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predictable, more localized, and more variable than in
humid-temperate streams'.
This greater variability in the morphological adjust-
ments of desert rivers is corroborated by Laronne and
Shulker (2002), who looked at several towns in the north-
ern Negev and found cases of spectacular channel en-
largement but also narrowing and a reduction in capacity.
Unsurprisingly, the geomorphological effects of urbani-
sation are continuous, synergistic and occur concurrently
with natural variability. A study of the Salt River in the
Phoenix metropolitan area of central Arizona found that
channel changes over more than 60 years were driven pri-
marily by both large-scale regional flood events and local
human activities (Graf, 2000). General changes in sinu-
osity of the low-flow channel were attributable to floods,
although islands were found to have remained remark-
ably consistent in location and size, while channel-side
bars waxed and waned. These changes occurred at the
same time as extensive sand and gravel mining, which
was the most important determinant of local channel form.
These mining operations moved with the expanding ur-
ban fringe, which serves as a market for sand and gravel
during construction.
the reduced plant cover brought about by heavy grazing.
Channels were found to be less braided, a higher pro-
portion of the river width consisted of unvegetated runoff
channels and the river system was generally dominated
by very sandy soil. Interestingly, although plant cover in-
creased along a section of the river system after removal
of livestock for a period of nearly three years, no changes
in river morphology were apparent during this time.
22.5
Cause and effect: the arroyo
debate continues
The object of this chapter has been to review instances
where human activities have effected geomorphological
change, whether intentional or otherwise. There are many
examples throughout the growing literature on environ-
mental change in general, both contemporary and in the
past, and the role played by people. The links between
cause and effect are more obvious in some cases than
in others. A landscape can be a complex place and the
relationship between cause(s) and effect(s) is often com-
plicated by feedbacks, timelags, thresholds and synergies.
On occasion, competing explanations for geomorpho-
logical phenomena persist for prolonged periods. Devel-
opments in our understanding of arroyo trenching in the
US southwest provides a good example. Deep incision
by valley-bottom gullies, or arroyos, occurred in many
valleys and plains in the southwestern USA over a fairly
short period between 1865 and 1915, a phenomenon that
had a detrimental impact on settlement and economic ac-
tivities in the area (Cooke and Reeves, 1976). Attributing
causation for arroyo trenching in this part of the world
has been the subject of lengthy debate and review (Elliott,
Gellis and Aby, 1999).
The arrival of European settlers in the region coincides
with arroyo trenching and a range of human activities
that could have triggered incision has been suggested, in-
cluding heavy grazing, logging, compaction along well-
travelled routes and the cutting of grass for hay in val-
ley bottoms. Conversely, a number of climatic drivers of
change have also been put forward, and an essentially
natural cause is supported by studies of valley fills that in-
dicate repeated phases of aggradation and incision, some
of which pre-date any possible significant human effects
(Waters and Haynes, 2001). Shifts in climate towards
more arid conditions, and indeed towards more humid
conditions, have both been proposed. Another climatic
explanation suggests that an increase in the frequency
of heavy rainfall events would have enhanced the power
of gullying. A third perspective (Schumm, Harvey and
22.4.3
Changes in vegetation
Any human activity that results in alterations to the veg-
etation found in an area is also likely to produce indirect
changes in local geomorphology. People disturb, modify
or clear the vegetation in a particular region for all sorts
of reasons (e.g. fuelwood collection, cultivation, herding,
construction, mining) and in many different ways (e.g.
fire, selective cutting, clear-felling). Rivers can be affected
by vegetation changes immediately adjacent to the chan-
nel itself, but also by more general changes in land use in
a catchment.
The establishment of saltcedar (Tamarix spp.) along
many rivers in the southwestern USA has caused sig-
nificant aggradation of floodplains. Saltcedar stands are
dense and have extensive root systems, leading to trap-
ping of sediment and bank stabilisation. On the Brazos
River in Texas, for instance, the plants encourage deposi-
tion on sandbars and at the edge of channels, where they
become established. Between 1941 and 1979 the width of
the channel was reduced in this way by nearly 90 metres as
sediment some 3 metres thick was deposited (Blackburn,
Knight and Schuster, 1982).
The effects of more general vegetation change, in this
case due to long-term grazing pressure, have been studied
by Allsopp
et al
. (2007) on an ephemeral river system
in Namaqualand, South Africa. Both the physical and
