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 rivers: 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 river 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 tributary 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.