Climate change in arid and tropical humid regions
661
In a dry stage the river decreases its discharge, land cover gets reduced and solid
transportation rises. The change toward wetter conditions produces an increment in the
discharge, width, depth, and in the amplitude of the meanders. The result represents a
complete transformation of the fluvial system. An example of fluvial system abandonment
and replacement by another system occurs in Oman, in western Arenas Wahiba (Maizels,
1987). There it is possible to recognize a five paleochannels system consisting of cemented
gravels, which is part of an alluvial fan, (Figure 25.17). The erosion of the fan produces
lowering, emphasized by aeolian deflation, leaving an exhumed landscape in the shape of a
cord, which sticks out about 30 m above the plain. Older channels can correspond to
humid stages in the early Pliocene-Pleistocene, whereas the exhumation is related to
semiarid stages during the Pleistocene, corresponding to lower sea levels.
In arid environments overlaps and interferences between aeolian and fluvial domains are
common. Thus, southwestern Egypt close to the frontier with Sudan and Algeria, is
constituted by the large sand layer of Selima. That is a hyperarid zone, in which the study of
radar images has shown that there is a system of big fluvial channels overlain by the sand
layer, having an age that probably corresponds to the early Miocene (McCauley et al., 1982).
The most classic and complex example of interference between alternating aeolian and
fluvial stages occurred in the middle course of the fiver Niger in Mali (Figure 25.18).
Fluvial accumulations took place in the humid period of the early Quaternary and totally
stopped during the subsequent stage of highest aridity, producing longitudinal dunes, or
trade winds ones (ENE-SSW), in which watercourses disappeared. Later, in a period
wetter than the present one, they crossed dune fields and filled the existing enclosed
depressions, producing the expansion of lakes, as demonstrated by the high quantity of
ceramic remains from the Neolithic. Nowadays, the discharge is enough to make the Niger
have an exorheic character, although present precipitation is lower than Neolithic and
much higher than the one existing during the stage of dune formation (Tricart, 1969;
1979). In the section about dune systems this aspect will be discussed in more detail
(Figure 25.18).
1.2. Paleolakes
The study of the shorelines of ancient lakes, together with the analysis of the sediments
deposited, provide very valuable data on the hydrologic modifications that have occurred.
Many desert lakes present clear geomorphological and stratigraphic evidence of having
had, during the late Quaternary in times with a higher effective humidity (Mabbut, 1977),
dimensions much larger than the present ones.
Shoreline mapping of lakes requires a detailed photogeomorphologic interpretation and
meticulous field work (Sack, 1994). Measures taken at different points around the basin of
the lake for a given shoreline normally present different heights. This can be due to coastal
geomorphologic processes and in deep lakes to hydroisostatic rebounds (Gilbert, 1890).
With these data, it is possible to determine the area and depth of a paleolake for different
periods. The existence of subbasins, separated by thresholds, as in Lake Bonneville
(Gilbert, 1890), controls the levels of the paleolake (Benson, 1978; Benson and Paillet,
1989) and makes their study difficult, because of the complex dynamics of each subbasin.
Another difficulty is related to the paleohydrologic variations that occurred within the
661
In a dry stage the river decreases its discharge, land cover gets reduced and solid
transportation rises. The change toward wetter conditions produces an increment in the
discharge, width, depth, and in the amplitude of the meanders. The result represents a
complete transformation of the fluvial system. An example of fluvial system abandonment
and replacement by another system occurs in Oman, in western Arenas Wahiba (Maizels,
1987). There it is possible to recognize a five paleochannels system consisting of cemented
gravels, which is part of an alluvial fan, (Figure 25.17). The erosion of the fan produces
lowering, emphasized by aeolian deflation, leaving an exhumed landscape in the shape of a
cord, which sticks out about 30 m above the plain. Older channels can correspond to
humid stages in the early Pliocene-Pleistocene, whereas the exhumation is related to
semiarid stages during the Pleistocene, corresponding to lower sea levels.
In arid environments overlaps and interferences between aeolian and fluvial domains are
common. Thus, southwestern Egypt close to the frontier with Sudan and Algeria, is
constituted by the large sand layer of Selima. That is a hyperarid zone, in which the study of
radar images has shown that there is a system of big fluvial channels overlain by the sand
layer, having an age that probably corresponds to the early Miocene (McCauley et al., 1982).
The most classic and complex example of interference between alternating aeolian and
fluvial stages occurred in the middle course of the fiver Niger in Mali (Figure 25.18).
Fluvial accumulations took place in the humid period of the early Quaternary and totally
stopped during the subsequent stage of highest aridity, producing longitudinal dunes, or
trade winds ones (ENE-SSW), in which watercourses disappeared. Later, in a period
wetter than the present one, they crossed dune fields and filled the existing enclosed
depressions, producing the expansion of lakes, as demonstrated by the high quantity of
ceramic remains from the Neolithic. Nowadays, the discharge is enough to make the Niger
have an exorheic character, although present precipitation is lower than Neolithic and
much higher than the one existing during the stage of dune formation (Tricart, 1969;
1979). In the section about dune systems this aspect will be discussed in more detail
(Figure 25.18).
1.2. Paleolakes
The study of the shorelines of ancient lakes, together with the analysis of the sediments
deposited, provide very valuable data on the hydrologic modifications that have occurred.
Many desert lakes present clear geomorphological and stratigraphic evidence of having
had, during the late Quaternary in times with a higher effective humidity (Mabbut, 1977),
dimensions much larger than the present ones.
Shoreline mapping of lakes requires a detailed photogeomorphologic interpretation and
meticulous field work (Sack, 1994). Measures taken at different points around the basin of
the lake for a given shoreline normally present different heights. This can be due to coastal
geomorphologic processes and in deep lakes to hydroisostatic rebounds (Gilbert, 1890).
With these data, it is possible to determine the area and depth of a paleolake for different
periods. The existence of subbasins, separated by thresholds, as in Lake Bonneville
(Gilbert, 1890), controls the levels of the paleolake (Benson, 1978; Benson and Paillet,
1989) and makes their study difficult, because of the complex dynamics of each subbasin.
Another difficulty is related to the paleohydrologic variations that occurred within the