have much greater yields (2-3 fold) than rivers draining other mountainous areas of the world, and an order of magnitude greater than rivers draining high-Arctic and European mountains (Figure 5.5).

The trend of increasing sediment yield with decreasing size of mountainous rivers becomes less pronounced in river basins less than about 4000 km2 in area, as seen by the relative number of rivers that fall >1 standard deviation from the mean (Table 1, Milliman and Syvitski, 1992). Some very small rivers in New Zealand and Taiwan, for example, have yields much lower than expected, while others have much higher yields; together they account for one-third of the deviating rivers designated in Table 1 (Milliman and Syvitski, 1992). Slaymaker (1987) noted a decreased sediment yield in rivers <1000 km in western Canada. This variance of sediment yield in very small river basins probably reflects the dominance of single types of geology or microclimate in small basins, whereas larger river basins are modulated by a greater range of conditions.

With the exception of the high Arctic, latitude does not appear important. Equatorial rivers (e.g., the Tana in Kenya) do not have significantly higher yields than rivers of similar size in higher latitudes e.g., the Susitna in Alaska). High-Arctic mountainous rivers whose headwaters rise in the Arctic (e.g., Colville, Babbage), however, have much lower yields than Arctic rivers whose headwaters are in lower latitudes (e.g., Copper, Yukon, MacKenzie). The reason is not clear, but it may be related to lower levels of precipitation and shorter periods during which the rivers can transport sediment (Milliman and Syvitski, unpublished data).

DISCHARGE OF SEDIMENT BY WORLD RIVERS

North America

Most rivers draining eastern North America are upland, lowland, or coastal plain rivers, with correspondingly low sediment loads. Much of the sediment leaving the contiguous United States and Canada comes from three large rivers—the Mississippi, MacKenzie, and Colorado (now dammed)—and smaller west coast rivers (e.g., Eel, Columbia, Fraser), most of which drain mountains. Large discharges of sediment also come from rivers draining western Canada and Alaska; the Susitna, Cooper, and Stekine rivers, for example, collectively drain an area <4% that of the Mississippi, but discharge nearly a third as much sediment (Table 1, Milliman and Syvitski, 1992); the many other rivers along this coast also must contribute large amounts of sediment: the average thickness of Holocene sediment on the southeast Alaskan shelf is 55 m (Molnia et al., 1978) and fjords into which many of these rivers discharge have Quaternary sediment thicknesses >500 m (Syvitski et al., 1987).

South America

Eastern South America is drained by four major rivers (Magdalena, Orinoco, Amazon, Parana) all having their headwaters in the Andes Mountains. Collectively they drain more than half the continent (10 of 17 million km2). In contrast, rivers draining the western Andes are less known, but collectively their sediment discharge may be of the same magnitude as the larger rivers draining eastward

FIGURE 5.3 Variation of runoff versus basin area for rivers within four topographic categories. Note the decreasing maximum runoff values with increased river basin area and with lower elevations.



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