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When referring to riverine material, many sedimentologists, geochemists, and even hydrobiologists consider global or continental averages. This tendency is now reinforced by the increasing use of present-day global river inputs to oceans to model past geochemical cycles (Berner et al., 1983; Wilkinson and Walker, 1989). Actually, global averages mask the extended chemical diversity of river water and particulates as well as the wide range of river transport rates.
The purpose of this chapter is (1) to assess the variability of river chemistry, (2) to review the main environmental factors that control these variations, and (3) to estimate the contributions to the global river loads from various geographic, geologic, and climatic environments. Because of the amount of information available, the focus is put on major elements, nutrients, and organic carbon. The anthropogenic influence is not considered here, [i.e., river data have been screened so that most polluted rivers have been discarded and only predamming data on total suspended solids (TSS) have been retained]. Extended compilations of river chemistry started in the 1960s (Durum et al., 1960; Livingstone, 1963; Turekian, 1969). More recent basic data have already been published or previously referenced (Meybeck, 1979, 1982, 1983, 1986, 1987, 1988; Kempe, 1982; SCOPE CARBON, 1982, 1983, 1985, 1987). In addition to these, some regional studies on river water chemistry in unpolluted or less polluted environments have been used: the Mackenzie river watershed (Reeder et al., 1972), Japan rivers in 1943 to 1957 (Kobayashi, 1960), Thailand rivers in 1956/1957 (Kobayashi, 1959), and the entire Amazon River basin (Stallard, 1980).
ENVIRONMENTAL FACTORS CONTROLLING CHEMISTRY OF WATER AND SUSPENDED MATTER
River water chemistry is controlled by many environmental factors, most of them known for a long time (Erikson, 1960; Gorham, 1961; Drever, 1982; Stallard and Edmond, 1981, 1983, 1987; Meybeck, 1984, 1986; Berner and Berner, 1987). They can be presented in three major groups: sources (lithosphere, atmosphere, biosphere), sinks (vegetation uptake, settling), and rate-controlling factors (temperature, water circulation). In addition, the river basin size plays a major part through the integration of diverse environments.
Lithology is a key factor for most major dissolved elements (Si, Ca, Mg, Na, Cl, S, C). Carefully selected, pristine monolithologic watersheds are characterized by distinctly different water quality compositions (Table 4.1A),
TABLE 4.1 Variability of Dissolved Major Elements in Pristine Stream Waters Draining Various Rock Types