442
product of Q, SP and VP is calculated separately for all
grid points in the basin (not as the product of .basin averages). In addition to these parameters, lithologIcal characteristics of the basins can be introduced into the forrnula. It
appears that the sediment yield of large wet drainage basins
can be weil approxirnated with this method. The forrnula
seems to be less suited to dry climates and small areas with
specific rock types.
All these attempts as well as the discussion in the
previous chapter demonstrate that the complex processes controlling mechanical denudation cannot be
expressed by a simple law. Each region reacts in a
somewhat different way, even if most of the variable
factors are comparable. Changes in climate and vegetation appear to be especially difficult to evaluate.
Cold and moist conditions during glacials, for exampIe, can reduce sediment yield from mountain ranges
if they lead to more widespread vegetation (Leeder et
al. 1998; also see Welt je et al. 1998). For all these
reasons, mechanical denudation rates are more difficult to predict than chemical denudation rates (shown
in Figure 9.10 for different morpho-climatic zones).
Total denudation, Dt = D~e + DReh' can be determined by using one of the above formulae for D~e
and adding a constant value for the regional chem~cal·
denudation D~h which does not depend on the rehef.
9.4
Modern Chemical vs. Mechanical
Denudation Rates
9.4.1 Global Values
On aglobai scale, the contemporary total river flux
of suspended sediment (TSS) to· the oceans is 15 - 20
x 10 9 t/a or 100 to l30 t/km 2 /a (Sect. 9.3.2). This corresponds to a mean global mechanical denudation
rate of -40 to -50 mmlka. Likewise, the mean global
chemical denudation rate is -25 t/km 2 /a or -10
mmIka after correction for atmospheric components
in the dissolved river load (Sect. 9.2.4). Thus, the
ratio of mechanical and chemical denudation,
D~/D~h' is 4 to 5. Globally, mechanical denudation and transport of particulate matter clearly exceeds solute transport.
Distributed evently over the entire ocean basins,
D~e would produce a pore-free clastic sediment
layer at a rate of -20 mmlka and DReh a "carbonatesilica-salt" layer at a rate of -4 mmlka.
9.4.2 Regional Variations and Trends
On a regional scale or for individual drainage basins,
the D~/D~h ratios vary considerably. In the river
basins discussed earlier, the ratios range from -0.3
(Siberian rivers) and 20 to 40 (Ganges, Brahmaputra,
Chapter 9 Denudation and Mass Transport
Fig. 9.12). If smaller catchments are included, the
deviation from the global mean is even greater. As
Walling and Webb (1983) have pointed out, there is
no clear relationship between the particulate (TSS)
and dissolved loads (TDS) of rivers. This may partly
result from the significantly differing transport mechanisms for the two load components. Most of the
particulate load is transported ?y peak .n0ws during
just a few days of the year, whJle the dlssolved load
moves continuously downriver. However, TDS varies seasonally and decreases during storm events due
to dilution. Furthermore, the specific transport rate of
suspended so lids commonly declines from the. upper
to the lower reaches of a fluvial system, whlle the
transport rate of dissolved solids generally does not.
Ifthe data plotted in Figs. 9.l2. 9.l3 and 9.14 are
considered together with information summarized by
Walling and Webb (1986), some ~eneral tren.ds f~r
regions of different climate and rehef can be Identlfied.
- In mountainous regions, due to incomplete soil
cover and high precipitation, both load compon~nts
usually increase as relief increases, but the speclfic
sediment yield or the mechanical denudation rate
increases much faster than the chemical denudation
rate. In the Himalayas the D~/D~h ratio is > 10, but
in the Alps it is only 1 to 2 because of widespread
carbonate rocks. In tropical mountain ranges,
D~jDReh can reach values ranging from 1 0 t~ 30;
the very high values commonly come from relatlvely
small study areas.
- In tropieal areas with low or mixed relief, the
D~/D~h ratios often vary between 5 and 10 (cf.
Fig. 9.l2). Similar ratios have been found for large
river basins of arid to semi-arid regions.
- In areas with temperate to subtropieal climates and
mixed relief the D~/DReh ratio varies between
about 0.5 and 4. Most of the rivers in northwestem
and central Europe (apart from those originating in
the Alps) have ratios <1, i.e., chemical denudation is
more effective thanmechanical denudation. With
increasing annual runoff (and no change in relief)
chemical denudation is enhanced, while mechanical
denudation remains approximately constant or even
declines due to increasing soil protection by dense
vegetation cover.
- The lowest reported D~/D~h ratios are from the
aretie to high-latitude rivers of Siberia which mainly
drain lowlands. With growing relief, this ratio also
increases and exceeds 1 (e.g., Mackenzie River, cf.
Fig. 9.12). For glaciated regions, DR.n/D~h can become very high due to intense mechanical erosion.
product of Q, SP and VP is calculated separately for all
grid points in the basin (not as the product of .basin averages). In addition to these parameters, lithologIcal characteristics of the basins can be introduced into the forrnula. It
appears that the sediment yield of large wet drainage basins
can be weil approxirnated with this method. The forrnula
seems to be less suited to dry climates and small areas with
specific rock types.
All these attempts as well as the discussion in the
previous chapter demonstrate that the complex processes controlling mechanical denudation cannot be
expressed by a simple law. Each region reacts in a
somewhat different way, even if most of the variable
factors are comparable. Changes in climate and vegetation appear to be especially difficult to evaluate.
Cold and moist conditions during glacials, for exampIe, can reduce sediment yield from mountain ranges
if they lead to more widespread vegetation (Leeder et
al. 1998; also see Welt je et al. 1998). For all these
reasons, mechanical denudation rates are more difficult to predict than chemical denudation rates (shown
in Figure 9.10 for different morpho-climatic zones).
Total denudation, Dt = D~e + DReh' can be determined by using one of the above formulae for D~e
and adding a constant value for the regional chem~cal·
denudation D~h which does not depend on the rehef.
9.4
Modern Chemical vs. Mechanical
Denudation Rates
9.4.1 Global Values
On aglobai scale, the contemporary total river flux
of suspended sediment (TSS) to· the oceans is 15 - 20
x 10 9 t/a or 100 to l30 t/km 2 /a (Sect. 9.3.2). This corresponds to a mean global mechanical denudation
rate of -40 to -50 mmlka. Likewise, the mean global
chemical denudation rate is -25 t/km 2 /a or -10
mmIka after correction for atmospheric components
in the dissolved river load (Sect. 9.2.4). Thus, the
ratio of mechanical and chemical denudation,
D~/D~h' is 4 to 5. Globally, mechanical denudation and transport of particulate matter clearly exceeds solute transport.
Distributed evently over the entire ocean basins,
D~e would produce a pore-free clastic sediment
layer at a rate of -20 mmlka and DReh a "carbonatesilica-salt" layer at a rate of -4 mmlka.
9.4.2 Regional Variations and Trends
On a regional scale or for individual drainage basins,
the D~/D~h ratios vary considerably. In the river
basins discussed earlier, the ratios range from -0.3
(Siberian rivers) and 20 to 40 (Ganges, Brahmaputra,
Chapter 9 Denudation and Mass Transport
Fig. 9.12). If smaller catchments are included, the
deviation from the global mean is even greater. As
Walling and Webb (1983) have pointed out, there is
no clear relationship between the particulate (TSS)
and dissolved loads (TDS) of rivers. This may partly
result from the significantly differing transport mechanisms for the two load components. Most of the
particulate load is transported ?y peak .n0ws during
just a few days of the year, whJle the dlssolved load
moves continuously downriver. However, TDS varies seasonally and decreases during storm events due
to dilution. Furthermore, the specific transport rate of
suspended so lids commonly declines from the. upper
to the lower reaches of a fluvial system, whlle the
transport rate of dissolved solids generally does not.
Ifthe data plotted in Figs. 9.l2. 9.l3 and 9.14 are
considered together with information summarized by
Walling and Webb (1986), some ~eneral tren.ds f~r
regions of different climate and rehef can be Identlfied.
- In mountainous regions, due to incomplete soil
cover and high precipitation, both load compon~nts
usually increase as relief increases, but the speclfic
sediment yield or the mechanical denudation rate
increases much faster than the chemical denudation
rate. In the Himalayas the D~/D~h ratio is > 10, but
in the Alps it is only 1 to 2 because of widespread
carbonate rocks. In tropical mountain ranges,
D~jDReh can reach values ranging from 1 0 t~ 30;
the very high values commonly come from relatlvely
small study areas.
- In tropieal areas with low or mixed relief, the
D~/D~h ratios often vary between 5 and 10 (cf.
Fig. 9.l2). Similar ratios have been found for large
river basins of arid to semi-arid regions.
- In areas with temperate to subtropieal climates and
mixed relief the D~/DReh ratio varies between
about 0.5 and 4. Most of the rivers in northwestem
and central Europe (apart from those originating in
the Alps) have ratios <1, i.e., chemical denudation is
more effective thanmechanical denudation. With
increasing annual runoff (and no change in relief)
chemical denudation is enhanced, while mechanical
denudation remains approximately constant or even
declines due to increasing soil protection by dense
vegetation cover.
- The lowest reported D~/D~h ratios are from the
aretie to high-latitude rivers of Siberia which mainly
drain lowlands. With growing relief, this ratio also
increases and exceeds 1 (e.g., Mackenzie River, cf.
Fig. 9.12). For glaciated regions, DR.n/D~h can become very high due to intense mechanical erosion.