Page 61 - Fundamentals of Geomorphology

44 INTRODUCING LANDFORMS AND LANDSCAPES

a
Table 2.4 Average composition of river waters by continents (mg/l)
Continent SiO 2 Ca 2+ Mg 2+ Na + K + Cl – SO 4 2– HCO 3 – i b

Africa 12.0 5.25 2.15 3.8 1.4 3.35 3.15 26.7 45.8
North America 7.2 20.1 4.9 6.45 1.5 7.0 14.9 71.4 126.3
South America 10.3 6.3 1.4 3.3 1.0 4.1 3.5 24.4 44.0
Asia 11.0 16.6 4.3 6.6 1.55 7.6 9.7 66.2 112.5
Europe 6.8 24.2 5.2 3.15 1.05 4.65 15.1 80.1 133.5
Oceania 16.3 15.0 3.8 7.0 1.05 5.9 6.5 65.1 104.5
World 10.4 13.4 3.35 5.15 1.3 5.75 8.25 52.0 89.2
Notes:
a The concentrations are exoreic runoff with human inputs deducted
b i is the sum of the other materials
Source: Adapted from Meybeck (1979)

and sodium, which are determined primarily by river runoff (itself related to climatic factors) and then on
evaporation and fractional crystallization and which lithology.
are exemplified by the Rio Grande and Pecos
rivers.
Regional and global patterns of
denudation
This classification has been the subject of much debate
(see Berner and Berner 1987, 197–205), but it seems Enormous variations in sediment and solute loads of
undeniable that climate does have a role in determining rivers occur within particular regions owing to the local
the composition of river water, a fact borne out by the effects of rock type, vegetation cover, and so forth.
origin of solutes entering the oceans. Chemical erosion is Attempts to account for regional variations of denuda-
greatest in mountainous regions of humid temperate and tion have met with more success than attempts to explain
tropical zones. Consequently, most of the dissolved ionic global patterns, largely because coverage of measuring
load going into the oceans originates from mountainous stations is better and it is easier to take factors other than
areas, while 74 per cent of silica comes from the tropical climate into consideration. Positive correlations between
zone alone. suspended sediment yields and mean annual rainfall and
Further work has clarified the association between mean annual runoff have been established for drainage
chemical weathering, mechanical weathering, lithology, basins in all parts of the world, and simply demon-
and climate (Meybeck 1987). Chemical transport, mea- strate the fact that the more water that enters the system,
sured as the sum of major ions plus dissolved silica, the greater the erosivity. Solute loads, like suspended
increases with increasing specific runoff, but the load sediment loads, exhibit striking local variations about
for a given runoff depends on underlying rock type the global trend. The effects of rock type in particu-
(Figure 2.5). Individual solutes show a similar pat- lar become far more pronounced in smaller regions. For
tern. Dissolved silica is interesting because, though the example, dissolved loads in Great Britain range from 10
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rate of increase with increasing specific discharge is to more than 200 t/km /yr, and the national pattern is
roughly the same in all climates, the actual amount influenced far more by lithology than by the amount
of dissolved silica increases with increasing tempera- of annual runoff (Walling and Webb 1986). Very high
ture (Figure 2.5b). This situation suggests that, although solute loads are associated with outcrops of soluble rocks.
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lithology, distance to the ocean, and climate all affect An exceedingly high solute load of 6,000 t/km /yr has
solute concentration in rivers, transport rates, especially been recorded in the River Cana, which drains an area of
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in the major rivers, depend first and foremost on specific halite deposits in Amazonia; and a load of 750 t/km /yr