Suspended load in rivers

Fig. 9-8 Histogram of dissolved solids of samples from the Orinoco and Amazon River basins and corresponding denudation rates for morpho-tectonic regions in the humid tropics of South America (Stal-lard, 1985). The approximate denudation scale is calculated as the product of dissolved solids concentrations, mean armual runoff (1 m/yr), and a correction factor to account for large ratios of suspended load in rivers that drain mountain belts and for the greater than average annual precipitation in the lowlands close to the equator. The correction factor was treated as a linear function of dissolved solids and ranged from 2 for the most dilute rivers (dissolved solids less than lOmg/L) to 4 for the most concentrated rivers (dissolved solids more than 1000 mg/L). Bedrock density is assumed to be 2.65 g/cm. (Reproduced with permission from R. F. Stallard (1988). Weathering and erosion in the humid tropics. In A. Lerman and M. Meybeck, Physical and Chemical Weathering in Geochemical Cycles," pp. 225-246, Kluwer Academic Publishers, Dordrecht, The Netherlands.)...

Direct evidence supporting the occurrence of reverse weathering has proven difficult to obtain for two reasons. First, the same kinds of clay minerals produced by this process are also transported to the ocean as part of the suspended load in river runoff. Second, the rate of reverse weathering is so slow that laboratory studies of this process are difficult to conduct. 

River transport of clay minerals into the ocean is spatially and temporally variable. The global annual suspended load of river sediment into coastal waters currently averages 12.6 X 10 ton. This flux is approximately 10% less than was delivered before humans began damming rivers. (One notable exception is the Mississippi River, whose sediment load has increased due to very high rates of soil erosion. The riverine sediments deposited in the mouth of the Mississippi River form one of the world s largest deltas.)... 

It can be seen in Table 9.7 that the particulate load constitutes by far the most important contribution (88%) of total river discharge of materials to the ocean. The amount carried as solids should be increased by bed load transport, which usually is considered to be about 10% of the total suspended load (Blatt et al 1980). The mean chemical composition of river suspended matter closely approximates that of average shale (Table 9.8). This resemblance is expected because suspended solids in rivers are derived mainly from shales. Sedimentary rocks constitute about 66% of the rocks exposed at the Earth s surface fine-grained rocks, like shales, comprise at least 65% of the sedimentary rock mass. Thus, roughly 50% of surface erosion products come from shaly rocks. 

Sampling suspended materials in rivers and lakes is particularly problematic. Not only does the suspended load tend to vary in space and time, but also it is necessary to take care not to introduce systematic error when taking the sample. For example, sampling devices which suck or pump water from depth along pipes can result in substantial loss of suspended materials by sedimentation. It is better therefore to have a sample bottle which may be both opened and then closed at the appropriate depth. 

Increased erosion due to forest clear-cutting and widespread cultivation has increased riverine suspended matter concentrations, and thus increased the riverine particulate-P flux. Dams, in contrast, decrease sediment loads in rivers and therefore diminish the phosphorus flux to the oceans. However, increased erosion below dams... 

Marine waters also receive some biogenic silica from the land. This material is transported to the sea as windblown dust and as part of the suspended load of rivers. Rivers also deliver about 0.43 Pg of dissolved silica annually to the oceans, and some fraction of this is undoubtedly derived from biological sources as well. Locally, terrigenous biogenic silica (in particulate form) may accumulate to significant concentrations on the sea floor. For example, Kolbe (1957) reported frequent occurrences of phytoliths and freshwater diatom frustules in deep-sea cores from the equatorial Atlantic, and one locahty contained diatom tests derived exclusively from freshwater species. 

Most of the PAHs in surface waters are believed to result from atmospheric deposition (Santodonato et al. 1981). However, for any given body of water, the major source of PAHs could vary. Jensen (1984) studied benzo[a]pyrene loading in a marine coastal area and determined that atmospheric deposition was indeed the major source of benzo[a]pyrene, with lesser amounts contributed by refinery effluent, municipal waste water, urban runoff, and rivers. Prahl et al. (1984) found that combustion-derived PAHs adsorbed to suspended sediments in rivers accounted for the major portion of PAHs in the waters of a Washington coastal area, and other studies have identified industrial effluents, road runoff, and oil spills as the major contributors in specific bodies of water (DeLeon et al. 1986 Santodonato et al. 1981). 

Andean rivers often acquire an intense red color only after crossing regions with red beds. Furthermore, the suspended load of rivers in the... 

In fact, the concentration of suspended solids in rivers and lake waters is often closely correlated with the percentage of land in the catchment that is devoted to agriculture. The suspended load per km in Asian rivers is 3 to 8 times the world... 

Rivers transport suspended sediments derived from the disintegration of basin surface layers. With reduced velocity, sediment is deposited in the river channel. The finest material is carried to the sea. It has been estimated that the average mechanical denudation rate for continents is 0.056 mm year (35). This is based on a total suspended load of 13.5 x 10 metric tons year (S). Presently, about two-thirds of the world s total suspended sediment load derives from Southern Asia and large Pacific Islands. Berner has estimated the increase in sediment loss in the U.S. and world since prehuman times to be approximately 200% (35). Current estimated erosion rate from the major land forms is provided in Table I. The relatively recent construction of large sediment trapping dams that normally caused sediment to be deposited in river valleys or transported to the ocean has drastically reduced sediment yields in great rivers. 

The material transported by rivers consists of dissolved ions (dissolved load), sediment suspended in the flow (suspended load), and sediment transported along the bed of the river (bedload). The total load and the proportion of the load represented by these phases varies widely among rivers in different environments. In particular, climate, topography, and erosion influence the amount and composition of riverine sediment loads. 

The physical transport of particles in a river occurs by two primary modes bedload and suspended load. Bedload consists of material moved along the bed of the river by the tractive force exerted by flowing water. Bedload may roll or hop along the bottom, and individual particles may remain stationary for long periods of time between episodes of movement. Suspended load consists of material suspended within the flow and that is consequently advected by flowing water. Rivers and streams are naturally turbulent, and if the upward component of turbulence is sufficient to overcome the settling velocity of a particle, then it will tend to remain in suspension because the particles become resuspended before they can settle to the bottom of the flow. Suspended load consists of the finest particles transported by a river, and in general is composed of clay- and silt-sized... 

The travel time for suspended load is controlled by the flow velocity and the distance to the basin outlet. Flow velocities do not change much downstream in a typical river system (Leopold, 1953) and typically range from 0.1 to several m/s. Hence, suspended load should be able to travel at least 10 to 100 km per day and the travel time for suspended sediment to traverse even the longest rivers in the world should be less than a season. Although some of the suspended load will be deposited in floodplains, the component of the suspended load that does not get sequestered in terrestrial depositional environments is delivered almost as fast as the water that it flows in. Bedload travels much more slowly. In mountain drainage basins, the velocity of individual bedload clasts is on the... 

The presence of unstable and cation-rich minerals in the suspended load and bed material of rivers that drain the Andes indicates that... 

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