Climate change in arid and tropical humid regions
659
1.1.5. Desert rivers
In order to deduce environmental changes, the modifications in the form of the channel and
in the type of the deposit are analysed. During historic periods many examples of human
intervention are known, which trigger important variations in morphology, as well as in
fluvial sediments (see Chapter 19). Furthermore, neotectonic activity is another factor that
can influence the modification and intensity of the fluvial processes in some areas. Due to
this, geomorphological works are carried out in cratonic areas where there is no tectonic
activity, and so the "noise" produced by such activity can be avoided. Rivers are relatively
insensitive to climate changes if these are not very marked (Reid, 1994). Rivers in arid and
wet regions, however, present important differences, however, not only in form but also in
the transported solid charge (Wolman and Gerson, 1978). In arid regions few hydrologic
records are available, or do not exist, so the recent variations and their relations with
climate are not well known. Some researchers, such as Schumm and Lichty (1963), have
studied temporal variations of the channel parameters to deduce climate changes. One
difficulty in drainage basins is the response of the variables of the fluvial system to climate
changes (Schumm and Lichty, 1965). These variables are interrelated and climate affects
mainly the weathering, pedogenesis, vegetation and hydrology (Schumm, 1977).
Furthermore, the variables susceptible to change can be more active in one basin than
in another.
The modifications that the fluvial system can undergo may be the result of changes in
the average characteristics of the climate and also of the modifications in the common
values of magnitude-frequency of the different events (Knighton, 1998). Similarly, small
climate changes have produced modifications in fluvial systems (Knox, 1983). The studies
of the floods of the Mississippi River for the last 7000 years reveal that there was a warm
and dry stage between 5000 and 3000 yr BP, with minor flooding, and a abrupt climate
change of a colder and wetter nature that produced important floods (Knox, 1993).
Moreover, this author indicated that the most important floods in the last 9000 years are
due to periodic changes in the atmospheric circulation systems (Knox, 1995).
When a channel increases due to a climate change, the width, depth and the inclination
increases and vice versa. If the sediment load increases, the erosion will rise in the
drainage basin. This increment in sediments can be produced by a climate change toward
more aridity or by deforestation. A fall in the solid load can be due to a climate change
toward wetter conditions or to soil conservation practices. In general, any modification in
the channel is attached to a change in the type of sediment charge. When a change into a
more humid climate takes place, an increment of increase is produced in the percentage of
the vegetal cover and, therefore, there is a decrease in the loss of soils. This increase in
precipitation turns ephemeral courses into perennial ones, the density of drainage grows
and erosion in the tributaries rises, which produces an increase in the solid load, mainly
during storms. If climate returns to semiarid conditions, soil loss and erosion rise,
meanwhile discharge falls. All these variations in discharge and solid load, which in this
case can be a climate change from subhumid to semiarid and vice versa, produce important
hydrologic modifications and changes in the morphology of the channels, termed channel
metamorphosis (Schumm, 1977; Chorley et al., 1984).
Many large drainage basins go through different climatic zones. The Nile fiver rises in
central Africa, where it presents an important discharge and transports a small load
659
1.1.5. Desert rivers
In order to deduce environmental changes, the modifications in the form of the channel and
in the type of the deposit are analysed. During historic periods many examples of human
intervention are known, which trigger important variations in morphology, as well as in
fluvial sediments (see Chapter 19). Furthermore, neotectonic activity is another factor that
can influence the modification and intensity of the fluvial processes in some areas. Due to
this, geomorphological works are carried out in cratonic areas where there is no tectonic
activity, and so the "noise" produced by such activity can be avoided. Rivers are relatively
insensitive to climate changes if these are not very marked (Reid, 1994). Rivers in arid and
wet regions, however, present important differences, however, not only in form but also in
the transported solid charge (Wolman and Gerson, 1978). In arid regions few hydrologic
records are available, or do not exist, so the recent variations and their relations with
climate are not well known. Some researchers, such as Schumm and Lichty (1963), have
studied temporal variations of the channel parameters to deduce climate changes. One
difficulty in drainage basins is the response of the variables of the fluvial system to climate
changes (Schumm and Lichty, 1965). These variables are interrelated and climate affects
mainly the weathering, pedogenesis, vegetation and hydrology (Schumm, 1977).
Furthermore, the variables susceptible to change can be more active in one basin than
in another.
The modifications that the fluvial system can undergo may be the result of changes in
the average characteristics of the climate and also of the modifications in the common
values of magnitude-frequency of the different events (Knighton, 1998). Similarly, small
climate changes have produced modifications in fluvial systems (Knox, 1983). The studies
of the floods of the Mississippi River for the last 7000 years reveal that there was a warm
and dry stage between 5000 and 3000 yr BP, with minor flooding, and a abrupt climate
change of a colder and wetter nature that produced important floods (Knox, 1993).
Moreover, this author indicated that the most important floods in the last 9000 years are
due to periodic changes in the atmospheric circulation systems (Knox, 1995).
When a channel increases due to a climate change, the width, depth and the inclination
increases and vice versa. If the sediment load increases, the erosion will rise in the
drainage basin. This increment in sediments can be produced by a climate change toward
more aridity or by deforestation. A fall in the solid load can be due to a climate change
toward wetter conditions or to soil conservation practices. In general, any modification in
the channel is attached to a change in the type of sediment charge. When a change into a
more humid climate takes place, an increment of increase is produced in the percentage of
the vegetal cover and, therefore, there is a decrease in the loss of soils. This increase in
precipitation turns ephemeral courses into perennial ones, the density of drainage grows
and erosion in the tributaries rises, which produces an increase in the solid load, mainly
during storms. If climate returns to semiarid conditions, soil loss and erosion rise,
meanwhile discharge falls. All these variations in discharge and solid load, which in this
case can be a climate change from subhumid to semiarid and vice versa, produce important
hydrologic modifications and changes in the morphology of the channels, termed channel
metamorphosis (Schumm, 1977; Chorley et al., 1984).
Many large drainage basins go through different climatic zones. The Nile fiver rises in
central Africa, where it presents an important discharge and transports a small load