654
Climatic Geomorphology
Figure 25.13. Aerial view of a dendritic drainage network consisting of flat floored valleys excavated in
Miocene gypsum and marls. Central part of the Ebro Depression of Spain.
incision. In the centre of the Ebro Depression in Spain, an intricate dendritic network of
flat valleys that appears incised, showing several levels with sediments from the Holocene,
has developed over Miocene marly gypsum material (Figure 25.13), (Pefia et al., 1993;
Arauzo and Guti~rrez, 1994). In the ephemeral courses, it is also possible to recognize
stages with a shortage of sedimentation that are recorded in incipient soil profiles.
Therefore, these ephemeral courses are characterized by lengthy periods of sedimentation,
alternating with a brief hiatus with shallow incised channels and the development of
entisols (Leopold, 1994). The aggradation processes described above are followed by a
stage of degradation that corresponds to the incision of the rambla that starts with a rapid
incision followed by an exponential decrease, due to the approximation of a new baselevel of erosion (Bull, 1997). For southern Arizona, Cooke and Reeves (1976) proposed a
model on the different causes of the beginning of the arroyo incision in which many
variables interact. Land-use changes, random variations in the frequency and magnitude of
the processes and the secular climate changes are the principals. The first and the last seem
to be the principal causes of the initiation of the arroyos. Nevertheless, the origin is a
classic example of equifinality or convergence of landforms, in which it is very difficult to
identify a unique mechanism (Cooke and Reeves, 1976).
In the analysis of the origin of trenching in arroyos it is very complicated to distinguish
between anthropic actions and secular or short-term climate changes. Another way to deal
with the problem is based on the fact that there is an increase of the erosion produced by
waters and a decrease in the resistance to erosion in the sediments of the arroyo. In any
case, vegetation seems to be the key to the initiation and development of the arroyos.
Climatic Geomorphology
Figure 25.13. Aerial view of a dendritic drainage network consisting of flat floored valleys excavated in
Miocene gypsum and marls. Central part of the Ebro Depression of Spain.
incision. In the centre of the Ebro Depression in Spain, an intricate dendritic network of
flat valleys that appears incised, showing several levels with sediments from the Holocene,
has developed over Miocene marly gypsum material (Figure 25.13), (Pefia et al., 1993;
Arauzo and Guti~rrez, 1994). In the ephemeral courses, it is also possible to recognize
stages with a shortage of sedimentation that are recorded in incipient soil profiles.
Therefore, these ephemeral courses are characterized by lengthy periods of sedimentation,
alternating with a brief hiatus with shallow incised channels and the development of
entisols (Leopold, 1994). The aggradation processes described above are followed by a
stage of degradation that corresponds to the incision of the rambla that starts with a rapid
incision followed by an exponential decrease, due to the approximation of a new baselevel of erosion (Bull, 1997). For southern Arizona, Cooke and Reeves (1976) proposed a
model on the different causes of the beginning of the arroyo incision in which many
variables interact. Land-use changes, random variations in the frequency and magnitude of
the processes and the secular climate changes are the principals. The first and the last seem
to be the principal causes of the initiation of the arroyos. Nevertheless, the origin is a
classic example of equifinality or convergence of landforms, in which it is very difficult to
identify a unique mechanism (Cooke and Reeves, 1976).
In the analysis of the origin of trenching in arroyos it is very complicated to distinguish
between anthropic actions and secular or short-term climate changes. Another way to deal
with the problem is based on the fact that there is an increase of the erosion produced by
waters and a decrease in the resistance to erosion in the sediments of the arroyo. In any
case, vegetation seems to be the key to the initiation and development of the arroyos.