Climate change in arid and tropical humid regions
657
Leopold (1994) carried out a synthesis of the climate history following the last
Wisconsinan ice retreat. The synthesis was based on much research carried out on the
alluvial deposits of southwestern USA. A period of aggradation can be recognized from
the last glaciation to the Altithermal Phase or Climate Optimum, some 4000 to 6000 years
ago. During dry conditions in this time period intense erosion was triggered, affecting
valley fills, which produced terraces up to 10 m above the channel. Later, another
aggradation period developed, which finished about 1200 to 1400 yr AD (Medieval Warm
Period or Secondary Climate Optimum). In this dry period an incision of the fluvial
deposits previously deposited occurred again. During the wet and cold stage
corresponding to the Little Ice Age (1450 to 1850 AD) aggradation occurred yet again.
From 1880 to 1920 there was a change toward aridity that produced erosion in the valleys.
This general proposal is based on existing data, but is not universal. This sequence of
aggradation and erosion stages can be different for tributary headwaters than for principal
channels (Balling and Wells, 1990). Furthermore, it is possible that a synchronism exists
among degradation stages within a vast region, because each fluvial system has a different
sensitivity to climate perturbations (Bull, 1997). This author calculated that a 100 years of
time were needed for the trenching, but at least 500 years were necessary to complete the
aggradation in incised channels.
In conclusion, it is too simplistic to ascribe channel trenching to only a single factor, as,
for instance, an increase in temperature. The changes in the density of vegetation of slopes
or riversides play an important role in the incision initiation. Vegetation can be modified
by changes in precipitation, temperature, overgrazing, fires, and other human activities.
Incision begins when these variables create an imbalance within the system, which can
trigger and maintain the process of trenching. Some explanations such as climate change,
however, have a regional significance, whereas others can only be locally referenced.
There are defenders and detractors of each hypothesis regarding the complicated problems
about rambla and arroyos, but it is necessary to take into account that different changes in
an area can produce similar geomorphic results.
1.1.4. Alluvial fans
These are semiconical forms that are produced when water flows and confined channel
sediments reach the mountain foot, where there is a reduction in the stream capacity,
absence of confinement, and sedimentation of the charge, mainly by the larger ephemeral
rivers. Although they are located in many climate environments (Rachocki, 1981), here
only those actually located in arid regions are considered in order to deduce the climate
changes that have occurred.
The study of alluvial fans has progressed a lot through the application of
sedimentological techniques (Mayer et al., 1984; Grossman and Gerson, 1987),
the analysis of palaeosoils (Bull, 1991), charcoal dating and the rock varnish method
(Dorn, 1994a). From that, it is possible to identify the climatic features from deposits
and palaeosols, as well as from correlations within the fan and in the region.
Thus, the objective is to establish the influence of the climate on the genesis of alluvial
fans.
For some authors, the fundamental stages of aggradation in alluvial fans occur in wet
periods (Glennie, 1970; Dorn et al., 1987; Maizels, 1987), whereas others think that
657
Leopold (1994) carried out a synthesis of the climate history following the last
Wisconsinan ice retreat. The synthesis was based on much research carried out on the
alluvial deposits of southwestern USA. A period of aggradation can be recognized from
the last glaciation to the Altithermal Phase or Climate Optimum, some 4000 to 6000 years
ago. During dry conditions in this time period intense erosion was triggered, affecting
valley fills, which produced terraces up to 10 m above the channel. Later, another
aggradation period developed, which finished about 1200 to 1400 yr AD (Medieval Warm
Period or Secondary Climate Optimum). In this dry period an incision of the fluvial
deposits previously deposited occurred again. During the wet and cold stage
corresponding to the Little Ice Age (1450 to 1850 AD) aggradation occurred yet again.
From 1880 to 1920 there was a change toward aridity that produced erosion in the valleys.
This general proposal is based on existing data, but is not universal. This sequence of
aggradation and erosion stages can be different for tributary headwaters than for principal
channels (Balling and Wells, 1990). Furthermore, it is possible that a synchronism exists
among degradation stages within a vast region, because each fluvial system has a different
sensitivity to climate perturbations (Bull, 1997). This author calculated that a 100 years of
time were needed for the trenching, but at least 500 years were necessary to complete the
aggradation in incised channels.
In conclusion, it is too simplistic to ascribe channel trenching to only a single factor, as,
for instance, an increase in temperature. The changes in the density of vegetation of slopes
or riversides play an important role in the incision initiation. Vegetation can be modified
by changes in precipitation, temperature, overgrazing, fires, and other human activities.
Incision begins when these variables create an imbalance within the system, which can
trigger and maintain the process of trenching. Some explanations such as climate change,
however, have a regional significance, whereas others can only be locally referenced.
There are defenders and detractors of each hypothesis regarding the complicated problems
about rambla and arroyos, but it is necessary to take into account that different changes in
an area can produce similar geomorphic results.
1.1.4. Alluvial fans
These are semiconical forms that are produced when water flows and confined channel
sediments reach the mountain foot, where there is a reduction in the stream capacity,
absence of confinement, and sedimentation of the charge, mainly by the larger ephemeral
rivers. Although they are located in many climate environments (Rachocki, 1981), here
only those actually located in arid regions are considered in order to deduce the climate
changes that have occurred.
The study of alluvial fans has progressed a lot through the application of
sedimentological techniques (Mayer et al., 1984; Grossman and Gerson, 1987),
the analysis of palaeosoils (Bull, 1991), charcoal dating and the rock varnish method
(Dorn, 1994a). From that, it is possible to identify the climatic features from deposits
and palaeosols, as well as from correlations within the fan and in the region.
Thus, the objective is to establish the influence of the climate on the genesis of alluvial
fans.
For some authors, the fundamental stages of aggradation in alluvial fans occur in wet
periods (Glennie, 1970; Dorn et al., 1987; Maizels, 1987), whereas others think that