Geology Reference
In-Depth Information
Table 2.4 Average composition of river waters by continents
a
(mg/l)
Ca
2+
Mg
2+
Na
+
K
+
Cl
-
SO
4
2-
HCO
3
-
i
b
Continent
SiO
2
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
evaporation and fractional crystallization and which
are
river runoff (itself related to climatic factors) and then on
lithology.
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
undeniable that climate does have a role in determining
the composition of river water, a fact borne out by the
origin of solutes entering the oceans. Chemical erosion is
greatest in mountainous regions of humid temperate and
tropical zones. Consequently, most of the dissolved ionic
load going into the oceans originates from mountainous
areas, while 74 per cent of silica comes from the tropical
zone alone.
Further work has clarified the association between
chemical weathering, mechanical weathering, lithology,
and climate (Meybeck 1987). Chemical transport, mea-
sured as the sum of major ions plus dissolved silica,
increases with increasing specific runoff, but the load
for a given runoff depends on underlying rock type
(Figure 2.5). Individual solutes show a similar pat-
tern. Dissolved silica is interesting because, though the
rate of increase with increasing specific discharge is
roughly the same in all climates, the actual amount
of dissolved silica increases with increasing tempera-
ture (Figure 2.5b). This situation suggests that, although
lithology, distance to the ocean, and climate all affect
solute concentration in rivers, transport rates, especially
in the major rivers, depend first and foremost on specific
Enormous variations in sediment and solute loads of
rivers occur within particular regions owing to the local
effects of rock type, vegetation cover, and so forth.
Attempts to account for regional variations of denuda-
tion have met with more success than attempts to explain
global patterns, largely because coverage of measuring
stations is better and it is easier to take factors other than
climate into consideration. Positive correlations between
suspended sediment yields and mean annual rainfall and
mean annual runoff have been established for drainage
basins in all parts of the world, and simply demon-
strate the fact that the more water that enters the system,
the greater the erosivity. Solute loads, like suspended
sediment loads, exhibit striking local variations about
the global trend. The effects of
rock type
in particu-
lar become far more pronounced in smaller regions. For
example, dissolved loads in Great Britain range from 10
to more than 200 t/km
2
/yr, and the national pattern is
influenced far more by lithology than by the amount
of annual runoff (Walling and Webb 1986). Very high
solute loads are associated with outcrops of soluble rocks.
An exceedingly high solute load of 6,000 t/km
2
/yr has
been recorded in the River Cana, which drains an area of
halite deposits in Amazonia; and a load of 750 t/km
2
/yr
