Input by Indian Rivers Into the World Oceans

Proc. lndian Acad. ScL, Vol. 87 A (E & P sciences-z), No.7, July 1978,pp. 77-88, @ printed in India

Input by Indian rivers into the world oceans

V SUBRAMANIAN School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067

MS received 16 March 1978; revised 31 May 1978

Abstract. The chemical, sediment and total load carried by the major river basins in India-Ganges, Brahmaputra, Indus (Jhelum), Godavari, Krishna, Narmada, Tapti, Mahanadi and Cauvery have been calculated, based partly on new set of data and partly on existing data. There is a significant amount of chemical load transported by all the Indian rivers, and for global mass transfer calculation, these cannot and should not be ignored. The chemical mass transfer during the monsoon is not surpri singly small, as would be expected for excess discharge and dilution controlled run off. The sediment mass transfer from non-Hirnalyan rivers, all within the same range of magnitude, accounts for less than a tenth of that of the Ganges but during the monsoon, except for Cauvery, all the Indian rivers carry a sediment load of greater than 1000 ppm. The total mass transfer from the Indian subcontinent accounts for 6'5 per cent of the global transfer. Except for the Ganges and the Brahmaputra, the erosion rates are similar for all Indian basins, independent of their size and these rates are agreeable with the conti nental earth average. The Ganges-Brahmaputra basin erosion rates are highest on the continental earth. Based on the average rate of denudation of the Indian sub continent, the mean elevation of this landmass will be that of the present day mean sea level in 5 million years from now. The average denudation rate of 2'1 cmjl00 years is different from the calculated average sedimentation rate of 6'7 cmjl00 years in the Bay of Bengal suggesting that an accurate erosion rate in the continent is needed to determine sedimentation rate in the oceans. The chemical and sediment mass transfer rates appear to have a logarithmic linear relationship on a global scale, as against the reported negative logarithmic trend for North America alone.

Keywords. Mass transfer: Indian rivers; chemical load; sediment load; total load; erosion rate; basin changes.

1. Introduction

Continental mass transfer into the oceans in the present day environment need to be understood well for a variety ofreasons: the principal one among them is the possible fluctuation in the mass transfer in the geological past with implications on the evolu tion of the oceans, on one hand, and the sedimentary rocks, on the other. The pre sently accepted proposition on the constancy of chemical composition of the sea water for the last 600 million years implies similar invariance of the mass transfer over the same period. The sedimentary recycling model, propounded by Garrels and Mackenzie (1971a) depends very much on the uniform rate of continental input into world oceans since Cambrian time; however, Gregor (1970) calculated an erosion rate for the paleozoic period to be nearly four times that for the present day environ ment; on the other hand, Garrels and Mackenzie (1971b) calculated a depositional rate which is less than one fourth of paleozoic erosion rate of Gregor. Everi though

77 78 V.Suaramanian sedimentary rocks constitute a very insignificant portion of the total rock mass in the crustal earth, more than 90 per cent of rocks on the surface, the part of the earth intimately associated with weathering, erosion and mass transfer, represent some type of sediments. Hence mass transfer can be thought to involve, recycling of all the recent and ancient sedimentary rocks, whose mass is estimated to be 32 x 10~o kg. Based on the chemical analysis of a number ofmajor rivers in the world, compiled by Livingstone (1963) and data on the suspended sediment transport, compiled by Holeman (1968), Gibbs (1972a) has tabulated chemical and sediment mass transfer for these rivers. There are several limitations to the data compiled by various wor kers: First, there is no common agreement among the various set of data probably because of different sources of data; there is no agreement on the average mass trans port which is being continuously revised; but the principal deficiency of the existing numbers is the lack of accurate information on a number of Asian drainage basins and the total neglect of basins in Asia which are big enough to be quantitatively significant but whose discharge may not qualify it to be among the -largest basins of the world. To this category belongs basins such as Godavari. As pointed out by Mebeck (1976), middle-size homogenous basins are ideal for the mass transfer studies since in these basins several environmental factors can be well defined. In India, except [or the Ganges and Brahmaputra, all other important basins, namely, Goda vari, Mahanadi, Narmada, Tapti, Krishna and Cauvery are middle-size of the order of (1-3)105 km2 basin area and together they add up to half the basin size of Ganges and Brahmaputra. Hence, any mass transfer studies ignoring these second order basins will be quantitatively not reliable; with this limitation in mind, an attempt has been made in this paper to understand the net mass transfer from the entire Indian subcontinent into the world oceans.

%. Sampling and source of additional data

For the eight major basins in India, water samples were collected at a number of stations. For each basin, one station in the watershed and other in the mouth (for example, Hoshingabad and Broach for Narmada) were chosen. Two litres water samples were collected in polyethylene bottles, pH and alkalinity measured immedi ately (pH with Phillips portable pH meter and carbonate alkalinity by micropipette titration techniques), sealed tight and with wax outside and sent to the laboratory. The samples were filtered through millipore 0·45 micron membrane filters to collect the suspended matter and the filtered water was used for elemental analysis. Water samples were collected once during May 1-15 and again during July 15-Aug.. 15, 1977. Chlorinity was measured by AgNOa titration, Na and K by emission mode and Ca and Mg by absorption mode of AAS-l spectrophotometer. S04-2, P04- 3 and silicates were analysed by standard complexation techniques for fresh waters (Golterman 1970) using ECIl spectrophotometer; conductivity was measured with II syntronic direct reading conductivity meter, standardised with 0·1 N KCl solution. In dilute waters such as fresh waters, the conductivity values can be converted to total dissolved salts, by certain multiplication factors or.can be numerically equated to TDS (Davis and Dewiest, 1967). Such conversions were checked with the actual chemical analysis and were used only where the numbers matched; otherwise, the actual chemical.analyses were summed to get TDS. Input by Indian rivers into the world oceans 19

Data for the winter season were taken from the file of Central Soil and Material Research Station, New Delhi. The average values for 1972 to 1976 were taken for calculations. The discharge values and basin areas for the rivers were taken either from Rao (1975) or the Newsletters of the Indian National Committee for IHD (1972-77). Pooling of data from such diverse sources puts some limitationson the interpretation but in this paper a beginning has been made by pooling such data with the ones generated by the author; this is still not satisfactory and interpretations presented here are bound by these limitations. It is further realised here that monthly, preferably daily, sampling will better reflect the natural process but sampling costs and logistics dictate optimisation of fieldwork. It is hoped to refine the data presented here with continued observations during the next few years. As such, interpretations presented in this paper do not represent the last word in the mass transfer studies.

3. Chemical mass transfer

Table I indicates the annual chemical and suspended load, total denudation and various rates calculations from the f3,W data (CSMRS, 1973 ar.d Subramanian, present manuscript) for the eight Indian river basins under investigation. For com parison, data for the major river basins, such as Amazon, Mississippi, _Congo, Mekong, etc. are computed in table 2. Continent-wise mass transfer are presented ill table 3. Based on their limited sampling, Carbonnel and Meybeck (197STcalcu1ated the chemical mass transfer from the Mekong basin to be a very significant portion ofthe total chemical load input into the world oceans; but a look into the respective values in tables I, 2 and 3 suggest that the entire Asian continent contributes a maximum 40 %of the chemical load and out of this, the Mekong's contribution is a mere 4 % and on the total input, Mekong accounts for less than 2% of the chemical load. All the Indian rivers together contribute about 6 %of the chemical load. Since the land mass ofIndia accounts for a mere 2 %of the continental earth, she contributes the chemical load much in excess of her size. In other words, compared to the chemi cal denudation rate of 2·3 x 104 kgjkm2 /yr for the continental earth, the chemical

Table 1. Denudation rates for major Indian rivers

Chemical Sediment Total Chemical Sediment Total Basinload loadrate rate denuda- lOt kg/yr load io- kg/km2/yr tion rate

Cauvery 7-60 0·71 8'31 8·70 0·87 9'57 Krishna 15·01 8'50 23-51 6·11 HI 9-42 Godavari 22'02 16·20 38'22 7-32 5-30 12-60 Narmada 11-03 6'20 17-23 12·11 7·00 i9·10 Tapti 5-31 2·70 8'01 8'75 4'30 13'00 Mahanadi 8-41 7-10 15·51 6'43 5-40 11·80 Ganges 71'04 460'10 531-14 7-32 47-21 54'51 Brahmaputra 81-03 711'20 792>23 12·26 110'11 122-31

India Total 221'45 1212·71 1434'16 8'61 46'81 55-42 80 V Subramanian

Table 1. Erosion rates of some important rivers in the world".

Sediment Chemical Sediment Total Chemical Erosion River load rate rate load 109 kg/yr load rate 10' kg/km2/yr.

Amazon 232 499 731 3-68 7-92 11-66 Congo 98'5 31 129'5 2'54 8 3-34 Mississippi 118'0 213 331'0 3-66 6'61 10·27 Colorado 17·29 297'79 315'08 2·55 46·7 49·25 Columbia 40'74 31·24 71-98 6·39 4·9 11·29 Ob 29;6 14·2 43'8 1·22 0·6 1-82 Yenesei 30·9 10'5 41-4 l-I4 0·4 1'54 Mekong 60'6 1000·0 1060'6 7'50 120'0 127-50 HuangHo 1601·8 215·0 Irrawady 300'0 69'8 Indus 45-9 400'0 445-9 4'95 42'0 46·95 Ganges 71·04 460'1 531'14 7-32 47-21 54'51 Brahmaputra 81·03 7II ·20 792-23 12-26 IIO·II 122-37

·Data for Ganges and Brahmaputra-this work; Colorado and Columbia-recalculated from Judon (1964) and for others, from Gibbs (1972).

Table 3. Erosion rates for the continental earth".

Chemical Sediment Total Chemical Sediment Total Contents load load load rate denudation 1010 kg/yr rate 10' kg/kml/yr rate

Asia 149'0 1450'0 1599·0 3'2 30·24 33'44 Africa 71·0 49'0 120'0 2-4 1'63 4·03 Europe 46'0 25·0 71·0 4·2 2-27 6·47 Australia 2'0 21·0 23'0 0·2 2'10 2'30 North America 70'0 178'0 248'0 3'2 8·48 11·78 South America 55'Q 110'0 165'0 2'8 5-60 8·40

COntinental earth 393'0 1830·0 2223'0 2·3 10'82 13'12

·Data from Garrels and Mackenzie (197Ib). rate for India, 8·61 x 104 kg/km~/yr., is nearly 3·5 times higher, and as such the intensity of chemical weathering is the highest for India compared to any other single landinass. Gibbs'{1972b) has very clearly demonstrated that refinement of the Amazon chemical load altered the estimated world average chemical load of Living stone (1963). Because of uncertainties associated with the data used in this paper, a similar calculation for Indian rivers' impact on Livingstone's average value is not being made here; however, with more frequent sampling, a single source data (to avoid intercalibration problems) can be used in future to check the effect of inclusion of medium Indian rivers on the world average river water composition of Livingstone. Even though the chemical load of Amazon is nearly thrice that of Brahmaputra and many times ,that of other Indian rivers, the erosion rates for any of the Indian river, say even Tapti (very small, size and discharge-wise to Amazon) as an example. is much higher; in other words, the size of the basin or the river discharge has no effecton the rate of chemical mass transfer.. Input by Indian rivers into the world oceans 81

Table I points out to certain important point: the chemical rate between Cauvery and the Ganges are not appreciably different even though the former basin. is definitely entirely in low latitude; this suggest that the geographical location of a basin similar to the size, may not be very important chemical rate determining factor. Except for Godavari and the Brahmaputra, all other Indian rivers have about the same chemical mass transfer rate; surprisingly, the calculated area of dominance of the mountain environment and the plains environment for these two basins are identical (60:40; see table 5, section 8). These data agree well with Gibbs (1967) observation for amazon whose chemical load is dominated by the mountain environment comprising only 12%of the basin area. On a global scale, except for Europe and Australia the chemical erosion rates for other continents are similar in the range (2,8 ± 0,4) x 10 kgjkm2/yr. The high rate for Europe and the low rate for Australia can be explained easily: Europe is a • sedi mentary carbonate' continent (Garrels and Mackenzie 1971), and the chemical weathering is expected to be intense whereas Australia is a mid to high latitude desert continent and the predominant eolian weathering is not effective for the chemical decomposition of primary rock forming minerals (Goldlich 1938). The close range of values for Africa, Asia, North and South America suggest that there is a natural uniformity in the chemical mass transfer process from the continental earth to world oceans.

4. Sediment mass transfer

The total suspended matter (TSM) transported by a river does not necessarily reflect the total sediment transfer to the oceans for two reasons: (1) the organic component of TSM is quantitatively important in many rivers and often 30-50% of TSM may in reality not represent the sediment component (Subramanian and d'Anglejan 1976). The organic component may not represent any weathering process but may be there due to addition of organic and plant debris during transport or due to the precipita tion of dissolved organic compounds as fine coatings on clay suspensions. Hence the actual suspended sediment mass transfer is likely to be less than values based on TSM. (2) Even though conventionally, 10%by weight of TSM is added to TSM calculation to take care of bed load, in certain rivers, such as the Brahmaputra, bed load may be much higher; hence actual mass transfer will be very much different. Calculations based on data given in Rao (1975) for several Indian rivers give an upper limit of 2-3 times of TSM for bed load. Hence, calculations for sediment load shown in tables 1-3 are only approximate. The sediment transport by Cauvery, Godavari, Krishna, Tapti, Narmada and Mahanadi are quite insignificant as compared to the Ganges and the Brahmaputra both ofwhich together account for nearly 90 %ofthe total sediment transfer from the Indian subcontinent; however, seasonally, all the Indian rivers are important. For example, during the monsoon, except for Cauvery, all the rivers carry TSM in excess of 1000 ppm and Narmada, near Broach, was observed to carry 3814 ppm (Subramanian 1971). Data presented in the tables very clearly point out to the dominance of the Asian continent in the global sediments mass transfer. Nearly 75 %of the sediment mass 82 V Subramanian transfer takes place from Asia but as pointed out by Holeman (1968), within Asia the non-Himalayan rivers such as Mekong, Huang Ho (Yellowriver), etc. contribute much larger amount of sediments than the four major Himalayan basins-Indus, Ganges, Brahmaputra and Irrawady, which are however, important for the south Asian region of the continent. The basin area of the Himalayan system is less than half that of Amazon but their sediment contribution is nearly thrice that of the Amazon, the sediment erosion rates show that the Himalayan region is being eroded at a very high rate compared to the rest of the world; these values perhaps reflect the fact that in the Amazon basin, only 12% of the area is in. mountain environment, whereas in the Himalayan basin system, up to 60% of the area lies in the mountains; this again support Gibbs (1967) concept of relief as a dominant environmental parameter in a drainage basin. Recalculations to metric units of Judson's (1964) data indicate that except the Colorado basin, all other North American basins have sediment mass transfer rates similar to that of the Indian basins excluding the Ganges and the Brahmaputra; he accounts for the high value for Colorado as due to low precipitation in a predomi nately sedimentary basin, such a relationship of sediment erosion rate to rainfall is not valid for India because the Ganges and the Brahmaputra, both having a very high erosion rate, have a very high precipitation also, yet the uniform rate of the sediment mass transfer rates for the six non-Himalayan rivers having variations in rainfall suggest that high rainfall and high sediment erosion rate do not necessarily follow each other. Congo, the second largest river basin in the world, carries a very small amount of sediment load and the erosion rate is comparable to Cauvery; as pointed out by Grove (1972) for some African rivers, the intense chemical weathering might be due to the very fine particles produced by intense physical weathering; as a result, a very low amount of sediment load coupled with a very high amount of chemical load may indicate the weathering process going to a stage of intimate relationship between the physical and chemical weathering; the mean elevation of Cauvery basin is 630 meters and 70%of the basin lies in the mountainenvironment: yet the sediment load is small, in the range of 10-34 ppm TSM even during monsoon (Subramanian, unpublished data). Hence, even relief may not be an important factor in sediment transport for basins in the low-latitude. However, on the scale of the continent, Asia has the highest mean elevation of about 900 meters and has the highest sediment denudation rate and Europe the lowest. Garrels and Mackenzie (1971) have predicted an expo nential relationship between relief and the sediment erosion rate; data presented in this paper in table I (sediment erosion rate) and table 5 (mean basin elevation) for the Indian river basins do not agree with such a prediction.

5. Total mass transfer

The total mass transfer from the Indian subcontinent to the world oceans, prima rily through Bay of Bengal, is 9 % of the input from the Asian continent and 6·5% of the total mass transfer from the continental earth. The total continental discharge into world oceans computed by Gibbs (1972) (1,1x lOIS mS/yr) differs by a factor of 3 from the values of Garrels and Mackenzie (197Ib) (3'2x lOIS m3/yr). Accepting Gibbs' value, the total discharge from the Indian subcontinent is approximately a Input by Indian rivers into the worldoceans 83 fifth of the global discharge. The total mass transfer is a much smaller value, but considering the rate of mass transfer, the annual denudation rate of 710 MT/km2/yr (Subramanian and Dalavi, in press) is the highest for India. Comparing to Judson's (1964) value of II to 43 MTjkm2jyr for various North American basins and to the global average of 131 MTjkm2jyr, the Indian continent can be considered to contribute total mass to the oceans out of proportion to her area and discharge. This fact again probably reflect the high average elevation of India, calculated to be about 1000meters as compared to the global average of 840 meters. The total mass transfer for the Indian basins, excepting the Ganges and the Brahms putra, are of the same order of magnitude independent of the size ofthe ba~lls; but Godavari, with a basin three times the average of other non-Himalayan rivers, show a net mass transfer rate highest next to the Ganges. On a global scale, however, the size of the basin has no relevance to the total mass transfer rate. Gibbs (1970) has demonstrated for a number of tributaries of Amazon that independent of their size and climate, mountain to plains ratio influences the amount and nature of the total, dissolved and suspended mass transfer. With the exception of Cauvery, the Indian rivers can be considered now, with the data presented in this work, to behave similar to the heterogeneous Amazon system.

6. Monsoon effects on the mass transfer

As already stated, the erosion effect by a river during monsoon is so intense that during the three to four months of monsoon, some of the Indian rivers carry most of their annual sediment load. In order to understand the monsoon effect on the erosion, Ganges, Jamuna and their tributaries, Brahmaputra, Jhelum (representing the Indus basin) and rivers south ofthe Vindhyas were sampled during July-September, 1977. As an illustration, data for the Himalayan system are computed in table 4. Surprisingly, the monsoon data do not reflect the maximum rate expected for these rivers; for example, Brahmaputra carries an annual load of792 X 109 kg, based on Government of India data for 1955 at Pandu and this data, shown in table], have been widely used

Table 4. Erosion rates in the Indo-Ganges-Brahmaputra system based on monsoon data".

Annual Annual Total Chemical Sediment Annual River chemical chemical erosion Physical/ load annual erosion erosion load load rate rate Chemical 10' kg/yr 104 kg/km"/yr rate

Ganges 43·01 270·00 313·11 4·01 25'03 29'04 6'25 Yamuna 34·00 74·00 108·00 9'03 20'12 29·15 2'21 Chambal 20'02 19'12 39'14 14·01 14'00 28·01 1·00 Betwa 3·50 2'00 5'50 8·00 4·00 12·00 0'50 Ken NO 1·90 5·30 12·00 7·00 19'00 0'58 Gandak 6·00 60·00 66'00 13·00 130·00 143'00 10·00 Son 15·00 272-56 287'56 21'00 381·85 402-85 18'10 Indus (Jhelum) 16·00 14·00 30·00 46·00 40'00 86'00 0'87 Brahmaputra 29·15 160'21 189·36 5·00 28'10 33'10 5-61

*Annual data calculated, based on observations for the monsoon season of 1977. 84 V Subramanian

by Gibbs (1967), Raymahasay (1970), Meybeck (1976) and many others. In absence of factual information on the methodology, it is not possible here to predict whether this truly represent the sediment transport in the river. Sampling during. the monsoon by the author gave a TSM load of only 160x 109 kg/yr. There are two explanations for the observed differences: I) Due to glacial malt, the discharge and TSM of Brahmaputra during summer should be enormous to give an annual rate of 711 x 109, as against the monsoon based rate of 160 x 109 kg; (2) The bed load of Brahmaputra is several times the TSM during monsoon so that the (1955) GOI values represent the total sediment load. The author is currently exploring the possibilities of seasonal and bed load sampling for Brahmaputra. Two small but seasonally important tributaries, of Ganges are the Gandak and Son. Near Ganga bridge at Patna, the Gandak, Son and the Ganges meet and the tri junction is extremely clear by the colour and quantity of the respective TSM, up to Allahabad, Jamuna carries a total load in excessof Ganges (Subramanian and Dalavi, in press) and at Patna, the monsoon addition from Son and Gandak to Ganges is much in excess of the load transported by Ganges between Allahabad and Patna, The extreme physical erosion with the resultant large sediment mass transfer in the Son and Gandak basins, is evident from the erosion rate values for these rivers. The decrease in the annual chemical load for Ganges and Brahmaputra based on monsoon data (table 4) compared to the seasonally adjusted annual average (table I) can be easily explained as due to the high discharge-low salinity relationship develop ed by Carbonnel and Meybeck (1975) for the Mekong basin. The very low ratios for physical to chemical weathering, shown in table 4, reflects the effect of chemically highly reactive basaltic (Ken and Betwa) or carbonate (Jhelum) terrain in the respec tive basins. Chambal basin shows a 'balanced' erosion characteristics due to equitable distribution of mountain to plains environment and of hard basaltic rocks to recent alluvium, such a 'balanced' pattern is approximate only by Africa, on a continental scale (table 3).

7. Chemical versus physical mass transfer

Onthe basis of data for seven major North American drainage basins, Judson (1964) derived a negative logrithmic relationship between the chemical and sediment erosion

(01 :: \. 10 ~

Figure la. Data from Judson (1964) relating chemical to sediment erosion rates for North American basin plotted on metric units. Curve visually fitted. Figure lb. Data for the Indus (Jheluml-Ganges-Brahmaputra system calculated based on their monsoon (1977)TSM and TDS values. Parabolic curve visually fitted. Note the discontinuity in the horizontal scale. Input by Indian rivers into the world oceans 85

t:. (01

11 13 15 • lbl

6 7 8 9 10 11 12 13 Chemical erasion rate (104kg/km2,fyrl

Figure 28. Relative erosion rates for major river basins of the world. Data from table 2. Note the discontinuity in the vertical scale. Dashed line visually fitted. Figure 2b. Relative erosion rates for major river basins in India. Data form table I. Note the discontinuity in the vertical scale. rate. His data have been recalculated to metric units and the relationship is shown in figure lao However, analysis of data for Indian rivers, data for major rivers of the world and data for single monsoon seasonal samples in the Indus-Ganges-Brahma putra system do not support Judson's proposition. In figure lb, data from table 4 have been plotted. Even though, a parabolic relationship is suggested, the limited number of data do not warrant any definite relationship. In figure 2a, data from table 2 have been plotted. A clear cut logarithmic but positive trend is evident on the global scale. In figure 2b data from table 1 have been plotted. The scatter of points indicates that in the Indian subcontinent, the chemical erosion rate and the sediment erosion rate have no definite relationship. Even though in times of high discharge, such as monsoon, the aqueous medium does not have sufficient time to react with the minerals in the basin and as a result, the chemical load will be small, further dilution decreases the dissolved load concent ration. On the other hand, the monsoon is a period of dynamic interaction with the basin lithology and a large quantity of sediments will be released for transportation. However, when discharge is very high, there will be definite addition to the dissolved load also. Figure Ib can thus be interpreted as a two phase system: the first one of moderate discharge and obeying Judson's predictions and the next phase accounting for the additive processes. Since the global data computed in table 2 are discharge weighted, the dissolved load-suspended load rate has a positive slope in figure 2a, as in the second part of figure lb. The scatter in figure 2b, probably reflects the uncertainty in the discharge data of Indian rivers. With refinement of this information, it is hoped that figures lb and 2b will merge into figure 2a. Data given in Gibbs (1967) for the major tributaries of Amazon and in Meybeck (1976) for a number of European and African rivers appear to agree with the interpretation of figure 2a.

8. Estimation of basin elevation changes Based on relief, average elevation for various drainage basins in India have been calculated and are shown in table 5. From the erosion rate values given in table 1, 86 V Subramanian

Table 5. Rate of decrease in mean elevation of river basins and that of Indian subcontinent

Time to Per cent Per cent Average Average Rate of reduce Per cent in moun- in plain mountain basin decrease basin to Basin of total tain en- environ- elevation elevation in eleva- M.S.L. basin vironment ment (m) (m) tion (Million Years)

Ganges 39'2 25 75 3000 750 20'1 Jo6 Brahmaputra 23'5 60 40 5000 3000 45'1 6'5 Godavari 12·1 60 40 650 400 2'2 8'1 Krishna 10·0 70 30 700 420 5·9 12'1 Narmada 3-6 90 10 850 760 8'4 10'4 Tapti 2-4 90 10 850 740 5-8 15'2 Mahanadi 5·2 50 50 1000 500 4'5 12'0 Cauvery 3·5 70 30 900 630 3'6 1-6 AlI India Average 100 67 33 1500 1000 21·0 5'0 the rates of decrease ofthe basin elevation to mean sea level were calculated and these values are also shown in the table. Judson (1964) estimated an average erosion rate of 6 mm/lOO years for seven North American basins, with a maximum of 15·8 mml 100years, for the Colorado basin and a minimum of 3·8 mm/lOOyears for the Colum bia basin. Table 5 shows that the average value of 21 mm/lOO years and the maximum of 45·1 mmlI00 years for Brahmaputra are 3 to 4 times the North American values. According to the author's calculations, the entire Indian subcontinent would have been levelled to the sea in five million years (equal to the time, since mid Pliocene) from now, though individual basins such as, Cauvery, would have long disappeared (1,5 m.y. from now), or basins such as Tapti would continue to survive (till 15·2 m.y. from now). Judson estimated a period of 11 to 12 million years for levelling the North American continent. There are, however, serious limitations and implications, in the above calculations: (1) Discharge, TDS and TSM values for various rivers are only approximate; in addition there are inherent error in observations for logistically difficult terrain such as the Himalayas; further, estimation of basins area are not accurate; there are overlapping basins such as Godavari and Krishna; Narmada and Tapti, etc. average basin elevation can never be estimated accurately but aerial photo or satellite imageries might refine the calculations. (2) TSM values obtained at the end of fresh water need not necessarily reflect the material transport to the oceans. Estuarine regions either substract or add to the mass transfer, depending on the dynamics of estuarine circulations. St. Lawrence estuary is observed to remove up to 75 per cent of the river transported materials (d'Anglejan and Smith 1973) whereas TSM values within the estuarine region will depend on resuspension and other mechanisms due to the tidal cycles (d'Anglejan et aI1975); there are some estuaries in the world where there could actually be input of TSM into the estuary from the sea (d'Anglejan and Ingram, 1976) under these constraints, it is very difficult to precisely estimate the mass transfer from the continents to the open oceans. (3) In the Bay of Bengal, Mullick (1976) have identified sediments transported longshore from as far down as Godavari mouth, to the mouth of the Ganges; if this Input by Indian rivers into the world oceans 87 is correct, then materials carried by this current will be moved inland during cross tidal cycles. The TSM estimation of Godavari, Mahanadi, Ganges and Brahma putra in their estuarine regions may not truly reflect the mass transfer, sampling of TSM even at the end of an estuary wil1 have to consider possible contribution due to turbidity currents and other resuspension mechanisms. Until a precise estimate ofthe total mass transfer could be made, taking care of the above problems, prediction of rate of transfer and basin elevation changes are at best speculative. If a period of five mil1ion years is accepted as time require to reduce the Indian sub-continent to sea level, then Bay of Bengal, being the principal recipient of the mass will flood the continent until such a time equilibrium is reestablished; mean sea level changes have taken place in the geological past (Mackenzie 1974) and the predicted event five million years from now may perhaps be the event that the common man in India is waiting for in the form of' Prahlai '.

9. Prediction of sedimentation rate in Bay of Bengal Based on field sampling and the known sedimentation pattern in the Ganges mouth, Mullick (1976) has indicated the rate of sedimentation in the Bay of Bengal to be very high. The total mass of about 1600x 109 kg (table 2) from the Indian subcontinent (Ganges-Brahmaputra-Irrawady) will be annually delivered to the Bay of Bengal. According to Mullick (1976), an area of 3000 Ian by 1000km in the Bay is influenced by the continental input, spreading the input over this region, assuming a sediment density of 0·8 gm/cc will give a layer of 6·7 em every 100 years. The annual mean erosion rate for India is calculated to be 2·1 cm/loo years (table 5). Because of the errors involved in various approximations and calculations both the numbers are higher than the average rate of sedimentation; thus an accurate rate of continental input is needed to be known in order to find out the sedimentation rate in the oceans. The estimated rates of erosion will be altered on a geological time scale if the man induced activities, such as mining, irrigation dams and barrages, etc. particularly in the eastern regions escalate beyond the rate at which nature can readjust itself. For lack of data, however, the input of man's activities on the natural mass transfer process cannot be quantitatively estimated at this stage.

10. Conclusions In the chemical mass transfer calculations, contributions from medium Indian basins are important. In the sediment mass transfer, the Himalayan drainage system is the single-most important one, while the chemical rate is reasonably uniform, the sediment rate is irregular for Indian basins. Based on the total mass transfer, it is estimated that in 5 million years' time, the average elevation of India will be the M.S.L. The current rate of input into the oceans is not balanced by the calculated rate of sedi mentation in the Bay of Bengal.

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