in Syria and Iraq. The lower reaches of the Shatt el Arab and the Karun River in Iran form an extensive wetland that has been greatly affected by conflicts, oil drilling, and drainage. The restoration of this world-class wetland remains uncertain. The upper Indus and its tributaries are impounded, and the Kotri dam in the lower reach also takes water for irrigation. As a result, the Indus water discharge to the Arabian Sea has been decreased by 80%, and its sediment load has been reduced even more. The upper Mekong, in China also, is now dammed in five places, as are some of its Thai tributaries. In the lower Mekong, a major tributary in Laos is being impounded. The water quality evolution influenced by irrigation is not much documented, but some increase of salt content is likely to occur in Pakistan and Iraq. In South and Southeast Asia, the degradation of river quality is particularly caused by faecal contamination and by organic pollution, which depends on population density and on the degree of urban and industrial sewage collection and treatment. The pressure on water quality is therefore very high during the pre-monsoon period in densely populated regions where urban development is growing faster than sewage collection and treatment. Such a situation is found in most of the Indian subcontinent (Himalayan headwaters excepted), in Sumatra and Java, and in populated lowlands of Myanmar, Thailand, and Viet Nam, and is likely to occur in the lower Shatt El Arab. Some of the largest and fastest growing cities are located in South Asia. Many of them are located inland on relatively small river courses with limited dilution or self-purification capacity, particularly during the pre-monsoon period, as is the case for Hyderabad (Pakistan) on the Indus; Lahore on the Ravi River; Hyderabad (India) on the Krishna River headwaters; Bangalore on Ponnayar River headwaters; New Delhi, Allahabad, and Varanasi on the Yamuna River, a Ganges tributary; Nagpur on the Godavari headwaters; Poona on the Bima headwaters, a branch of the Krishna River; Calcutta on the Hoogly, a branch of the Ganges Delta; Ahmedabad on the Sabarmati River; Dacca in the Bengal Delta; Baghdad on the Tigris; Ho Chi Minh City on the Saigon River; and Hanoi on the Song Koi (Red River).
Industrial activities are developing rapidly in the Indian subcontinent and in Southeast Asia. They may result in hotspots of pollution involving heavy metals already evidenced on the Yamuna River from Delhi to Varanasi, and on the Gomati River and on the Loni River at Lucknow, both Ganges tributaries, on Saigon River and delta branches of Song Koi River (Viet Nam). Other possible threats are associated with conflicts (e.g., defoliant use in Viet Nam, leakage from destroyed industrial facilities in Iraq). However, the levels of toxic contaminants remain largely undocumented (see GEMS Water, a global program on river quality). The effect of rapid deforestation in Southeast Asia (e.g., Malaysia, Indonesia, Thailand) on water quality and aquatic species biodiversity should also be assessed.
Perspectives River basins and water resources of South and Southeast Asia region are changing very rapidly in response to increasing water demand. From Turkey to Pakistan, shared basins, already sensitive to climate variations, are exposed to regional water management conflicts between water-rich upstream countries and water-poor downstream users. In this region, most of the river water is already being used for irrigation and water supply for large cities. The upper river courses are heavily fragmented by large dams and seasonal drought can now be observed in the lower courses, such as the lower Indus. Although human influences on river quality are not yet adequately documented with regard to the water use and demand, the fast growth of urbanization, intensive agriculture, and industrialization could cause important problems in the future, particularly in densely populated floodplains (Ganges, Meghna, Irrawaddy, Chao Phraya, Mekong, Song Koi), and where water resources have already been much used (Shatt El Arab, Indus).
Glossary Allogenic river – Any river the main stem of which is sustained almost entirely by waters derived from the uppermost regions of the drainage basin, because the upper part of the drainage basin is well watered, whereas the lower part of the basin is arid. Water tower – In a hydrologic context, a water tower is a large source of water that feeds the lower reaches of a river, often because the upper reaches are mountainous and receive abundant precipitation.
Rivers and Streams _ Asia – Monsoon Asia 325
Suspended particulate matter – Suspended particulate matter (SPM), which is also referred to as total suspended solids (TSS), consists of all material carried by water that can be separated from the water by use of a filter. SPM consists of both inorganic and organic particles. TZþ – Sum of cations dissolved in water, expressed as microequivalents per liter (meq l1). TZþ is an index of total salinity or total dissolved solids. See also: Africa; Asia – Eastern Asia; Flood Plains; Fluvial Export; South America.
Further Reading Ansari AA and Singh B (2000) Importance of geomorphology and sediment processes for the metal dispersion in sediments and soils of the Ganga Plain. Chemical Geology 162: 245–266. Carbonnel JP and Guiscafre´ (1965) Grand Lac du Cambodge, Sedimentologie et Hydrologie, 1962–1963, 401pp. Paris: Rapport de Mission , Ministe`re des Affaires Etrange`res. Jennerjahn T, et al. (2004) Biogeochemistry of a tropical river affected by human activities in its catchment: Brantas River estuary and coastal waters of Madura Strait, Java, Indonesia. Est. Coastal Shelf Science 60: 503–514.
Korzun VI (ed.) (1978) World Water Balance and Water Resources of the World. Studies and Reports. Hydrology vol. 25, 663 pp. Paris: UNESCO (þatlas). Le Thi Phuong Q, Garnier J, Billen G, Thery S, and ChanVan M (2007) Hydrological regime and suspended load of the Red River (Viet Nam), observation and modelling. Journal of Hydrology 334: 199–214. Meybeck M, Du¨rr HH, and Vo¨ro¨smarty CJ (2006) Global coastal segmentation and its river catchment contributors: A new look at land-ocean linkage. Global Biogeochemical Cycles 20, GB IS 90, doi 10.1029/2005 GB 002540. Milliman JD and Syvitski JPM (1992) Geomorphic/tectonic control of sediment discharge to the oceans, the importance of small mountains. Journal of Geology 95: 751–762. Salomons W, Kremer HH, and Turner RK (2006) The catchment to coast continuum. In: Crossland CJ, et al. (eds.) Coastal Fluxes in the Anthropocene, pp. 145–200. Springer. UNESCO (1969) Discharges of selected rivers of the world. Studies and reports in Hydrology, no. 5, 70pp. Paris: UNESCO. UNESCO (1996) Global river discharge database (Riv Dis), Technical Documents in Hydrology, 41pp. Paris: UNESCO.
Relevant Website http://www.gemsstat.org/descstats.aspx – Gems Water program UNEP, global river water quality data.