Physiography of Nepal

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.