. "Geomorphic/Tectonic Control of Sediment Discharge to the Ocean: The Importance of Small Mountainous Rivers." Material Fluxes on the Surface of the Earth. Washington, DC: The National Academies Press, 1994.
The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
by Milliman and Meade. Similar percentage increases might hold for southeastern Alaska, western South America, the southern Alps-Caucasus orogen, and NW Africa (e.g., Walling, 1985).
There is another way to calculate the flux: The rivers (listed in Table 1, Milliman and Syvitski, 1992) are greater than 10,000 km2 in drainage basin area, and collectively they discharge (before dam construction) slightly more than 8 bt of sediment annually. River basins <10,000 km2 account for slightly >20% of the total drainage area to the ocean (20 x 106 km2; Figure 5.1). Assuming that the mean drainage basin area of these rivers is 1000 km2, an additional 20,000 rivers would be required to account for the entire 20 x 106 km2. If we assume that 10% of these rivers (i.e., 2000) are mountainous and that of these half drain high mountains and or Asia/Oceania and the other half drain mountains exclusive of the Arctic and non-alpine European, the combined loads of these rivers would be (8 mt/river/yr x 1000 rivers) + (1.5 mt/river/yr x 1000 rivers) (see Figure 5.5), or a total of 9.5 bt/yr. This number is surprisingly close to our estimate for the rivers (mostly small) draining Oceania, but since it does not include southern Asia or western North and South America, our calculation may be too conservative. Although the yields for similar-sized upland and lowland rivers are significantly lower (900 and 90 t/km2/yr, respectively), there are more of them, and the combined small upland and lowland rivers might contribute another 1-2 bt annually. Adding undocumented rivers larger than 10,000 km2 probably would add another 1-2 bt. The combined total suspended discharge conservatively might be 20 bt.
A regional example of the influence of small mountainous rivers in sediment discharge can be seen in southern Europe. Milliman and Meade (1983) pointed out that the rivers draining south from the Alps have much higher yields than those rivers draining northern Europe. Assuming a yield of 120 t/km2/yr and a combined drainage of 0.55 x 106 km2, Milliman and Meade calculated that the southern rivers discharge 66 mt/yr to the Mediterranean Sea. In fact, the sediment loads of southern alpine rivers are much greater: the 24 mountainous rivers listed in Table 1 (Milliman and Syvitski, 1992) drain only 0.22 x 106 km2, but collectively they discharge more than 140 mt of sediment annually. If the values are similar for the remainder of the combined drainage area, total sediment discharge would be 350 mt/yr, five times the value calculated by Milliman and Meade.
Unfortunately, calculating world-wide discharge is more complicated, because not all sediment carried by large rivers reaches the sea: some is stored along the lower reaches of rivers and adjoining deltas. If subsidence rates in the Bengal Delta are 1-2 cm/yr (cf. Milliman et al., 1987; J.R. Curray, oral communication, 1991), for example, 40-80% of the sediment load carried by the Ganges/ Brahmaputra may be sequestered in the subaerial portion of the delta, perhaps explaining the relative lack of Holocene sediment accumulating on the adjacent shelf (Kuehl et al., 1989) and the lack of net progradation of the delta front (Alam, 1987). As a result, it is entirely possible that the present sediment discharge of large rivers has been overestimated.
Because rivers are being dammed at an increasing rate, many of the numbers given in this paper are probably out of date. Pearce (1991) states that 13% of all fluvial discharge is presently dammed. Ironically, with their high sediment yields and therefore (at least relatively) high sediment loads, Asian rivers can fill their dammed reservoirs quickly thereby shortening the lives of these dams more quickly than calculated by the engineers who designed them. But since pre-dam sediment loads for most rivers were artificially high due to human activities in the drainage basins, dam construction, for example on the southeastern US rivers, probably has offset anthropogenically enhanced erosion, and post-dam discharges may not be too different from those prior to European colonization (Meade and Parker, 1985).
Even if the present global flux of river sediment could be calculated, the significance of such a number to either future or past river discharge is questionable. Mid-twentieth century river discharge (to the sea) may have been about 20 bt/yr, nearly half of this amount coming from oceania and another third from southern Asia. But because sediment loads may have increased by a factor of 2-10 since humans began farming (see Saunders and Young, 1983; Berner and Berner, 1987), the annual sediment discharge 2000-2500 yr ago may have been considerably < bt. Extensive human influence in Oceania and southern Asia suggests that sediment loads in this area are disproportionately elevated.
Active Versus Passive Margin Rivers
All rivers with large sediment loads originate in mountains. Most large rivers discharge to the sea along passive continental margins, and they act as point-sources for sediment influx; as a result, large deltas (e.g., Mississippi, Nile, Amazon, Ganges, Indus, Yangtze) form on passive margins or in marginal seas (Audley-Charles et al., 1977; Inman and Nordstrom, 1971; Potter, 1978).
In contrast, rivers that drain mountainous islands and the active edges of continental margins (e.g., western North and South America) or collision margins (southern Europe, southern Asia) are generally much smaller, but collectively they may transport similar amounts of sediment