9.3 Mechanical Denudation
9.3.2 Modern Denudation Rates
The River-Load Method and Its Limits
A eommon method for determining modem rates of
denudation (and soi1 erosion) is the measurement of
the sediment diseharge of rivers. As will be diseussed further be1ow, the easiest way to do this is the
measurement of the suspended river load, whieh normaHy aecounts for the majority of the total solid
load.
Measurements of the suspended river loads are usually
accomplished by taking water sampies within the crosssection of a river and analysing their contents for total suspended solids, TSS, expressed as concentrations (e.g. in
g/m 3 ). With the aid of river discharge-weighted concentrations, annual freights of TSS (tla) are found. Taking into
ac count the river drainage areas, these values can be converted into specific transport rates or specific sediment
yields, SY, (tlkm 2 /a), or mean mechanical denudation rates;
DRne, for the average rock density in the study area (cf.
Fig.9.5).
Data gained by this method must be viewed with eaution for severa1 reasons: (1) These measurements
often do not inc1ude rare events, for examp1e eatastrophie floods, whieh may oceur onee in 10 or 100
years and earry more suspended and bed10ad than a
10ng period of normal river discharge. Therefore,
average va1ues based on measurements of a few
years are often not representative of 10nger periods.
(2) It is on1y the suspended river load whieh ean be
easi1y determined; the bed10ad (midd1e to eoarsegrained sand and grave1), which is almost exc1usive1y
transported during peak flows, is diffieu1t to measure.
The best way to determine bedload transport is to observe
delta prograding in lakes and artificial reservoirs. Small
and middle-sized rivers leaving mountain ranges may carry
bedloads of 50% or more of the total solids. However, the
bedload of major rivers having passed wide valleys and
floodplains makes up only a few percent of the total solid
load.
(3) Many of these measurements are strong1y affeeted by human aetivities (e.g., deforestation, agricu1ture, construetion of dams and roads, etc.) and
therefore yie1d denudation rates mueh higher than
those of prehistorie times.
Present-day loads are often higher by a factor of 2 to 5 for
land of mixed use in a humid temperate climate, and by a
factor of 5 to 10 in plain regions in subtropical to tropical
climates (Dedkov and Mozzherin 1996), but locally up to a
factor of 100 and more! (e.g., Toy 1982; Douglas 1996;
Walling and Webb 1996; Einseie and Hinderer 1997). Because this text focuses on sediment sources of ancient sedimentary basins, this problem is not discussed further here.
435
On the other hand, modern solid loads of middle and
lower river courses can also be too low compared with natural conditions. Artificial dams and reservoirs upstream
may trap large portions of the sediment provided by the
upper parts of the drainage areas. The sediment load arriving in the Ebro delta, Spain, for example, has been drastically reduced during the past hundred years (Guillen and
Palanques 1997).
Moreover, a considerable number of rivers draining previously glaciated areas pass ice-formed lakes, where they
deposit most of their solid load. Such sediment traps are
short-lived and did not exist during long periods of the
Earth's history. Hence, many measurements in rivers represent present-day sediment delivery rates into the oceans
and not denudation rates in the source areas (Leeder 1991).
(4) Data coHected at gauging stations upstream of a
river mouth do not necessari1y record tlte actua1 input
of sediment into the eorresponding depositiona1 basin
because of storage or uptake of sediment downstream
of the measuring point.
A striking example of the first case is the Yellow River in
China which deposits as much as 75% of its suspended
load before it reaches the open sea (Milliman et al. 1987).
In the middle reaches ofthe Amazon River in Brazil, large
amounts of sediment are exchanged between the river and
its flood plain (Dunne et al. 1998). In aperiod of >10
years, deposition on the river bars and flood plain exceeded
bank erosion by 500 x 10 6 tla at irregular time intervals. A
specific investigation of flood-plain sedimentation along
the Brahmaputra River, using 137C as tracer, has shown that
about 40-70% of the solid river load is trapped landward of
the river mouth (Allison et al. 1998).
The best modern values for TSS and SY transported by
rivers can be obtained from the sediment masses trapped in
artificial reservoirs (Fig. 9. 13b). The sediment volume is
determined by bathymetric surveys and can be converted
into units of mass using porosity values of undisturbed
sampies. This method renders mean values for time periods
of tens of years which also include river bedloads. However, the drainage areas of these sediment traps, mostly 10cated in mountainous areas, are often of limited size. A
number of values from modern reservoirs are little affected
by human activities in their drainage areas and therefore
can also be used for long-term sediment mass balances
(Sects .. 11.2 and 11.3).
(5) The suspended loads of rivers inc1ude various
amounts of particu1ate organic matter which does not
come from eroded rocks. As an average, about 10%
of the suspended material consist of organic matter
(Ludwig et al. 1996; also see Sect. 10.3).
9.3.2 Modern Denudation Rates
The River-Load Method and Its Limits
A eommon method for determining modem rates of
denudation (and soi1 erosion) is the measurement of
the sediment diseharge of rivers. As will be diseussed further be1ow, the easiest way to do this is the
measurement of the suspended river load, whieh normaHy aecounts for the majority of the total solid
load.
Measurements of the suspended river loads are usually
accomplished by taking water sampies within the crosssection of a river and analysing their contents for total suspended solids, TSS, expressed as concentrations (e.g. in
g/m 3 ). With the aid of river discharge-weighted concentrations, annual freights of TSS (tla) are found. Taking into
ac count the river drainage areas, these values can be converted into specific transport rates or specific sediment
yields, SY, (tlkm 2 /a), or mean mechanical denudation rates;
DRne, for the average rock density in the study area (cf.
Fig.9.5).
Data gained by this method must be viewed with eaution for severa1 reasons: (1) These measurements
often do not inc1ude rare events, for examp1e eatastrophie floods, whieh may oceur onee in 10 or 100
years and earry more suspended and bed10ad than a
10ng period of normal river discharge. Therefore,
average va1ues based on measurements of a few
years are often not representative of 10nger periods.
(2) It is on1y the suspended river load whieh ean be
easi1y determined; the bed10ad (midd1e to eoarsegrained sand and grave1), which is almost exc1usive1y
transported during peak flows, is diffieu1t to measure.
The best way to determine bedload transport is to observe
delta prograding in lakes and artificial reservoirs. Small
and middle-sized rivers leaving mountain ranges may carry
bedloads of 50% or more of the total solids. However, the
bedload of major rivers having passed wide valleys and
floodplains makes up only a few percent of the total solid
load.
(3) Many of these measurements are strong1y affeeted by human aetivities (e.g., deforestation, agricu1ture, construetion of dams and roads, etc.) and
therefore yie1d denudation rates mueh higher than
those of prehistorie times.
Present-day loads are often higher by a factor of 2 to 5 for
land of mixed use in a humid temperate climate, and by a
factor of 5 to 10 in plain regions in subtropical to tropical
climates (Dedkov and Mozzherin 1996), but locally up to a
factor of 100 and more! (e.g., Toy 1982; Douglas 1996;
Walling and Webb 1996; Einseie and Hinderer 1997). Because this text focuses on sediment sources of ancient sedimentary basins, this problem is not discussed further here.
435
On the other hand, modern solid loads of middle and
lower river courses can also be too low compared with natural conditions. Artificial dams and reservoirs upstream
may trap large portions of the sediment provided by the
upper parts of the drainage areas. The sediment load arriving in the Ebro delta, Spain, for example, has been drastically reduced during the past hundred years (Guillen and
Palanques 1997).
Moreover, a considerable number of rivers draining previously glaciated areas pass ice-formed lakes, where they
deposit most of their solid load. Such sediment traps are
short-lived and did not exist during long periods of the
Earth's history. Hence, many measurements in rivers represent present-day sediment delivery rates into the oceans
and not denudation rates in the source areas (Leeder 1991).
(4) Data coHected at gauging stations upstream of a
river mouth do not necessari1y record tlte actua1 input
of sediment into the eorresponding depositiona1 basin
because of storage or uptake of sediment downstream
of the measuring point.
A striking example of the first case is the Yellow River in
China which deposits as much as 75% of its suspended
load before it reaches the open sea (Milliman et al. 1987).
In the middle reaches ofthe Amazon River in Brazil, large
amounts of sediment are exchanged between the river and
its flood plain (Dunne et al. 1998). In aperiod of >10
years, deposition on the river bars and flood plain exceeded
bank erosion by 500 x 10 6 tla at irregular time intervals. A
specific investigation of flood-plain sedimentation along
the Brahmaputra River, using 137C as tracer, has shown that
about 40-70% of the solid river load is trapped landward of
the river mouth (Allison et al. 1998).
The best modern values for TSS and SY transported by
rivers can be obtained from the sediment masses trapped in
artificial reservoirs (Fig. 9. 13b). The sediment volume is
determined by bathymetric surveys and can be converted
into units of mass using porosity values of undisturbed
sampies. This method renders mean values for time periods
of tens of years which also include river bedloads. However, the drainage areas of these sediment traps, mostly 10cated in mountainous areas, are often of limited size. A
number of values from modern reservoirs are little affected
by human activities in their drainage areas and therefore
can also be used for long-term sediment mass balances
(Sects .. 11.2 and 11.3).
(5) The suspended loads of rivers inc1ude various
amounts of particu1ate organic matter which does not
come from eroded rocks. As an average, about 10%
of the suspended material consist of organic matter
(Ludwig et al. 1996; also see Sect. 10.3).