658
Climatic Geomorphology
sedimentation increases in drier stages (Wells et al., 1987; Bull, 1991). An intermediate
solution is adopted by other researchers who have indicated that aggradation can
be produced in both climate conditions (Williams, 1973; Ponti, 1985). All this shows
the complexity of the interpretation and the extreme difficulty in carrying out correlations.
Four conceptual models have been proposed, however, to relate climate changes with
alluvial fans (Dorn, 1994b). When a transition to a drier climate takes place, there is
produced a greater denudation of the accumulated deposits in slopes and floodplains in
the upstream basin, previously retained by a greater land cover, creating an aggradation
stage (Knox, 1983; Wells et al., 1987; Harvey, 1990; Bull, 1991; Harvey, 1997). The
paraglacial model is significant due to the great quantity of erodible materials and the
large volumes of water released during melting epochs which affect alluvial fans. In this
way, glaciations produce great quantities of sediments that supply alluvial fans, lasting
for thousands of years after the glacial stage. In the deserts of the western USA and
central Asia, many examples can be recognized in which upstream basins of alluvial fans
have been affected by glaciers. Another alternative, previously described, is the direct
relation between a wet stage and aggradation. Some authors relate the latter with glacial
conditions in which water availability is higher (Bull, 1991). Finally, the periglacial
model implies an important production of gelifracts, which when they are eroded,
generate a stage of aggradation in the alluvial fan (Williams, 1973; Wasson, 1977).
These periglacial processes are very marked in some mountains, in which upstream
basins of alluvial fans develop. In the southeastern part of Spain, as indicated, the main
aggradation periods coincide with the cold phases of the Quaternary and the incision
periods with times of scarce sediments, which correspond to warmer phases (Harvey,
1978, 1990, 1996, 1997).
The construction of an alluvial fan requires a long period of time and, therefore, during
its formation there will be changes in the climatic variables. Despite the diversity of the
models proposed, it is really difficult to differentiate the climate influence from other
"noise," such as the tectonic or the intrinsic factors of the system.
Nevertheless, analysing the paleo-vegetation record of the Mohave Desert, climate
correlations between different areas with alluvial fans have been carried out (Spauldin,
1990). In this type of climate correlation it is necessary to take into account that the areas
analysed usually have highly variable tectonic histories of fairly recent age.
At present, researchers agree that the construction of the alluvial fans in arid regions is
due to storms of high magnitude and low frequency (Beaty, 1974; Baker, 1977; Blair and
McPherson, 1994a,b; Dorn, 1994b; Harvey, 1997), which can have occurred in any
climatic period of its history. Therefore, it is difficult to forecast the effects of the climate
change (Harvey, 1996). As pointed out by this author, threshold conditions and sensitivity
to environmental change (Brunsden and Thormes, 1979) vary from one alluvial fan to
another one. In the semiarid zone of the southeastern part of Spain, during the period
1985 to 1990, a significant increment in the number of storms occurred. The erosion was
limited to the alluvial fan apex, where marked trenches were produced, while eroded
sediment was deposited downstream, without any evidence of new incisions in the middle
and distal zones of the alluvial fans (Harvey, 1996). Therefore, it is very complicated to
reconstruct the evolutionary history or, even ascertain data on the climate changes that can
have affected the alluvial fans (Blair and McPherson, 1994a,b).
Climatic Geomorphology
sedimentation increases in drier stages (Wells et al., 1987; Bull, 1991). An intermediate
solution is adopted by other researchers who have indicated that aggradation can
be produced in both climate conditions (Williams, 1973; Ponti, 1985). All this shows
the complexity of the interpretation and the extreme difficulty in carrying out correlations.
Four conceptual models have been proposed, however, to relate climate changes with
alluvial fans (Dorn, 1994b). When a transition to a drier climate takes place, there is
produced a greater denudation of the accumulated deposits in slopes and floodplains in
the upstream basin, previously retained by a greater land cover, creating an aggradation
stage (Knox, 1983; Wells et al., 1987; Harvey, 1990; Bull, 1991; Harvey, 1997). The
paraglacial model is significant due to the great quantity of erodible materials and the
large volumes of water released during melting epochs which affect alluvial fans. In this
way, glaciations produce great quantities of sediments that supply alluvial fans, lasting
for thousands of years after the glacial stage. In the deserts of the western USA and
central Asia, many examples can be recognized in which upstream basins of alluvial fans
have been affected by glaciers. Another alternative, previously described, is the direct
relation between a wet stage and aggradation. Some authors relate the latter with glacial
conditions in which water availability is higher (Bull, 1991). Finally, the periglacial
model implies an important production of gelifracts, which when they are eroded,
generate a stage of aggradation in the alluvial fan (Williams, 1973; Wasson, 1977).
These periglacial processes are very marked in some mountains, in which upstream
basins of alluvial fans develop. In the southeastern part of Spain, as indicated, the main
aggradation periods coincide with the cold phases of the Quaternary and the incision
periods with times of scarce sediments, which correspond to warmer phases (Harvey,
1978, 1990, 1996, 1997).
The construction of an alluvial fan requires a long period of time and, therefore, during
its formation there will be changes in the climatic variables. Despite the diversity of the
models proposed, it is really difficult to differentiate the climate influence from other
"noise," such as the tectonic or the intrinsic factors of the system.
Nevertheless, analysing the paleo-vegetation record of the Mohave Desert, climate
correlations between different areas with alluvial fans have been carried out (Spauldin,
1990). In this type of climate correlation it is necessary to take into account that the areas
analysed usually have highly variable tectonic histories of fairly recent age.
At present, researchers agree that the construction of the alluvial fans in arid regions is
due to storms of high magnitude and low frequency (Beaty, 1974; Baker, 1977; Blair and
McPherson, 1994a,b; Dorn, 1994b; Harvey, 1997), which can have occurred in any
climatic period of its history. Therefore, it is difficult to forecast the effects of the climate
change (Harvey, 1996). As pointed out by this author, threshold conditions and sensitivity
to environmental change (Brunsden and Thormes, 1979) vary from one alluvial fan to
another one. In the semiarid zone of the southeastern part of Spain, during the period
1985 to 1990, a significant increment in the number of storms occurred. The erosion was
limited to the alluvial fan apex, where marked trenches were produced, while eroded
sediment was deposited downstream, without any evidence of new incisions in the middle
and distal zones of the alluvial fans (Harvey, 1996). Therefore, it is very complicated to
reconstruct the evolutionary history or, even ascertain data on the climate changes that can
have affected the alluvial fans (Blair and McPherson, 1994a,b).