9.3 Mechanical Denudation
From a mass balance of glacial and glaciofluvial sediments left behind by the fonner Laurentide continental ice
sheet, the mean rate of glacial erosion in Canada was 40-70
mmlka during the past 3 Ma (Bell and Laine 1985). The
volumes of glacio-marine sediments in the Orphan Basin
(northeast of Newfoundland) and deep-sea fans in the
northern North Atlantic point to glacial denudation rates
ranging from 100 mrnIka (in the Holocene, Svalbard) to
130 to 400 mrnIka in the Pleistocene (Newfoundland,
Svalbard and the Barents Sea, Hiscott and Aksu 1996;
Elverooi et al. 1995). Modern ice f10w records and glacial
sediments investigated in some fjords of eastern Greenland
gave a mean Holocene denudation rate of 10 mm/ka (Andrews et al. 1994).
3. Small drainage basins in mountainous areas have
a higher specific sediment yield than larger river systems draining regions of both high imd lower relief
(Fig. 9.13). The denudation rate ofan area 100 times
srnaller may be ab out 10 times higher than that of the
large area. This rule applies to most examples studied, but in humid forested areas with dominating
bank erosion the sediment yield mayaiso increase
with growing drainage area (Walling and Webb
1996).
The upper river Rhine and smaller rivers draining central
parts of the Swiss Alps provide about 1000 t/km 2/a, which
are deposited in peri-Alpine lakes or reservoirs (cf. Fig.
11.4). The transport rates of the rivers Rhone and Danube
measured farther downstream are much lower (110 and 150
tlkm 2 /a, respectively). The Alps in total are thought to produce 150 tlkm 2 /a. The major rivers draining the Himalayas
have average sediment yields from about 100 tlkm 2/a
(Indus river) to 600 and 900 t/km 2/a (Ganges and
Brahrnaputra rivers, respectively), but their drainage areas
also include lowlands.
4. Lowland rivers of the humid temperate zone show
denudation rates of 3-5 mmlka and deliver sediment
on the order of 10 tikm 2 /a. With increasing portions
of hills and mountainous regions in their drainage
areas, DR.ne may reach values of 20 to 40 mm1ka
(Figs. 9.12 and 9.13). Rivers draining arctic or highlatitude regions. of mostly low relief have low specific sediment yields, SY, of approximately 5 to 10
t/km 2 /a.
The Siberian rivers Ob, Yenisei, Lena, and many rivers of
the Canadian Northern Territories and northwestern Europe belong to this category. If areas of high relief make up
a large proportion of the drainage area, the sediment yield
may increase to about 50 tlkm 2 /a (e.g., Mackenzie and Columbia rivers).
Fig. 9.13. a Specific sediment yields sr versus drainage areas (overview, logarithmic scale!) of small
and large river "basins" all over the world. (After
Milliman and Syvitski 1992). Note the wide ran~e of
SY values mainly controlled by relief. The lInear
regression lines do not show the large scattering of
439
5. To a large extent clirnate controls soil formation
and vegetation. Dense, permanent vegetation cover
prevents or strongly reduces soil erosion. Many studies in forested areas in the temperate humid zone
have shown that even on relatively steep slopes
hardly any soil erosion takes place, apart from specific sites where gravity mass movements, rill and
gully erosion occur. In these areas erosion is normally restricted mainly to the banks of stream channels.
In contrast, in regions of sparse or missing vegetation, erosion also operates on the surface of slopes.
Thus, even little but irregular annual rainfall, can
deliver comparatively large amounts of soil and rock
debris to river systems. In addition, wind erosion
plays some role. Therefore, denudation rates and sediment yields from limited mountainous areas in arid
to semi-arid zones are often surprisingly high (some
100 to more than 1000 mmlka, Fig. 9.13b) and may
even approach the same magnitude as values . from
tropical to subtropical regions. Extremely high rates
are typical of badlands which usually develop in
weak, easily erodible rocks.
Larger drainage systems of mixed relief under different climate zones (but mostly arid to semi-arid) are
characterized by low to moderate specific sediment
yields or mean denudation rates (10 to about 100
tikm 2 /a, corresponding to denudation rates of 4 to 40
mmIka).
The rivers Nile, Niger, Murray, and others have SY values
of 10 to 50 tlkm 2 /a. Large areas in southern Europe, central
America, and central China provide suspended solids
within the range of 100 to 500 tlkm 2 /a (Figs. 9.12,9.13 and
9.14).
A special investigation in lowlands has revealed that
the sediment yield significantly decreases and
reaches a minimum value at the transition from
sparse to dense vegetation (e.g. Ohmori 1983) associated with increasing annual precipitation (Fig. 9.13c).
However, if seasonal or annual rainfall surpasses the
lower limit necessary for a dense vegetation cover,
the specific sediment yield and thus mechanical denudation again markedly increase.
The highest sediment yield of rivers (1000 up to
10 000 t/km 2 /a) is reported from relatively small
mountainous areas in the tropics, where annual or
seasonal (monsoon) precipitation is in the order of
2000 to 3000 mm (e.g., Indonesia, Taiwan, New
Guinea, Philippines, Japan, the southem slopes ofthe
Himalayas, Hawaii). In these cases, both rapid
data. b Diagram similar to a, but data come mainly
from artificial dam sites and are plotted for principal
climate zones. Insert indicates general trends. c Influence of annual precipitation and vegetation on SY
from lowlands. (After Ohmori 1983)
From a mass balance of glacial and glaciofluvial sediments left behind by the fonner Laurentide continental ice
sheet, the mean rate of glacial erosion in Canada was 40-70
mmlka during the past 3 Ma (Bell and Laine 1985). The
volumes of glacio-marine sediments in the Orphan Basin
(northeast of Newfoundland) and deep-sea fans in the
northern North Atlantic point to glacial denudation rates
ranging from 100 mrnIka (in the Holocene, Svalbard) to
130 to 400 mrnIka in the Pleistocene (Newfoundland,
Svalbard and the Barents Sea, Hiscott and Aksu 1996;
Elverooi et al. 1995). Modern ice f10w records and glacial
sediments investigated in some fjords of eastern Greenland
gave a mean Holocene denudation rate of 10 mm/ka (Andrews et al. 1994).
3. Small drainage basins in mountainous areas have
a higher specific sediment yield than larger river systems draining regions of both high imd lower relief
(Fig. 9.13). The denudation rate ofan area 100 times
srnaller may be ab out 10 times higher than that of the
large area. This rule applies to most examples studied, but in humid forested areas with dominating
bank erosion the sediment yield mayaiso increase
with growing drainage area (Walling and Webb
1996).
The upper river Rhine and smaller rivers draining central
parts of the Swiss Alps provide about 1000 t/km 2/a, which
are deposited in peri-Alpine lakes or reservoirs (cf. Fig.
11.4). The transport rates of the rivers Rhone and Danube
measured farther downstream are much lower (110 and 150
tlkm 2 /a, respectively). The Alps in total are thought to produce 150 tlkm 2 /a. The major rivers draining the Himalayas
have average sediment yields from about 100 tlkm 2/a
(Indus river) to 600 and 900 t/km 2/a (Ganges and
Brahrnaputra rivers, respectively), but their drainage areas
also include lowlands.
4. Lowland rivers of the humid temperate zone show
denudation rates of 3-5 mmlka and deliver sediment
on the order of 10 tikm 2 /a. With increasing portions
of hills and mountainous regions in their drainage
areas, DR.ne may reach values of 20 to 40 mm1ka
(Figs. 9.12 and 9.13). Rivers draining arctic or highlatitude regions. of mostly low relief have low specific sediment yields, SY, of approximately 5 to 10
t/km 2 /a.
The Siberian rivers Ob, Yenisei, Lena, and many rivers of
the Canadian Northern Territories and northwestern Europe belong to this category. If areas of high relief make up
a large proportion of the drainage area, the sediment yield
may increase to about 50 tlkm 2 /a (e.g., Mackenzie and Columbia rivers).
Fig. 9.13. a Specific sediment yields sr versus drainage areas (overview, logarithmic scale!) of small
and large river "basins" all over the world. (After
Milliman and Syvitski 1992). Note the wide ran~e of
SY values mainly controlled by relief. The lInear
regression lines do not show the large scattering of
439
5. To a large extent clirnate controls soil formation
and vegetation. Dense, permanent vegetation cover
prevents or strongly reduces soil erosion. Many studies in forested areas in the temperate humid zone
have shown that even on relatively steep slopes
hardly any soil erosion takes place, apart from specific sites where gravity mass movements, rill and
gully erosion occur. In these areas erosion is normally restricted mainly to the banks of stream channels.
In contrast, in regions of sparse or missing vegetation, erosion also operates on the surface of slopes.
Thus, even little but irregular annual rainfall, can
deliver comparatively large amounts of soil and rock
debris to river systems. In addition, wind erosion
plays some role. Therefore, denudation rates and sediment yields from limited mountainous areas in arid
to semi-arid zones are often surprisingly high (some
100 to more than 1000 mmlka, Fig. 9.13b) and may
even approach the same magnitude as values . from
tropical to subtropical regions. Extremely high rates
are typical of badlands which usually develop in
weak, easily erodible rocks.
Larger drainage systems of mixed relief under different climate zones (but mostly arid to semi-arid) are
characterized by low to moderate specific sediment
yields or mean denudation rates (10 to about 100
tikm 2 /a, corresponding to denudation rates of 4 to 40
mmIka).
The rivers Nile, Niger, Murray, and others have SY values
of 10 to 50 tlkm 2 /a. Large areas in southern Europe, central
America, and central China provide suspended solids
within the range of 100 to 500 tlkm 2 /a (Figs. 9.12,9.13 and
9.14).
A special investigation in lowlands has revealed that
the sediment yield significantly decreases and
reaches a minimum value at the transition from
sparse to dense vegetation (e.g. Ohmori 1983) associated with increasing annual precipitation (Fig. 9.13c).
However, if seasonal or annual rainfall surpasses the
lower limit necessary for a dense vegetation cover,
the specific sediment yield and thus mechanical denudation again markedly increase.
The highest sediment yield of rivers (1000 up to
10 000 t/km 2 /a) is reported from relatively small
mountainous areas in the tropics, where annual or
seasonal (monsoon) precipitation is in the order of
2000 to 3000 mm (e.g., Indonesia, Taiwan, New
Guinea, Philippines, Japan, the southem slopes ofthe
Himalayas, Hawaii). In these cases, both rapid
data. b Diagram similar to a, but data come mainly
from artificial dam sites and are plotted for principal
climate zones. Insert indicates general trends. c Influence of annual precipitation and vegetation on SY
from lowlands. (After Ohmori 1983)