Applied geomorphology and arid regions
477
Perkins, Oklahoma. When it was built, the fiver was straight and braided in the area of the
bridge, but there was a large meander one mile upstream. The position of this meander
drifted, and in 1968 it affected the structure of the bridge (Chorley et al., 1984). Gravel
extraction from fiver beds can also cause headcut erosion and the undermining of bridge
pillars, as occurred on the Salt River in Phoenix, Arizona (Chorley et al., 1984).
Dams are perhaps the clearest form of human interference in fiver systems. They cause
the deposition of sediment in the reservoir while downstream channels become degraded
and rapids can form. This occurred after the construction of the dams on the Colorado
River (Graf, 1980).
The effects of the Aswan dam on the Nile have been truly important. The downstream
delivery of silt has been reduced and the delta has retreated because of a lack of water in its
channels. It is estimated that this retreat might be as much as 30 km in some places by
2100: this would have dire consequences for the area's 1,000,000 plus inhabitants. There
has also been a generalized fall in the water table and an increase in the salinity of nearly
all the delta lands (Mainguet, 1991; Beaumont, 1993).
The River Ebro in northeast Spain also suffered an important fall in its bedload during
the 20th century due to the construction of numerous dams. During the first third of that
century, the sediment load was estimated at 1,000,000 metric t/yr. During the period
1950-1970, however, it had dropped to some 730,000 metric t/yr, and between 1970 and
1990 it was down to 300,000 metric t/yr. For 1990-2000, the mean was below
100,000 metric t/yr. When these results are compared with the transport capacity of the
coastal part of the Ebro Delta (320,000 metric t/yr), a negative sedimentation balance can
be appreciated which, since about 1970, has caused the acceleration of the delta's erosion
(Guill~n et al., 1992). Further, the horizontal and longitudinal instability of its channels -
many of which are found in arid regions - are frequently accompanied by quite serious
vertical instability. This is shown in successive stages of accumulation and incision, which
are reflected in the morphology of the terraces. Human activity in semi-arid areas has also
caused the formation of gullies. Several examples are analysed in the chapter on climatic
change in arid areas.
As a consequence of the spectacular increase in the settlement of arid areas, fiver
systems have suffered important changes. During construction, natural vegetation is
removed and the surface is strongly modified by earth-moving machinery. This increases
soil erosion. When construction is over, soils are impermeable and consequently there is
an increase in runoff during times of rainfall and an increase in the depth of the water
carried in the channels (Leopold, 1968, 1994). Flash floods can also occur.
4.3. Alluvial fans, debris flows and pediments
Alluvial fans, characteristic features of piedmonts in arid and semi-arid areas, are formed
by the discharge of sediments transported by rivers from mountainous areas with gentle
slopes. Their morphology is triangular and coalescence is very frequent.
Commonly, alluvial fans are controlled by tectonic, climatic or vegetation changes that
lead to modifications in the production of sediments and runoff (Harvey, 1997). Deposition
occurs locally when the supply of sediments is greater than the transport capacity, i.e.
when the transport potential of the fan is below that necessary to move the sediment
477
Perkins, Oklahoma. When it was built, the fiver was straight and braided in the area of the
bridge, but there was a large meander one mile upstream. The position of this meander
drifted, and in 1968 it affected the structure of the bridge (Chorley et al., 1984). Gravel
extraction from fiver beds can also cause headcut erosion and the undermining of bridge
pillars, as occurred on the Salt River in Phoenix, Arizona (Chorley et al., 1984).
Dams are perhaps the clearest form of human interference in fiver systems. They cause
the deposition of sediment in the reservoir while downstream channels become degraded
and rapids can form. This occurred after the construction of the dams on the Colorado
River (Graf, 1980).
The effects of the Aswan dam on the Nile have been truly important. The downstream
delivery of silt has been reduced and the delta has retreated because of a lack of water in its
channels. It is estimated that this retreat might be as much as 30 km in some places by
2100: this would have dire consequences for the area's 1,000,000 plus inhabitants. There
has also been a generalized fall in the water table and an increase in the salinity of nearly
all the delta lands (Mainguet, 1991; Beaumont, 1993).
The River Ebro in northeast Spain also suffered an important fall in its bedload during
the 20th century due to the construction of numerous dams. During the first third of that
century, the sediment load was estimated at 1,000,000 metric t/yr. During the period
1950-1970, however, it had dropped to some 730,000 metric t/yr, and between 1970 and
1990 it was down to 300,000 metric t/yr. For 1990-2000, the mean was below
100,000 metric t/yr. When these results are compared with the transport capacity of the
coastal part of the Ebro Delta (320,000 metric t/yr), a negative sedimentation balance can
be appreciated which, since about 1970, has caused the acceleration of the delta's erosion
(Guill~n et al., 1992). Further, the horizontal and longitudinal instability of its channels -
many of which are found in arid regions - are frequently accompanied by quite serious
vertical instability. This is shown in successive stages of accumulation and incision, which
are reflected in the morphology of the terraces. Human activity in semi-arid areas has also
caused the formation of gullies. Several examples are analysed in the chapter on climatic
change in arid areas.
As a consequence of the spectacular increase in the settlement of arid areas, fiver
systems have suffered important changes. During construction, natural vegetation is
removed and the surface is strongly modified by earth-moving machinery. This increases
soil erosion. When construction is over, soils are impermeable and consequently there is
an increase in runoff during times of rainfall and an increase in the depth of the water
carried in the channels (Leopold, 1968, 1994). Flash floods can also occur.
4.3. Alluvial fans, debris flows and pediments
Alluvial fans, characteristic features of piedmonts in arid and semi-arid areas, are formed
by the discharge of sediments transported by rivers from mountainous areas with gentle
slopes. Their morphology is triangular and coalescence is very frequent.
Commonly, alluvial fans are controlled by tectonic, climatic or vegetation changes that
lead to modifications in the production of sediments and runoff (Harvey, 1997). Deposition
occurs locally when the supply of sediments is greater than the transport capacity, i.e.
when the transport potential of the fan is below that necessary to move the sediment