FIGURE 5.6 Variation of sediment yield with basin area for the seven topographic categories of river basins listed in Table 1 (Milliman and Syvitski, 1992). For all river types, except lowland and coastal plain rivers, the correlation is strong (r2 ranging from 0.70 to 0.89).

Africa

Rivers draining Africa discharge a disproportionately small amount of sediment to the sea, although the discharge calculated by Milliman and Meade (1983) is probably low (Walling, 1985). At first it seems incongruous that Africa, one of the highest-standing continents (in terms of average elevation), has a low sediment flux. Only when viewed in terms of drainage basin morphology does the discharge pattern make sense; some large rivers with low loads (e.g., Senegal, Niger) are nonmountainous, and many small rivers in western Africa are lowland rivers, with correspondingly low sediment loads/yields. The major sediment discharge comes from rivers draining the rift mountains in eastern Africa (Nile, Zambesi, Limpopo, Rifiji) or rivers draining the mountains in Morocco, Algeria, and Tunisia (e.g., Mouloura, Sebou, Cheliff). The loads and yields of these rivers compare well with other mountainous rivers of similar size. The lack of rainfall throughout most of central Africa contributes to the low discharge rates (Walling, 1985).

Asia and Oceania

With notable exceptions of the loess-imparted Yellow River basin, the high sediment yield in Asia is restricted to rivers draining the Himalayan Mountains in southern Asia. These loads and yields are substantially higher than all other mountainous rivers of the world (save Oceania). Rivers draining eastern Asia have normal (Korea) or low (Japan) sediment loads relative to other mountainous rivers; dams in Japan may be important in these values. The



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