INTRODUCTION

Estimating the flux and fate of fluvial sediments discharged to the ocean has proved to be difficult, as rivers for which we have at least some data account for only about two-thirds of the land area draining into the ocean. Small rivers (drainage basins <<10,000 km 2) drain only about 20% of the land area, but they number in the many thousands (Figure 5.1) and, as will be seen in this paper, collectively they may contribute much more sediment than previously estimated. Previous attempts (e.g., Holeman, 1968; Milliman and Meade, 1983) assumed that global sediment flux could be calculated by extrapolating the yield of large and medium-sized rivers over large regions. By failing to take into account adequately smaller rivers, however, this assumption led to mistaken conclusions regarding seaward flux of fluvial sediment.

To predict the sediment load of a small river, we need to understand the interaction of numerous factors, including climate, precipitation (both average and peak), discharge (volume and velocity), basin geology, human impact, and the size of the drainage basin. Many workers have tried relating sediment load (or yield-load normalized for basin area) to net and/or gross precipitation, with varying results (see review by Walling and Webb, 1983). For small basins in the western United States, Langbein and Schumm (1958) showed that yields are high with low precipitation (where vegetation is too sparse to retard the erosive capacity of heavy rain and runoff), decrease in areas of medium precipitation, and then increase with higher levels of precipitation. A better relationship was seen between the annual variability of rainfall and sediment transport (Douglas, 1967), with basin relief also having an effect (Fournier, 1960). Other workers, however, have noted a variety of sediment transport trends relative to precipitation (e.g., Ahnert, 1970), leading Walling and Webb (1983, p. 84) to conclude that, "Current evidence concerning the relationship between climate and sediment yield emphasizes that no simple relationship exists."

In this paper we explore fluvial sediment discharge with respect to basin area and basin elevation. Both of these factors have been analyzed previously, but separately. For example, Ruxton and McDougall (1967) found that denudation rates in the Hydrographers Range (Papua New Guinea) are directly related to local relief. Pinet and Souriau (1988) found that the solid load of a river correlated well with mean basin elevation but not with environmental factors (such as rainfall). Potter (1978), Inman and Nordstrom (1971), and Audley-Charles et al. (1977, 1979) showed that large rivers (and their deltas) drain orogenic belts, but mostly discharge into intracratonic basins and trailing edge margins (see Dickinson, 1988, for a detailed review). These latter papers seem to have been overlooked by most geologists and oceanographers.

An inverse relationship between sediment yield and drainage basin area also has been noted (e.g., Schumm and Hadley, 1961), and Wilson (1973) suggested that sediment yield depends mainly on land use and basin area (not precipitation). Milliman and Meade (1983) reported that sediment yield increases by about seven-fold for every order of magnitude decrease in drainage basin area, but this correlation considered only rivers with sediment loads >15 million tons (mt)/yr, thereby excluding rivers with smaller sediment loads.

FIGURE 5.1 Cumulative drainage basin area of the world's 400 largest rivers with decreasing basin size. Data from Unesco (1978), this paper, and various IAHS publications. The largest river basin (Amazon) accounts for about 6 x 106 km2 of the 90 x 106 km2 land area (estimated by Milliman and Meade, 1983) draining into the oceans; the next nine largest rivers drain an additional 32 x 106 km2 of land. (Numbers by the dots indicate the drainage basin area for that particular river.) The remaining 20 x 106 km2 of the land surface are probably drained more than 10,000 small rivers.



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