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than pre-eruption levels (Major et al. 2000; Major
2003). At Mount St Helens post-1980 sediment
yields declined non-linearly for a decade but
increased abruptly in response to higher than
normal runoff in the late 1990s (Major 2003).
Even after 20 years some drainage basins still
have sediment yields 10 to 100 times greater
than the pre-eruption levels. Although sediment
sources in small drainage basins tend to stabilize
fairly rapidly (Collins & Dunne 1986), sediment
stored in larger river valleys remains active and
is unlikely to be stabilized for several decades
(Major 2003).
2.4.1 Anthropogenic impact on upland sediment
systems - deforestation in Nepal and the
'Himalayan environmental degradation theory'
The so-called Himalayan environmental degra-
dation theory (HEDT) as proposed by Ives &
Messerli (1989) neatly illustrates how changes
in population structure and pressure (human
impacts) on a mountain environment can lead
to a change in the natural sedimentary system
(Fig. 2.15). What is most significant about
the theory is the short time-scale over which it
appears to have developed and the very large
scale of the mountain area it potentially affects.
This qualitative model (Fig. 2.15) is deceptively
simple and can be summarized as follows
(Gerrard 1990):
• Population growth. Introduction of new
health care from 1950 has produced rapid
population growth. This population explosion
has been amplified by migration from the Indian
plain and Nepal with consequent greater de-
mands on fuel, construction materials, fodder
and agricultural land.
• Deforestation. Population expansion results
in massive deforestation.
• Soil erosion and landslides. Deforestation on
marginal and steep mountain slopes leads to
catastrophic soil erosion and landsliding with a
break down in the normal hydrological cycle.
• Increased runoff and flooding. The change in
slopes has resulted in increased runoff during
the summer monsoon with increases in flood-
ing and sedimentation in the Ganges and
Brahamaputra rivers resulting in the extension
of these great river deltas (Fig. 2.15, macro-
level).
• Positive feedback - accelerated deforestation
and greater soil loss. Loss of agricultural land in
the mountains results in greater pressure and
more deforestation and a switch to animal dung
as fuel, depriving the land of much needed
fertilizer.
• Degraded soil structure. Crop yields decline
and soil structure is degraded leading to further
soil erosion and regolith instability.
Although conceptually attractive this simple
model is fraught with difficulties. These relate
2.4
PROCESSES AND IMPACTS
-
ANTHROPOGENIC
INFLUENCES
A natural hazard can be defined as 'A phys-
ical event which makes an impact on human
beings and their environment' (Alexander 1998).
Mountainous environments are highly active
geomorphologically and increasing human use
of mountains has led to an increase in natural
hazards (Hewitt 2004). In the context of fluvial
hazards in mountain environments 'Mountain
rivers become a hazard only when they threaten
human life or property, by inundation, erosion,
sediment deposition or destruction' (Davies
1991). Although a large range of fluvial hazards
occur (e.g. glacial outburst floods, Alpine debris
flows, etc.) they generally result from extreme
temporal and spatial variability in fluvial and
hydrological processes. Because of these char-
acteristics mountain environments pose unique
problems for hazard assessment, prediction and
mitigation. More data are required on the fre-
quency and magnitude of hazardous events in
mountain regions.
This section considers two main examples of
human interaction with mountain sediment sys-
tems. The first outlines the debate surrounding
human-induced accelerated soil erosion in a steep
upland environment - deforestation in Nepal.
The second examines human infrastructure con-
struction in an unstable mountain environment
and discusses issues regarding the Karakoram
Highway.
