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FIGURE 4.1 Evolution of stream water chemistry in the Landes forest (southwest France) as a function of the average distance of the watersheds from the ocean. The ocean and continental (forest origin?) aerosols that mask the chemical weathering of quartz and feldspars sands are evident from the exponential decrease of Na+ and Cl-.
Temperature influence on weathering is best evidenced for silica and for total cations (TZ+) originating from crystalline rocks (Figure 4.2). This positive relation is probably due to the combined influence of feldspars dissolution kinetics and of higher bacterial activity in soil of the tropical belt. For carbonate rocks, an inverse relationship between temperature, Ca2+ and HCO3- has been observed (Harman et al., 1975).
Contact between water and rock (water residence time, nature or contact) is an important factor. As a result, phreatic groundwaters are usually one to three times more mineralized than surface waters depending on the element considered (Table 4.1B).
Geomorphologic features control both TSS and dissolved organic carbon (DOC). In similar climatic types with identical dominant vegetation, those river basins with a greater proportion of wetlands (peat bogs, swamps, etc.) have much higher DOC levels. In the well-drained Mamai basin, New Zealand, the average DOC is 4.5 mg/l compared to 43.5 mg/l for the proximate Larry basin draining a peat bog (Moore, 1987). Although much less documented, the geomorphologic influence on nitrogen and phosphorous levels and transport rates by rivers is likely. Relief pattern, particularly the slope distribution, is a key factor in mechanical erosion: all mountain ranges are drained by rivers with high TSS levels (Milliman and Meade, 1983).
Vegetation influence on river chemistry is still poorly known, except for the well-known nutrient increase in waters after a forest fire. Concerning TSS, the vegetal cover is one of the four major controlling factors in mechanical erosion, along with bedrock, slopes, and rainfall pattern: unprotected bare soils are sensitive to rainsplash impact, and developed root system lower mass wasting and subsequent erosion.
Past geological history in a river basin is seldom considered although post glacial features may affect chemical and physical weathering in many ways. Glacial deposits in those mountain previously glaciated (Alps, Rockies, Himalayas) are still an important source, if not the major one, of river particulates. The Quaternary eolian loess deposits are a peculiar case: they form the major TSS source of the Huang He River, the most turbid river (TSS average > 20 g/l, about 6 percent of the present global TSS input to oceans). Past geological events may also limit chemical weathering: (1) when former glacial abrasion has left only bare rock, as in Canadian and Scandinavian Shields, the chemical weathering is limited; (2) in lowland regions exposed to erosion for millions of years, as is common in Africa, the resulting soil layer extends over meters or more and is highly depleted in soluble elements and the subsequent chemical erosion is limited. When these important soil layers have developed, the river TSS is generally low and enriched in the less soluble elements (Al, Fe, Ti, Mn), whereas Ca, Mg, and Na are strongly depleted; Si and K are generally similar to parent rock. The average TSS composition of active mountain ranges is close to the average shale (Table 4.2B). The Huang He TSS has a loess composition.