Geoscience Reference
In-Depth Information
Pawing, rolling and trampling in roughly circular depres-
sions significantly increases the bulk density of soil in
wallows relative to the adjacent landscape and the com-
paction reduces infiltration, so that wallows become lo-
cal ponds that can retain water for several days after a
rainstorm. The number of bison wallows in North Amer-
ica prior to European contact has been put at more than
100 million and each displaced up to 23 m
3
of sediment.
This example can be contrasted with the impacts of the
introduced European rabbit (
Oryctolagus cuniculus
L.) in
Australia. Feral rabbits have had widespread effects across
the semi-arid landscapes of the continent, directly through
burrowing and trampling but also indirectly via profound
alterations to the vegetation of Australian rangelands. The
effects on microtopography, infiltration, sediment move-
ment and hydrology are largely unmeasured.
or bamboo. Li
et al
. (2003) report that this approach has
been used successfully along the Baotou-Lanzhou rail-
way in the Tengger Desert since the late 1950s. The straw
checkerboards, which remain intact for 4 or 5 years, aid
the establishment of xerophytic shrubs by increasing the
roughness of the sand surface by 400-600 times and re-
ducing wind velocity by 20-40 % at a height of 0.5 m
above the surface. The stabilised sand surface also pro-
vides good conditions for colonisation by annual plants
and the formation of biogenic soil crusts.
22.4
Human impacts on rivers
Rivers have played an important roll in moulding the ge-
ography of human activities in all biomes, but their sig-
nificance in deserts has been particular, affecting not only
the location of societies but also their political, economic
and social development (Wittfogel, 1958; Butzer, 1976).
The diversity of causes of change to the geomorphology of
river systems is great and people have undoubtedly played
a role in many cases. Societies have deliberately manip-
ulated many river channels in arid areas, particularly to
assist with flood control and as an aid to irrigation and
navigation. Examples of such intentional changes include
dams, channelisation, water extraction and river diversion,
but there are also many examples of human action being
responsible for the modification of river channel geometry
unintentionally, and in numerous ways.
22.3
Human impacts on sand dunes
There are numerous examples of human activities in dry-
lands modifying the forms of sand dunes and the processes
operating on them. Dunes that support some vegetation,
particularly common in semi-arid desert areas, frequently
attract both farmers and herders, but these systems can be
easily tipped towards degradation of both vegetation and
dune soils. Any reduction in vegetation cover - by factors
such as grazing, cultivation and burning, but also due to
natural factors such as drought - can increase the poten-
tial for sediment transport and surface change. The in-
teraction with drought was noted by Kumar and Bhandari
(1993) in their examination of semi-arid sites in northwest
Rajasthan, India. This study identified a positive feedback
of vegetation destruction, increased dune mobility and ex-
acerbated surface instability as pressure from human and
livestock populations has intensified.
Increasing human activities in drylands has also meant a
rise in the number of instances in which mobile dunes be-
come hazardous to economic interests. Dunes can affect
transport networks, agricultural productivity, residential
areas and water supplies. In the early years of the Trans-
Caspian railway, each train carried a special team of work-
ers to clear the tracks of drifting sand, a problem that led
directly to the creation in 1912 of the Desert Research
Station at Repetek in Turkmenistan (Babaev, 1999). Re-
search at Repetek and elsewhere has spawned a variety of
techniques to stabilise sand dunes, ranging from shelter-
belts, barriers or fences to surface treatments with chem-
icals. The approach favoured in many Chinese deserts
where dunes threaten economic activities is to plant veg-
etation on them using straw checkerboards initially pro-
22.4.1
Large dams
Given the importance to society of a reliable water supply
it is unsurprising that people have been building dams
to manage water resources for up to 5000 years. Two of
the oldest were built in desert areas to control flooding
and to supply water for irrigation and domestic purposes:
the Jawa Dam in Jordan and the Sadd al-Kafara in Egypt
were both built on wadis some 5000 and 4600 years ago,
respectively. However, the Sadd al-Kafara was destroyed
by floods before it was ever used.
Flood control and water supply remain important rea-
sons for dam construction today, along with hydroelectric-
ity generation and the regulation of river flow. Structures
above 15 m in height above their foundations are defined
as large by the International Commission on Large Dams
and major if they exceed 150 m. By the end of the twentieth
century, there were more than 45 000 large dams in over
140 countries and approximately another 750 000 smaller
dams globally (World Commission on Dams, 2000). What
