Water for the Future: Hydrology in Perspective (Proceedings of the Rome Symposium, April 1987). IAHS Publ. no. 164, 1987.

Material transport by the world's : evolving perspectives

D, E, WALLING & B, W, WEBB Department of Geography, University of Exeter, Exeter EX4 4RJ, Devon, UK

ABSTRACT Measurements of material transport by rivers were first undertaken more than 150 years ago and the significance of the resultant data has become increasingly apparent. Current knowledge concerning the magnitude of particulate and dissolved loads, the associated global patterns and total material transport to the oceans is reviewed and some future needs identified. Transport des matières par les fleuves mondiaux: perspectives d'évolution RESUME Les mesures du transport de matières solides et dissoutes par les fleuves ont été faites depuis 150 ans et l'importance des données qui en résulte est devenue de plus en plus apparente. Les connaissances actuelles en ce qui concerne l'ampleur des charges des sédiments et des matières solubles, leurs distributions mondiales, et le transport total aux océans sont examinés et des besoins pour l'avenir sont identifiés. INTRODUCTION

Interest in the measurement of the transport of dissolved and particulate material by rivers can be traced back to the first half of the nineteenth century. For example, an attempt to estimate the annual suspended and dissolved load of the Ganges was made as early as 1831 (Everest, 1832), measurements of suspended sediment transport were commenced on the Mississippi in 1845 (Nordin, personal communi­ cation) , and Livingstone (1963a) reports analyses of the chemical composition of water from the Rhine undertaken in 1837. Increasing awareness of the significance of such measurements for estimating rates of erosion and denudation (e.g. Reade, 1876; Penck, 1894; Dole & Stabler, 1909; Fournier, 1949) and for assessing material transport to the oceans and geochemical cycling (e.g. Kuenen, 1950; Gilluly, 1955; Livingstone, 1936b) as well as their significance to practical problems, such as reservoir sedimentation, subsequently encouraged the expansion of measurement programmes in many areas of the world. This growth of measurement activity in turn stimulated a series of global-scale assessments of loads (e.g. Fournier, 1960; Lisitzin, 1962; Livingstone, 1963a; Strakhov, 1967; Holeman, 1968; Meybeck, 1976,1979; Jansson, 1982; Milliman & Meade, 1983; Walling & Webb, 1983a) to which the International Association of Hydrological Sciences made an important contribution through its activities in promoting the compilation of load data for the world's rivers (e.g. Durum et al., 1960; Fournier, 1969; UNESCO, 1974). _ 314 D.E.Walling S B.W.Webb

It is now more than 150 years since Everest's pioneering work in estimating the annual load of the Ganges and his estimate of 360 x 10 t year- still compares quite favourably with the value of 600 x 106 t year-1 cited by Meybeck (1976) in his review of material transport by world rivers. In many other instances, however, recent expansion of data availability has occasioned more significant revisions of existing estimates and ideas. Looking specifically at the global-scale, this paper attempts to review existing information and some of the recent improvements in our knowledge of global river loads and to highlight some requirements for future research. Attention will be restricted to overall values of particulate and dissolved material transport rather than the loads associated with individual mineral or organic constituents.

PARTICULATE LOADS

Suspended sediment yields Global minima for specific suspended sediment yield in areas — 2 — 1 evidencing significant annual runoff lie well below 2 t km year For example Douglas (1973) cites a yield of 1.7 t km- year for the Queanbeyan River (172 km ) in the Southern Tablelands and Highlands of New South Wales, Australia, and loads of <1.0 t km- year- have been documented for several rivers in Poland (Branski, 1975). Increased data availability can do little to modify our view of minimum levels of suspended sediment yield, but it has significantly changed our perception of the upper bound in recent years. In their reviews of global sediment transport rates, Strakhov (1967) refers to a maximum of 2000 t km-2year- for the Sulak River in the USSR and Fournier (1960) cites a maximum of 6068 t km-2year_1 for the Lo Ho River in China. Values considerably in excess of 10 000 t km year have, however, now been reported for several rivers, and Table 1 lists a number of rivers characterized by such extreme values. The highest value in Table 1 is a mean annual yield of 53 500 t km-2year_1 for the Huangfuchuan River (3199 km2) in China. This river is a of the Yellow River (Hwang Ho) draining the gullied loess region which is now well known for its high sediment yields (cf. Long & Qian, 1986). In the past, there have been several attempts to combine the limited sediment yield data available at the time with notions concerning the influence of relief, climate, geology, tectonic stability and other factors, to produce global maps of sediment yield. The work of Fournier (1960) and the Soviet scientist Lopatin, reported in Strakhov (1967), are two such studies which have been frequently cited. Often it has not been fully appreciated that these maps were based on a very small number of actual obser­ vations of sediment yield (60 in the case of Lopatin and 96 for Fournier) and that they reflect very considerable subjective interpolation and extrapolation. These problems and uncertainties are clearly demonstrated by a comparison of the maps of these two workers (Fig.l). In terms of general levels, the sediment yields depicted on Fournier's map are frequently an order of magnitude greater than those shown by Strakhov. Furthermore, there are Material transport by the world's rivers 315

TABLE 1 Maximum values of mean annual specific suspended sediment yield reported for world rivers

Country River Drainage Mean annual Source area sediment yield (km2) (t km 2year 1 )

China Huangfuchuan 3199 53 500 Yellow River Conservancy Commission (Personal Communication) Dali 96 25 600 Mou and Meng (1980) Dali 187 21 700 Mou and Meng (1980) Taiwan Tsengwen 1000 28 000 Milliman and Meade (1983) Kenya Perkerra 1310 19 520 Dunne (1975) Java Cilutung 600 12 000 Hardjowitjitro (1981) Cikeruh 250 11 200 Hardjowitjitro (1981) North Island, Waiapu 1378 19 970 Griffiths (1982) New Zealand Waingaromia 175 17 340 Griffiths (1982) South Island Hokitika 352 17 070 Griffiths (1981) New Zealand Cleddau 155 13 300 Griffiths (1981) significant contrasts in the overall patterns demonstrated by the two maps. Recent improvements in data availability have inevitably permitted updating and improvement of these maps and two more recent attempts to produce global maps are presented in Figs 2 and 3. Figure 2 presents a map produced by the authors based on data assembled from nearly 2000 rivers and Fig.3 depicts a recent map produced by the Soviet scientists Dedkov & Mozzherin (1984) using a data-base which included more than 3000 measuring stations. Both maps refer to the sediment yields associated with intermediate-sized basins of the order of lO^km2, but in the latter case the global map refers essentially to plains rivers and no attempt has been made to map the yields occurring within the major mountain regions. Comparison of Figs 2 and 3 reveals many broad similarities between the two maps, indicating that considerable progress has been made towards producing a consistent and generally acceptable map of the global pattern of sediment yields. Furthermore, many of the patterns suggested by Fournier and Strakhov can be seen to be unsubstantiated by the recent improvement in data availability. For example, no evidence of the areas of very high sediment yield depicted by Fournier for West Africa is provided by Figs 2 and 3, and whereas 316 D.E.Walling S B.W.Webb

FIG.l Global patterns of suspended sediment yield according to (a) Strakhov (1967) and (b) Fournier (1960) .

Strakhov's map suggests that sediment yields are low throughout Africa, both these more recent maps represent relatively high values in parts of East and North Africa. Many workers have attempted to account for global variations in sediment yield in terms of climatic controls (e.g. Langbein & Schumm, 1958; Fournier, 1960; Douglas, 1967; Wilson, 1969) but recent work has increasingly demonstrated the complexity of the controls involved (e.g. Walling & Webb, 1983a). Any explanation of the Material transport by the world's rivers 317

FIG.2 A generalized map of global suspended sediment yields produced by the authors.

I D I Deserts i i Mountain 1 ' regions

FIG.3 The map of global suspended sediment yields produced by Dedkov & Mozzherin (1984). 318 D.E.Walling & B.W.Webb generalized pattern depicted in Fig.3 must, for example, take account of the influence of rock type, relief, tectonic stability, land use and human activity as well as that of climate. A number of authors have suggested that sediment yields will be highest in areas of semiarid climate (e.g. Langbein & Schumm, 1958), and the high yields mapped for the Mediterranean, Southwest United States and parts of East Africa may be largely ascribed to this tendency. Equally, however, the high sediment yields occurring throughout much of Asia and in the Pacific Islands reflect the high annual rainfall of these areas, although the steep terrain and tectonic instability are also very important influences. The close association between high sediment yields and mountain belts is also evident from Fig.3, with large areas in the Andes, the Himalayas, Alaska and the Mediterranean producing high yields. The influence of topography and geology is also demonstrated by the low yields mapped for much of the northern regions of Eurasia and North America. Here the subdued relief, widespread coarse glacial deposits and the resistant basement geology are important controls. The extensive areas of low sediment yields in equatorial Africa and South America are also a reflection of the subdued topography and the dense cover of tropical vegetation. Despite its potential value, there have been few attempts to produce a detailed analysis of the various patterns described above in terms of global morphoclimatic zones, but a notable exception is the recent work of Dedkov & Mozzherin (1984) referred to earlier. These authors firstly subdivided their data-base into plains rivers and mountain rivers. Specific suspended sediment yields associated with the latter were on average three times greater than those for the former. Each of the morphoclimatic zones within these two areas were subsequently characterized by typical sediment yields. Yield values were given for both small (<5000 km2) and large (>5000 km ) rivers in order to take account of the scale factor involved in sediment delivery (cf. Walling, 1983). The results of this analysis, which are presented in Fig.4, are heavily dependent upon the representativeness of the sediment yield data available for the different morphoclimatic zones because the values depicted for each zone are simple averages of the available data. Figure 4 cannot therefore be viewed as a definitive representation of the global zonation of suspended sediment yields, but it, nevertheless, affords a valuable indication of the global patterns involved. In the case of plains rivers, it emphasizes the relatively low specific suspended sediment yields encountered in the temperate and equatorial belts and the occurrence of much higher values in subtropical and tropical regions. A similar pattern is evident for mountain rivers, but in this case rivers in the glacial zone produce the highest yields.

Bed load transport The numerous problems associated with the measurement of bed load transport by rivers have resulted in a paucity of information on this component of particulate transport. Faced with this situation and yet needing to produce estimates of the total sediment load of a river (i.e. suspended plus bed load) many workers have assumed that Material transport by the world's rivers 319 (a) PLAINS

BELT ZONE SMALL RIVERS LARGE RIVERS Tundra and Subarctic forest tundra Taiga and mixed forest , Broadleaved forest W//A Temperate Forest steppe W, Steppe Semi-desert PI Semi-desert No data Steppe Subtropical Mediterranean W^^m^ Forest W/////////////////AW/mmmmm Tropical Forest and Sub- W///////////M Equatorial Savanna Equatorial Forest M i i i i i H 100 200 300 400 500 0 100 200 300 400 Mean annual suspended sediment yield (t kni2year~') (b) MOUNTAINS

ZONE SMALL RIVERS LARGE RIVERS Glacial «^««^ Subnival Taiga and V////////A mixed forest '///. Broadieaved forest W//À Forest-steppe Steppe Semi-desert Subtropical semi-desert ÉP Subtropical steppe m Mediterranean ^^^«m ^ Subtropical rainforest Tropical forest w& Savanna Tropical W/////////A montane forest 0T^^ 200 400 60,0 80,0 100, 0 0 200 400 600 Mean annual suspended sediment yield (t krrf2year~') FIG.4 The global zonation of suspended sediment yields proposed by Dedkov S Mozzherin (1984). bed load represents only a small proportion of the total load and have arbitarily increased the measured suspended load accordingly. For example, Gregory & Walling (1973) suggest that in many rivers bed load will represent about 10% of the total sediment load. Similarly, ASCE (1975) provide a table for estimating bed load as a proportion of the suspended load from information on the concentra­ tion and texture of the suspended sediment and the channel forming material, which suggests that the proportion can range between 2 and 150% but commonly falls in the range 5-15%. In the absence of more definitive information, Milliman & Meade (1983) employed this approach in their study of world-wide delivery of river sediment to the oceans and estimated the bed load component as 7-13% of the total sediment load. These authors, however, stressed the many uncertain- 320 D.E.Walling & B.N.Webb ties associated with this estimate and cited evidence to suggest that the proportion could be very much higher in major rivers such as the Zaire and Brahmaputra. Recent expansion in data availability has done little to resolve this problem or indeed to indicate that the often-quoted value, of 10% is greatly in error. However, new data have emphasized the great variability that may occur in the magnitude of the bed load contribution to the total load (e.g. Table 2). Furthermore, in what

TABLE 2 Examples of the relative importance of bed load and suspended load to total sediment transport

River Country i Bed load Suspended Source (%) load (%)

Torlesse Stream New Zealand 92.9 7.1 Hayward (1980) (3.85 km2) Arctic streams Baffin Island, 80-95 5-20 Church (1972) Canada Usumacinta Mexico 33 67 Wundt (1962) Shangyou China 30.5 69.5 Qian & Dai (2750 km2) (1980) Minjiang China 11.7 88.3 Qian & Dai (18 900 km2) (1980) Zaire Zaire 10.7 89.3 Dedkov & Mozz- herin (1984) Hanshui China 9.0 91.0 Qian S Dai (95 217 km2) (1980) Snake and Clear­ Idaho, USA 5 95 Emmett (1984) water Rivers Kola USSR 3 97 Dementev & Malyutina (1970) Yalong China 2.8 97.2 Qian & Dai (110 750 km2) (1980) Dadu China 2.4 97.6 Qian & Dai (76 400 km2) (1980) Tanana Alaska 1.5 98.5 Emmett (1984) must be seen as a very useful advance, Dedkov & Mozzherin (1984) have attempted to examine the variation of the relative importance of bed load (expressed as a proportion of the suspended load) in different morphoclimatic zones (Table 3). These authors suggested that bed load expressed as a proportion of the suspended load averaged 8 and 23% in plains and mountain rivers respectively.

DISSOLVED LOADS

Information on the dissolved loads of the world's rivers is more limited than that for suspended sediment loads, but a number of Material transport by the world's rivers 321

TABLE 3 The relative contribution of bed load to the sediment loads of large rivers (>5000 km2) in different morphoclimatic zones (based on Dedkov & Mozzherin, 1984)

Zone Bed load/suspended load

I PLAINS RIVERS Tundra and forest tundra 3% Taiga and mixed forest 20% Broadleaved forest 6% Steppe 10% Semi-desert 4% Tropical forest 50% Savanna. 16% II MOUNTAIN RIVERS Glacial 36% Subnival 26% Taiga and mixed forest 30% Broadleaved forest 9% Steppe 6% Humid subtropical forest 24% Tropical forest 12%

generalizations are now possible. A review of existing data produced by Walling & Webb (1986) and which assembled information from nearly 500 rivers in different areas of the world provides a useful indica­ tion of the range of values likely to be encountered (cf. Fig.5). This review reported minimum values for areas with significant runoff of <1.0 t km~2year-1, a maximum value of 500 t km-2year-1

120

120 140 160 — 2 -1 Mean annua! dissolved load (tkm year ) FIG.5 Frequency distribution of mean annual total dissolved load for a sample of 496 world rivers (based on Walling & Webb (1986)) . 322 D.E.Walling S B.W.Webb for the River Dranse in the Chablais region of , and a mean — 2 —1 value for the data set of 39.5 t km year . Other authors have recently cited higher loads and Meybeck (1984) refers to values of 6000 t km" year- for the Cana River in Amazonia which drains an area of halite deposits, and of 750 t km year for an area of karst in Papua New Guinea. The former represents a local anomaly due to the halite deposit, but the latter probably provides a realistic indication of the global maximum representative of more normal geological conditions. Dissolved loads can be seen to span a considerably smaller range than suspended sediment loads. Insufficient dissolved load data are available to produce a generalized global map of solute yields comparable to Figs 2 and 3. However, in Fig.6 the available data for major world river basins

FIG.6 The specific dissolved loads associated with the major river basins of the world (based on data presented by Meybeck (1976,1984) and other authors). compiled by Meybeck (1976, 1984) and several other workers have been mapped. The resulting pattern is in general somewhat easier to decipher than that for suspended sediment yields, and can be seen as largely reflecting the influence of runoff amount, lithology, and to a lesser extent the temperature regime. The high load values mapped for many Asian rivers are a response to their high runoff totals which provide greater opportunity for removal and transport of dissolved material from their drainage basins. The relatively high loads associated with several European rivers can equally be linked to the predominance of sedimentary strata, including lime­ stones, within their catchments. Conversely, the low dissolved loads characteristic of Africa and Australia reflect the existence of ancient basement rocks with a low susceptibility to chemical weathering. The extremely high values for the Irrawaddy and for the Material transport by the world's rivers 323

Fly and Putari Rivers in Papua New Guinea undoubtedly reflect both high runoff totals and the presence of readily weathered sedimentary strata, but the high temperatures associated with their tropical climates may also be important in promoting rapid chemical weather­ ing.

MATERIAL TRANSPORT TO THE OCEANS

In the past, attempts to estimate the total annual transport of material by rivers from the land surface of the globe to the oceans have been severely hampered by lack of data for many of the major rivers of the world. Appreciable differences exist between many of the associated estimates (Table 4). Data availability for these rivers has, however, improved considerably in recent years and although uncertainties still exist, meaningful estimates can now be advanced. In the case of suspended sediment, the most up-to-date and rigorous compilation of available data is that undertaken by Milliman & Meade (1983). This provides an estimate of the total transport of suspended sediment to the oceans of 13.5 x 109 t year-1 This estimate excludes sediment deposited in major reservoirs which would formerly have reached the oceans and it must be increased to

TABLE 4 Some existing estimates of suspended sediment and dissolved load transport to the oceans

Author Estimated mean annual load (10* t)

I SUSPENDED SEDIMENT Fournier (1960) 51.1 Kuenen (1950) 32.5 Gilluly (1955) 31.7 Jansen & Painter (1974) 26.7 Pechinov (1959) 24.2 Schumm (1963) 20.5 Holeman (1968) 18.3 Goldberg (1976) 18.0 USSR National Committee for the IHD (1974) 15.7 Sundborg (1973), Walling S Webb (1983a) 15.0 Milliman S Meade (1983) 13.5 Lopatin (1952) 12.7 MacKenzie S Garrels (1966) 8.3 II DISSOLVED LOAD Golterman et al. (1983) 3.5-4.0 Goldberg (1976) 3.9 Livingstone (1963a) 3.8 Meybeck (1979) 3.7 Clarke (1924) 3.7 Meybeck (1976) 3.3 Alekin & Brazhnikova (1960) 3.2 (Section I based partly on Holeman (1968)). 324 D.E.Walling & B.W.Webb approximately 14.0 x 109 t year" to provide an estimate of the gross transfer, or that which would occur in the absence of such deposition. This value is in close agreement with estimates of 15.0 x 109 t year-1 produced by Walling & Webb (1983a) and of 15.7 x 109 t year-1 cited by the USSR National Committee for the IHD (1974), both of which essentially refer to the gross transport. Gross suspended sediment transport to the oceans can therefore currently be estimated with reasonable confidence at 14-15 x 109 t year-1. The estimates of total dissolved load transport to the oceans listed in Table 4 show less variability than those for suspended sediment, but in this case the most reliable would appear to be that produced by Meybeck (1979), i.e. 3.7 x 109 t year-1. As indicated previously, no meaningful estimate of the bed load component of the particulate transport is available, but if this is assumed to be about 10% of the gross suspended sediment load, the total material transport from the land to the oceans can be estimated at about 19-20 x 109 t year-1. Of this total, about 80% is contri­ buted by the particulate load and about 20% by the dissolved load. Because of problems of comparability between the conventions employed by different researchers (e.g. continental areas and boundaries), it is difficult to provide a reliable breakdown of the suspended sediment and dissolved loads transported to the oceans from the individual continents, but in Table 5 an attempt has been made to apportion the global estimates of Milliman & Meade (1983) for suspended sediment and of Meybeck (1979) for dissolved load. In terms of absolute magnitude of the suspended sediment loads for individual continents, Asia provides the highest load and Europe the lowest. Taking account of the relative size of the continents, however, maximum specific yields are associated with Oceania and the Pacific Islands and minimum values with Africa and Europe, with yields from Oceania and the Pacific Islands exceeding those from Africa by an order of magnitude. The absolute values of dissolved load for the individual continents listed in Table 5 indicate a maximum for Asia and a minimum for Africa. However, the specific yields provide a somewhat different ranking, with maximum values for Europe, although the overall range of yields is much less than that for suspended sediment. Table 5 also lists the suspended/dissolved ratio for each continent and these exhibit considerable variability. In Europe, the dissolved load exceeds that of suspended sediment, whilst the suspended sediment load is an order of magnitude greater than the dissolved load for Oceania and the Pacific Islands.

THE FUTURE

Knowledge concerning material transported by the world's rivers has improved greatly in the past decade as a result- of an expansion .of measurement activity and the interchange of data promoted by UNESCO and other international bodies. Meaningful representations of the generalized global pattern of suspended sediment yield and estimates of total material transport to the oceans are now available. However, considerable scope exists to refine these representations and estimates. The data available for many rivers are of limited Material transport by the world's rivers 325

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