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265
Abstract The Pokhara valley in the central part of
Nepal is one of the few Himalayan intramontane
valleys that permits one to decipher the way the land-
forms of the world’s highest mountain range evolve.
The valley is attractive to tourists for the scenic maj-
esty of its glaciated mountains, gorges, caves, and
lakes, the formation of which results from a complex
yet recent and dramatic evolution of the valley. For a
long time, most of the inhabitants believed that the
valley originated from the drying up of a huge lake
similar to those of the Kathmandu and Kashmir val-
leys. Careful observations of the sediments filling the
basin indicate that the Pokhara valley was affected by
a giant, catastrophic debris flow five centuries ago.
It is an emblematic site, where the steepness of the
still rising front of the very Himalaya (“the abode
of snow”) is maintained by sporadic collapses of the
mountain walls controlled by a combination of both
glacial and seismo-tectonic dynamics.
Keywords Debris ows • glaciation • Himalaya •
mountain building
27.1 Introduction
The Pokhara valley at the foot of the Annapurna Himal
presents one of the sharpest contrasts in relief in the
world. Located in the central part of Nepal, it stands
out as a distinctive feature of the Himalayan landscape.
It is an abnormally broad plain with an area of ~125
km
2
, and confined by hills ranging from 1,200 to 3,000
m in elevation. Towering above it to the north is the
Annapurna Himal (8,091 m), only 35 km as the crow
flies from Pokhara town (800 m a.s.l.). It is drained by
the Seti Khola (khola meaning “river” in Nepali language)
and its tributaries, originating from the glacial cirque
of Sabche, surrounded from west to east by the peaks
of Macchapuchare (6,993 m), Annapurna III (7,555 m),
and Annapurna IV (7,525 m). Well known for the
outstanding scenic majesty of these “snowy mountains”
(“Himal” in Nepali), the Pokhara valley is also attractive
to tourists for its gorges, caves, and lakes, the forma-
tion of which results from the complex yet recent and
dramatic evolution of the valley.
In fact, for a long time, the formation of the Pokhara
valley was a mystery. Inhabitants were struck by the
very many large rocks found in the upstream part of
the valley, and had their own explanation of these fea-
tures, referring to legends involving the action of God.
Most of them believed that the Pokhara valley origi-
nated from the drying up of a huge lake similar to those
of the Kathmandu and Kashmir valleys (Hagen 1961).
Careful observations of the sediments filling the valley,
together with a better understanding of the geomor-
phological processes in action, have led to new inter-
pretations involving the very recent evolution of the
glaciated mountain front of the Annapurna Range.
This makes the Pokhara valley one of the few
Himalayan basins that permits one to decipher the way
the landforms of the world’s highest mountain range
evolve and explains why this particular part of the
Himalaya has been selected for this volume.
27.2 Geographical and Geological
Setting
The Pokhara valley is located in the midland region
(or Pahar zone) of the Himalayan Range, a low-lying
belt, which is sandwiched between the Lesser
Himalaya (or Mahabharat Range) in the south and the
Chapter 27
The Pokhara Valley: A Product of a Natural Catastrophe
Monique Fort
P. Migon´ (ed.), Geomorphological Landscapes of the World,
DOI 10.1007/978-90-481-3055-9_27, © Springer Science+Business Media B.V. 2010
266
M. Fort
Great (or Higher) Himalaya to the north. It belongs
to a series of intermontane basins, the formation of
which is closely related to the formation of the
Himalayas as a whole.
The Himalayan Arc results from the collision
between the Asian and Indian plates, which occurred
about 50–45 million years ago. The compressional
motion between the two plates has been, and continues
to be, accommodated by slip on a suite of major thrust
faults, connected at depth along a major detachment
plane. The extreme elevations were acquired by stack-
ing of crust units in a continuing continental subduc-
tion régime. The Himalayan Range is still rising at a
rate estimated to be between several millimeters per
year to more than 1 cm/year. The collision process cre-
ated a series of elongated, more or less parallel and
asymmetrical ridges, developed to the north of the
Indo-Gangetic plains as a fold-and-thrust belt (Lesser
Himalaya), which controls the drainage pattern.
To the north, the steep barrier of the Greater Himalaya
reaches more than 8,000 m above sea level (Fig. 27.1).
It represents a more than 10 km thick thrust unit made of
crystalline, gneissic rocks, bounded at depth by the
Main Central Thrust, and overtopped by sedimentary
rocks, mostly limestones, that now underlie the
Himalayan peaks. It separates two very contrasted ter-
rains: the humid, tropical, monsoon-influenced Indian
subcontinent and the cold, arid, and rugged Tibetan and
Central Asian High Plateaus. Nowhere in the Himalayan
Range is this bioclimatic gradient greater than along the
Annapurna Range, north of Pokhara.
The Pahar zone has developed between the Lesser
and the Higher Himalayas, and may locally widen in the
form of intermontane depressions such as the Pokhara,
Kathmandu, or Kashmir valleys. They correspond to
large confluences of river valleys and/or to fluvio-lacus-
trine basins, developed on the back of thrust units of the
Lesser Himalaya. They were initiated by tectonic dam-
ming as a result of a relatively faster uplift rate in the
Lesser Himalaya than along the Pahar, transitional zone.
In contrast to the flat, perched, lacustrine basins of
the Kashmir and Kathmandu valleys (Burbank and
Raynolds 1984), the Pokhara valley floor is charac-
terized by an extensive, gravel-covered surface, the
formation of which has been influenced by a long
period of fluvial modeling and dissection, and was the
Fig. 27.1 Northern part of the Pokhara valley, developed at the
foot of the High Himalayan Front, dominated by Macchapuchare
(6,993 m), Gangapurna (7,454 m), Annapurna III (7,555 m),
Annapurna IV (7,524 m), Annapurna II (7,937 m), and Lamjung
Himal (6,983 m), from left to right. The Seti Khola, in the fore-
ground, issues from deep gorges cut below the glaciated cirque
of Sabche. The river is responsible for the accumulation of the
indurated Gaunda conglomerates, which underlie the two high-
est and very flat terrace levels, for the catastrophic accumulation
of Pokhara gravels, deposited about 500 years ago, and for their
recent dissection into several (here at least five) strath terraces
(Photo M. Fort)
26727 The Pokhara Valley: A Product of a Natural Catastrophe
locus for catastrophic events and processes developed
in relation to the proximity of the steep Main Central
Thrust Front (Fort 1987).
27.3 Landforms and Landform Diversity
Besides magnificent mountain views, the Pokhara basin
is characterized by several specific landforms. Firstly,
the wide, flat morphology of the Pokhara “plain” is
striking, a view reinforced by the sharp contact between
the plain and the surrounding hills. The plain slopes to
the south; the general longitudinal gradient varies from
32‰ upstream to 9‰ downstream. The plain also slopes
laterally from a central axis, a feature that has caused
diversions of the tributary streams. The surface of the
plain displays a braided-channel morphology (Fig. 27.2).
These characteristics are those of a large alluvial out-
wash fan. In fact, this plain is underlain by the so-called
Pokhara gravels (Gurung 1970), excavated by the pres-
ent Seti Khola River, hence providing numerous sec-
tions that permit one to analyze and interpret the nature
of the gravels and their mode of deposition.
Another distinctive landform of the Pokhara valley is
the dramatic set of terraces shaped by the Seti Khola and
its tributaries by both vertical incision and lateral erosion
(Fig. 27.1). The number of terraces increases down-
stream, whereas their relative height above the Seti
thalweg increases upstream, from 60 to 50 m in the
south to >100 m to the north. Most of the terrace levels
are unpaired, a feature distinctive of meandering streams
and rapid incision, this last statement being reinforced
by the fresh appearance of the topographic surface and
the absence of soil developed on terrace surfaces.
The canyons of the Seti Khola and its tributaries are
among the most intriguing features of the valley
(Fig. 27.3). Interrupting the long sections of terraces,
they occur along limited reaches, a few hundred meters
to 1 km long, and are deeply (up to 50 m) entrenched into
a material made of gravels and boulders, cemented
together into a hard conglomeratic bedrock with a rich
limestone matrix, locally known as “gaunda” (Hormann
1974). These gorges are locally so narrow that only the
sound of water can be heard; in some cases the stream
has even disappeared in underground tunnels. One of
these canyons is crossed by the Mahendra Pul (bridge),
along the main Kathmandu road before entering the old
bazaar of the city. These gorges are often associated with
potholes and caves, such as the Mahendra and Chamere
caves, close to Batulechaur village, the Jogi cave in
Balam Hill, or the Gupteshwor and Davis falls caverns in
Fig. 27.2 Surface, braided morphology of the Pokhara gravels, as highlighted by the rice-fields pattern, east of Pokhara airport
(Photo M. Fort)
268
M. Fort
Chorepatan, south of the Phewa lake: there, the Pardi
River disappears into a tunnel, 200 m long, with a natural
bridge on which the Sidartha highway passes. The devel-
opment of these karst-like features provides additional
evidence for the combined action of water dissolving the
limestone and abrading the conglomerates.
The last feature that makes Pokhara famous is the many
lakes (“tal” in Nepali) that are found along the edges of the
valley (Fig. 27.4). Most of them (Dipang, Maidi, Khalte,
Kamalpokhari, and Gunde lakes) have virtually disap-
peared due to siltation. The lakes Rupa, Begnas, and
Phewa are larger, and their sinuous shorelines clearly sug-
gest that they are drowned valleys (Gurung 1970). Close
to Pokhara city, the largest of them, the Phewa Tal, is an
important tourist attraction, with its waters reflecting the
entire Annapurna Himalayan peaks during clear days. The
occurrence of these lakes in juxtaposition with the grav-
elly plain of Pokhara reflects their origin, as a response to
the deposition of the Pokhara gravels.
27.4 The Pokhara Gravels
The Pokhara gravels (Gurung 1970; Yamanaka et al.
1982) are the main component of the basin fill: their
top layer underlies the geomorphological surface upon
which Pokhara town has been built. They extend from
Bharabhure at the mouth of the Seti Khola gorge,
downstream to Dhoban, at the foot of the Mahabharat
Lekh. The total thickness of their accumulation
decreases downstream, from an average thickness of
over 100 m to about 60 m, although local variations are
observed depending on the buried topography. The
sections, well visible from along the Prithvi Narayan
Road across the Seti valley or its tributaries such as the
Bijaypur, reveal the main characteristics of the Pokhara
gravels and their modes of deposition.
The Pokhara gravels consist of a rapid succession of
beds, decimeters or meters in thickness, with flat basal
contacts (Fort and Freytet 1979). The material is mainly
composed of layered, sub-angular to sub-rounded,
mostly calcareous gravels, centimeters to decimeters in
size, embedded in a muddy, calcareous matrix present
in variable proportions (Fig. 27.5). Blocks of a meter or
more in size – mostly gneisses (Higher Himalaya) or
quartzites (Lesser Himalaya) – can also be found ran-
domly within the whole accumulation package. Blocks
of exceptional size, such as the Bhimsen Kali Boulder
(32 m in diameter) visible on the University campus of
Pokhara (Fig. 27.6), are restricted to the top-most lay-
ers where they are distributed upon the surface of the
Pokhara plain. All these sedimentological characteris-
tics indicate that the Pokhara formation is an alluvial
Fig. 27.3 Major terrace levels south of Pokhara valley,
underlain by the Pokhara gravels. They have buried the older,
indurated “gaunda” conglomerates, which are exhumed by
the recent erosion of the Seti River, as observed in the fore-
ground, where potholes and small canyons have developed
(Photo M. Fort)
269
27 The Pokhara Valley: A Product of a Natural Catastrophe
Fig. 27.4 The Phewa Lake, seen from the west, with the
Pokhara valley in the background. This lake is a remnant of a
drowned valley dammed downstream by the catastrophic depo-
sition of the Pokhara gravels. This lake, like many others located
along the edges of the Pokhara valley, is nowadays subjected to
intense siltation by tributary streams (Photo M. Fort)
Fig. 27.5 A 25-m high section of the Pokhara gravels. Noticeable are the flat beds that include random occurrences of big boulders.
This accumulation typically exhibits irregular alternations of debris-flow and mud-flow layers (Photo M. Fort)
fan deposit, transported alternately by muddy flows,
debris flows, and torrential discharge (Fort 1987). The
occurrence of the largest boulders in the final stage of
deposition can be explained as a sorting phenomenon
distinctive of the dense, debris laden, muddy flow of
the Pokhara discharge.
This discharge was first considered to be a product
of glacio-fluvial outwash (Gurung 1970; Hormann 1974;
270
M. Fort
Fort and Freytet 1979), because most of the clasts are
limestones derived from the glaciated cliffs of the
Annapurna Range. However, the gneissic nature of the
largest boulders indicates that materials derived from
the adjacent valley walls located between the glacial
cirques and the Pokhara plain were also incorporated
by this massive discharge. Moreover, the volume of
Pokhara gravels (estimated as >4 km
3
; Fort 1987) and
their dating between 400 and 1,100
14
C years
(Yamanaka et al. 1982; Fort 1987) suggest the occur-
rence of a short-lasting, historical event that led to the
rapid filling of the Pokhara valley by a giant debris
flow and to the damming, and hence flooding of the
adjacent valleys. The present lakes are the relicts of
this exceptional event.
Among the various origins and formation condi-
tions that can explain this sudden, huge supply of both
water and debris, it seems that ice and rock avalanches
and/or landslide-dammed lake outbursts are capable of
liberating the greatest volumes of water and debris
simultaneously, in such a catastrophic way. Hence,
these processes are the most likely causes of the excep-
tional Pokhara gravel discharge. The sources of the
debris are the very steep slopes of the south face of
Annapurna III and IV (Fig. 27.7). The triggering factor
of an event of such a magnitude has probably to be
related to a tectonic-induced instability, i.e., to an
earthquake, the only mechanism capable of bringing
instantaneously a slope into disequilibrium and setting
into motion so huge a quantity of ice and rock material
(Fort 1987).
27.5 Evolutionary History
The Pokhara gravels have buried an irregular topogra-
phy, which includes the relicts of former stages of
Pokhara valley evolution. The evolutionary history of
the valley can be summarized as follows.
The formation of the intermontane basin is the
result of a long-lasting process. As already pointed
out, the basin belongs to the Pahar belt, which is rising
at a slower rate than the Mahabharat Lekh (Lesser
Himalaya) to the south and the Higher Range to the
north. More specifically, it is situated along an anticlinal
Fig. 27.6 The famous Bhim Kali boulder, 3,000 t large, pre-
served on the Pokhara University campus. A local legend tells
that this rock was thrown down from Machapucchare Peak by
the powerful hero Bhim. In fact, it represents the final stage of
Pokhara gravels deposition, and was brought by a competent,
highly muddy flow nourished by coarse debris detached from
the High Himalayan Front. Most of these boulders have nowa-
days disappeared as they have been quarried (Photo M. Fort)
271
27 The Pokhara Valley: A Product of a Natural Catastrophe
structure, mostly carved out into schists with intercala-
tions of quartzite and dolomitic limestone bands. The
distinctive z-shape of the basin and of the Seti Khola
course and its tributaries, together with the arrange-
ment of the surrounding hills, may be interpreted as the
superficial expression of deformation – namely fault
scarps – induced by the oblique, northward conver-
gence of the Sub-Himalaya underthrusting the Lesser
Himalaya.
The present, large-scale morphology of the valley
is the result of a complex alternation of aggradational
and erosional stages, developed under a tropical,
seasonally contrasted climate. In fact, the Pokhara
valley has experienced several stages of dissection,
separated by brief, yet intense periods of aggradation
(Fig. 27.8). A few patches of perched calcareous
breccia, like those preserved along the Sarankot
ridge, represent early remnants of slope deposits,
spread over an aggradational pediment, presently
reduced to sharp, karstic ridges modeled into towers
by the dissolution of carbonates. After a period of
basin dissection to a depth of 50 m or more, perched
weathered gravels provide evidence of a previous
Seti Khola course. Another period of dissection also
followed which, in the central part of the basin, pen-
etrated lower than the present level of the Seti Khola
river bed (Fig. 27.9).
This evolution was interrupted by the deposition
of the “gaunda” (or “Gachok”) conglomerates that
are the first extensive deposits in the Pokhara valley.
They underlie the prominent terrace benches of the
northern part of the valley, near Lachok and Gachok
villages (Fig. 27.8). Their well-rounded gravels and
sand grains, dominated by limestone, are derived
from the south-facing, upper cliffs of the Annapurna
Himal. They were until recently considered to be
glacio-fluvial outwash deposited by the melting of ice
after the last glaciation, but are now explained by
similar, catastrophic processes as were involved in
the formation of the Pokhara gravels. After their
deposition and lithification, these conglomerates
were dissected to form a stepped topography of fluvial
terraces, well preserved north of the valley, upstream
of the Mardi Khola confluence, whereas to the center
and south of the valley, they disappear under the
Pokhara gravels.
The more recent and rapid deposition of the Pokhara
gravels has resulted in burial of the former topography,
disorganization of the entire drainage system of the
valley, and creation of the peripheral lakes. Soon after,
the Seti Khola started incising its bed again to readjust
its longitudinal profile to the base level of the
Mahabharat Range. In the central part of the valley, the
new Seti course, superimposed on the Pokhara gravels,
locally cut through the hard “gaunda” conglomerates
(present beneath the Pokhara gravels), so that it pre-
vented the river to widen its beds, thus leading to the
formation of canyons instead (Fig. 27.4). Elsewhere,
the presence of the loose Pokhara gravels favored the
shift of the Seti River and the subsequent development
of unpaired flights of terraces (Fig. 27.10).
Fig. 27.7 Close-up view of the northern part of the Pokhara
valley, Seti Khola gorge, and the upper glaciated cirque of
Sabche. The terraces developed on the foreground are underlain
by the Pokhara gravels. Upstream the Seti gorge, entrenched
within the High Himalayan gneisses, is particularly narrow and
steep. It channelized the giant, catastrophic debris flow of
Pokhara gravels, originated from the southwest face of
Annapurna IV (Photo M. Fort)
272
M. Fort
Fig. 27.8 Map of the major deposits of the Pokhara valley. The
catastrophic accumulation of the Pokhara gravels buried a dis-
sected topography carved into the older “gaunda” conglomer-
ates. Since the giant debris flow, the Seti River cut through the
Pokhara infill, and found locally the “gaunda” conglomerates
beneath, so it was forced to incise deep canyons
273
Fig. 27.9 Cross section and altitudinal distribution of the
various formations deposited in the Pokhara valley in the last
100,000 years. To the north of the valley, the oldest perched
slope deposits (limestone breccia), the old, deeply weathered
alluvium and the Gaunda-Gachok formations are stepped
above the most recent Pokhara gravels filling, whereas in the
center of the valley, the Pokhara gravels have buried the
Gaunda-Gachok conglomerates. This particular setting clearly
indicates the rising of the High Himalayan Front relative to
the basin
Fig. 27.10 Since the deposition of the Pokhara giant debris
flow, the Seti Khola started incising its bed again at a rate vary-
ing between 20 cm/year upstream and about 10 cm/year down-
stream of the basin. Locally, the presence of loose Pokhara
gravels favored the shift of the Seti River and the development
of many, unpaired terraces (Photo M. Fort)
274 M. Fort
27.6 Conclusions
The catastrophic episode of the Pokhara debris-flow
aggradation serves as a model for the geomorphic evolu-
tion of the High Himalayan Front (Fort 1987; Fort and
Peulvast 1995). It shows how a huge mass of debris almost
instantaneously delivered from the front may temporarily
be stored in an intramontane basin of the Pahar before
transit to the Himalayan foothills. It also demonstrates
how the steepness of the still rising front of the very
Himalaya (“the abode of snow”) is maintained by spo-
radic processes of collapse of the mountain walls involv-
ing a combination of both glacial and seismo-tectonic
factors. This makes the basins and the valley trenched
across the High Himalayan Front, the areas most prone to
unpredictable, catastrophic geomorphological hazards,
and creates a permanent, low-recurrence, but significant
threat for the growing population living in these valleys.
The Author
Monique Fort is a Professor of Geomorphology and
Environmental Sciences, Natural Hazards and Risks, in
the Department of Geography of Paris Diderot – Paris
7 University. She worked extensively in various high
mountains of the world (Alps, Central Asia, and
Himalaya). Her research interests evolved from the
relations of landforms with respect to geological struc-
tures, then to glacial and climatic fluctuations, and
palaeoenvironmental reconstructions. Ongoing field
work includes studies on current instabilities and natural
hazards (large-scale landslides, catastrophic floods) in
the Himalayas and Pamir mountains, floods impacts,
and their prevention in various parts of France. She
published more than 50 peer-reviewed papers. She was
the Vice President of the International Association of
Geomorphologists (2005–2009), and member of the
Commission on Mountain Response to Global Change
of the International Geographical Union (2008–2012).
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