Geoscience Reference
In-Depth Information
Table 22.3
Some examples of playas where sediments have been mobilised by wind as a result of human activity.
Site
Country
Cause
Effects
Reference
Aral Sea
Kazakhstan/
Uzbekistan
Water diversion
Severe dust storms
Stulina and
Sektimenko (2004)
Sebkha Kourzia
Tunisia
Deforestation
Wind erosion, lunette
formation
Perthuisot (1989)
Coastal sabkhas on
Persian Gulf
Saudi Arabia
Grazing, vegetation loss
Blowing sand, dune
formation
Anton (1982)
Kappakoola Swamp
Australia
Vegetation loss
Sand dune formation
Smith, Twidale and
Bourne (1975)
Owens Lake
USA
Water diversion
Severe dust storms
Gill and Cahill (1992)
Lop Nor
China
Water diversion
Blowing sand and
saline dust storms
Zhao (1986)
Source:
after Gill (1996).
These dramatic changes have had far-reaching effects.
The delta areas of the Amu Darya and Syr Darya Rivers
have been transformed due to the lack of water, affecting
flora, fauna and soils, while the diversion of river water has
also resulted in the widespread lowering of groundwater
levels. The receding sea has had local effects on climate
and the exposed bed of the Aral has become a major re-
gional dust source, from which an estimated 43 million
tonnes of saline material is deposited on surrounding ar-
eas each year. The resulting hazards to local populations,
which include contamination of agricultural land up to
several hundred kilometres from the sea coast and sus-
pected adverse effects on human health, are covered in
detail in Chapter 23.
promotes invasion by desert shrubs, in turn leading to a
further localisation of soil resources under shrub canopies
in a process of positive feedback. In the barren areas be-
tween shrubs, soil fertility is decreased by erosion and
gaseous emissions (Schlesinger
et al
., 1990). Increased
runoff and erosion strip the soil surface layer, promoting
the formation of desert pavement in intershrub areas and
the development of rills. In a similar way, grazing may
contribute to the formation of banded patterns in vege-
tation found across desert-marginal landscapes in many
parts of the world (Barbier
et al
., 2006), which are of-
ten refered to as
brousse tigree
after the name coined in
Sahelian West Africa.
Domesticated herds have been blamed for notable trans-
formations of landscapes in several parts of the world. One
example is the series of dramatic changes in the range-
land vegetation of southern New Mexico, which have
been documented since the 1880s and the beginning of
intensive grazing by cattle and sheep. Typically, native
grasslands have been displaced by shrublands with con-
comitant increases in soil erosion, stream channel cutting
and the emergence of shrub coppice dunes on sandy soils
(Bahre, 1991).
Other, no less significant, zoogeomorphological effects
can occur through less direct human influence. A couple
of examples highlighted by Butler (2006) illustrate the
potential scale and magnitude of impacts, although much
work remains to be done on quantifying the effects. The
huge reduction through hunting of the native population
of bison (
Bison bison
) in North American rangelands, and
their widespread replacement with domesticated cattle,
will have had significant geomorphological implications.
The trampling effects of bison may be similar to those
22.2.4
Grazing
The effects of animals on the landscape, so-called zoo-
geomorphology (Butler, 1995), is relatively understudied
but is probably important in certain places. Among dry-
land environments, rangelands come particularly to mind.
Herds of domesticated ungulates can affect geomorphol-
ogy directly, their hooves causing soil compaction, re-
duced infiltration, increased runoff and sediment yield.
In riparian zones, trampling can also directly degrade
banks, with consequent effects on erosion (Trimble and
Mendel, 1995).
A range of indirect geomorphological impacts also oc-
cur through the effects of grazing on vegetation. One
widespread effect of intensive grazing is the encroach-
ment of unpalatable or noxious shrubs into rangelands.
Long-term grazing of semi-arid grasslands can typically
lead to an increase in the spatial and temporal hetero-
