Physiography of
Nepal
A
profile across the Himalaya illustrates the subdivisions
•
Terai
•
Siwalik Range (or briefly Siwaliks) with dun valleys
•
Mahabharat Range
•
Midlands
• Fore
Himalaya
•
Great Himalaya
•
Inner Himalayan valleys
•
Tibetan marginal ranges.
Terai
The
Terai Plain is represented by Pleistocene to Holocene sediments and is
part of the Ganga foreland basin. In Nepal there are three separate areas where
the alluvium is observed south of the Siwalik hills. Its width varies from about
30 km in west Nepal to about 40 km in central Nepal, and from 25 to 50 km in
east Nepal. the Terai slopes towards the south and the
altitudinal difference between the Siwalik
foothills and the Indian border ranges from 100
to 200 m. The Terai is subdivided into the
three zones, respectively, from north to south.
Upper Terai or
Bhabar Zone: The Bhabar (or Bhabhar) zone is observed at the foot
of the Siwaliks and it is made up of coalescing alluvial fans, which are
crosscut by major rivers and streams. It is 10–15 km wide. The Bhabar
zone gently (a few degrees) slopes towards the south and consists of poorly
sorted boulders, cobbles, pebbles, and sand derived from the Siwalik or older
rocks. It is the zone of groundwater recharge for the Terai.\
Middle Terai or
Marshy Land: The middle Terai lies at the distal end of the
coalescing alluvial fans of the Bhabar zone. Most of the region is made up of fine
silt and clay but there are also alternating beds of sand and fine
gravel. The beds are very gently (less than 1°) tilted to the south.
Frequently, a spring line separates
the middle Terai from the Bhabar zone
Lower Terai or
Gangetic Alluvium: Most of the lower Terai lies in the zone
of interaction between the Gangetic alluvium and the deposits of its meandering
and braided tributaries. It is almost flat and the gradient is
less than 0.1 %. The sediments are represented by sand, silt, and
clay with some pebbles.
Siwaliks and Dun
Valleys
The first
topographic rise north of the Terai is called the Siwaliks or Chure (Churia)
hills in Nepal. The Siwalik hills range in altitude from about 300 m to
2,000 m and they are generally higher than 1,000 m. Most of the Siwalik
ridges extend in the east–west direction, parallel to the Himalayan trend.
Generally, the Siwalik belt is 10–15 km wide.
It is about 20 km
wide at the western border (i.e., the Mahakali River) of Nepal and less than 1
km near its eastern extremity (i.e., the Mechi River). On the other hand, it is
from 30 to 50 km wide in the areas with intermontane valleys.
When
there are multiple Siwalik ranges, they are not parallel to each other, but
converge or diverge here and there. These ranges are broadly concave or
convex towards the foreland. As a result, some space is left between them and
that space is occupied by tectonic (or dun) valleys (Hagen 1969, p. 20). This kind of divergence or
convergence is related mainly to the orientation of branching imbricate faults.
One of the
remarkable features of frontal Siwalik hills is their abrupt disappearance in
the Terai Plain.
This is observed
east of Karnali, around the bend of the Rapti River (east of Nepalgunj), west
of Amiliya, north of Mirchaiya, and south of Beltar. These are the closures of
the Siwalik beds forming plunging anticlines or synclines. The landforms of
the Siwaliks are controlled predominantly by tectonic processes and subordinately
by weathering and mass wasting.
Parallel and
rectangular drainage patterns (controlled by bedding and joints) are frequent
in the Siwaliks. Centripetal and rectangular drainage patterns are
common in the core of synclines and a radial pattern is characteristic of an
anticlinal core.
Mahabharat Range
The Main Boundary
active fault system frequently delimits the Mahabharat Range from the Siwaliks.
The Mahabharat Range rises higher than the Siwalik hills and reaches an
altitude of 3,000 m, but most of it is less than 2,000 m.
This range is
discontinuous and includes a variety of Lesser and Higher Himalayan rocks. The
geomorphic classification of mountains into the Siwalik and Mahabharat ranges
does not always coincide with the geological Subdivisions.
The Mahabharat
Range is divisible into three main segments, separated by two major antecedent
rivers. The stretch west of the Karnali (near the Seti confluence) as well
as the portion east of the Narayani (Gandaki) is constituted mainly of
metamorphic and crystalline rocks, whereas the central portion extending
between the Karnali and Narayani is composed of sedimentary or slightly
metamorphosed sequences. The Mahabharat Range is rather discontinuous,
dissected also by the rivers originating from the south face of the Himalaya,
and relatively subdued in topography. Intense erosion in the central segment of
the Mahabharat Range exposed the underlying rock sequences when the region was
denuded of the metamorphic cover.
The Mahabharat Range
acts as a barrier to the rivers originating from the Midlands or farther north
and flowing into the Ganga. It is evident from their abrupt orientation change
while approaching the range: those flowing essentially perpendicular to it
suddenly take a parallel course. antecedent
rivers are deflected to the east or west by this range and they make
narrow and deep gorges while crossing it. All these observations point to a
recent upheaval of the Mahabharat Range.
Midlands
The
Midlands, the heart of Nepal, compose the scenic lowlands between the
Mahabharat Range to the south and the high mountains to the north (Hagen 1969, p. 21). Their altitude varies between 600
and 2,000 m, and they are generally covered with alluvial, colluvial,
and residual soils
Most of the
densely populated valleys are located within the Midlands. Chaurjahari, Kusma,
Pokhara, Kathmandu, Panchkhal, and Tumling Tar are some of the valleys in the
Midlands.
The
Midlands display diverse drainage patterns including trellis, rectangular,
parallel, and dendritic. The Kathmandu Valley (Fig. 3.8) is a textbook example of the
centripetal drainage pattern. The Midlands have
been bestowed a variety of alluvial terraces
suitable for agriculture. They were produced by constant
lateral channel shifting and vertical incision of the
snaking rivers. A range of sedimentary, low-grade metamorphic, and crystalline rocks belonging mainly to the Higher and Lesser Himalayan sequences constitutes
the Midlands. When a rock is exposed to prolonged
sunshine for tens of thousands to hundreds of
thousands of years in a humid subtropical
region of the Midlands, a residual red soil develops.
The red color is due to oxidation of iron-containing
minerals (viz., pyrite, chlorite, biotite, hornblende) of the bedrock. Such a soil is almost like a
laterite found in South India and tropical
regions. The red soil ranges from a few
meters to tens of meters deep and is distributed in foothills,
ridges, spurs, and convex slopes of the Midlands.
Fore Himalaya
The
Midlands region exceeding the average altitude of 3,000 m is defined as
the Fore Himalaya (Hagen 1969, p.
22). This region lies in the inner belt of the Midlands, that is, at the foot
of the Great Himalayan Range. This portion of the Himalaya falls within the
crystalline thrust sheets or in the valleys surrounded by them. It is also marked by an increased microseismic activity. The
valleys of Jumla in west Nepal and Solu in
east Nepal are two examples. This highland,
where the altitude may exceed 4,000 m, is covered with
thick snow in winter and gets some rain in summer.
Great
Himalayan Range
The
Great Himalayan Range is the showpiece of Nepal where there are eight summits
exceeding 8,000 m. The region is made up of metamorphic rocks and crystallines
capped by sedimentary rocks with some granite intrusions.
Generally, the
south face of the Great Himalaya is steeper
and shorter than its north slope. A peculiar
feature of the Great Himalayan Range is that although
it stands as a barrier to the humid air coming from the
Indian Ocean and prevents the moisture from entering the
Tibetan Plateau, the range allows the passage of some
deep trans-Himalayan rivers through it. These mighty rivers originate in the Tibetan marginal mountains with an
altitude of less than 6,000 m but cross a
range whose altitude exceeds 8,000 m.
Most of the
landscape is carved by rivers, glaciers, snow, ice, and wind. The glaciers are
the reservoirs of frozen water that feeds a number of large rivers of Nepal.
Cirques, lateral and terminal moraines, hanging valleys, and terraces of fluvio-glacial
or lacustrine origin are the common landforms.
There are also
numerous glacier lakes posing a threat of outburst flood to the
community living in the Midlands.
Snow and rock
avalanches as well as various other glacial disasters have struck
this region.
Inner
Himalayan Valleys
Because
there is no single continuous Great Himalayan Range, the intermediate areas are
occupied by the Inner Himalayan valleys (Hagen 1969, p. 23). Such valleys are surrounded by the Great Himalayan and
Tibetan marginal ranges. The rivers flowing through the valleys make
deep gorges while breaking the lofty Himalayan
barrier. Such gorges facilitate the inflow
of moisture-laden warm winds from the south
and the valleys get some precipitation. The Thakkhola,
Manang, and Khumbu are some examples.
Tibetan
Marginal Ranges
The
Tibetan marginal ranges constitute the northern boundary of the Inner Himalaya
(Hagen 1969, p. 35). The major Himalayan rivers
originating from these ranges flow towards the Indian subcontinent.
