Physiography of Nepal

 

A south to north profile across the Himalaya shows the following physiographic division:.

• Terai

• Siwalik Range  with dun valleys

• Mahabharat Range

• Midlands

• Fore Himalaya

• Great Himalaya

• Inner Himalayan valleys

• Tibetan marginal ranges.

 

Terai

The Terai is the plain land on the southernmost part. It consists of young river sediments (Pleistocene to Holocene sediments). It is also considered as the part of the  Ganga foreland basin.. The aerial width of the Terai  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 southern Indian side and the elevation difference between the Siwalik foothills and the Indian border ranges from 100 to 200 m. 

Within the Nepal territory, there are three separate zones of alluvium observed south of the Siwalik hills based on which the Terai is subdivided into the three zones from north to south.

• Upper Terai or Bhabar Zone: The Bhabar (or Bhabhar) zone is observed at the foot of the Siwalik hills and it is made up of union of several large alluvial fan bodies that are crosscut by major rivers and streams flowing through them.This zone is  10–15 km wide and gently slopes towards the south. This zone consists of poorly sorted boulders, cobbles, pebbles, and sand derived from the Siwalik ranges and other ranges to the north . It is the zone of groundwater recharge for the Terai.\
• Middle Terai or Marshy Land: The middle Terai lies at the southern end of the alluvial fans of the Bhabar zone.  The region is made up of fine silt and clay also some sand and gravel beds are frequent. The beds are very gently (less than 1°) tilted to the south. Also a spring line is observed separating the middle Terai from the Bhabar zone
• Lower Terai or Gangetic Alluvium: The lower Terai lies in the zone of interaction between the Gangetic alluvium and the deposits of its meandering and braided rivers. It is almost flat and the gradient is less than 0.1 %. The sediments in this zone are sand, silt, and clay with some pebbles.

 Siwaliks and Dun Valleys

The first significant elevation north of the Terai region is known as the Siwalik or Chure hills in Nepal. These hills range in altitude from approximately 300 meters to 2,000 meters, generally averaging over 1,000 meters in height. Oriented predominantly in an east-west direction parallel to the Himalayan trend, the Siwalik ridges form a belt that is typically 10-15 kilometers wide.

At its widest near Nepal's western border along the Mahakali River, the Siwalik belt narrows to less than 1 kilometer towards its eastern end at the Mechi River. In areas with intermontane valleys, the belt widens considerably to between 30 and 50 kilometers. Multiple Siwalik ranges in these regions often exhibit non-parallel orientations, occasionally converging or diverging. This dynamic results in concave or convex formations towards the foreland, interspersed by tectonic valleys known as dun valleys  influenced by branching imbricate faults.

A distinctive feature of the frontal Siwalik hills is their abrupt disappearance into the Terai Plain. This disappearance is notably 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 areas mark where Siwalik beds form plunging anticlines or synclines. Tectonic processes primarily govern the landforms of the Siwaliks, with weathering and mass wasting playing secondary roles.

Drainage patterns in the Siwaliks often follow parallel and rectangular routes, influenced by bedding and joints. Centripetal and rectangular patterns are prevalent within synclines, while a radial pattern characterizes the core of anticlines.

 Mahabharat Range

The Main Boundary active fault system frequently marks the boundary between the Mahabharat Range and the Siwaliks. Rising higher than the Siwalik hills, the Mahabharat Range peaks at 3,000 meters, although most of it stands below 2,000 meters.

This range is patchy and comprises a mix of Lesser and Higher Himalayan rock formations. The division of these mountains into the Siwalik and Mahabharat ranges doesn't always align with geological subdivisions.

The Mahabharat Range can be divided into three primary sections, separated by two significant rivers that have flowed there for years. The section to the west of the Karnali (near the Seti confluence), and the stretch to the east of the Narayani (Gandaki) mainly consist of metamorphic and crystalline rocks. Meanwhile, the central segment that lies between the Karnali and Narayani contains sedimentary or slightly metamorphosed sequences. The Mahabharat Range is quite discontinuous and is also cut by rivers that originate from the southern face of the Himalayas. Its topography is relatively modest. In the central segment of the Mahabharat Range, strong erosion revealed the underlying rock formations when the region's metamorphic cover was stripped away.

The Mahabharat Range serves as an obstacle to the rivers that flow from the Midlands or further north into the Ganga. This is apparent in their abrupt change in direction as they near the range: rivers that were running more or less perpendicular to it suddenly change to a parallel course. These antecedent rivers are diverted east or west by the range, forming narrow and deep gorges as they cut through it. All these features indicate recent geological activity in the Mahabharat Range.

 Midlands

The Midlands, situated centrally in Nepal, form the picturesque lowlands nestled between the Mahabharat Range to the south and the towering mountains to the north . Their elevation varies from 600 to 2,000 meters, and they are predominantly covered with alluvial, colluvial, and residual soils.

Many of Nepal's densely populated valleys are found within the Midlands, including Chaurjahari, Kusma, Pokhara, Kathmandu, Panchkhal, and Tumling Tar.

The Midlands exhibit a diverse array of drainage patterns, such as trellis, rectangular, parallel, and dendritic. The Kathmandu Valley serves as a classic example of centripetal drainage.

A variety of alluvial terraces in the Midlands are well-suited for agriculture, formed through continuous lateral channel migration and vertical river incision. The Midlands consist mainly of sedimentary, low-grade metamorphic, and crystalline rocks belonging to the Higher and Lesser Himalayan sequences.

Under the prolonged influence of sunlight over tens to hundreds of thousands of years in the humid subtropical climate of the Midlands, residual red soils have developed. This distinctive coloration arises from the oxidation of iron-containing minerals like pyrite, chlorite, biotite, and hornblende in the bedrock. Comparable to laterite found in South India and tropical regions, the red soil ranges from a few meters to tens of meters in depth, distributed across foothills, ridges, spurs, and convex slopes throughout the Midlands.

Fore Himalaya

The Midlands region, which surpasses an average altitude of 3,000 meters, is referred to as the Fore Himalaya. This area is situated within the inner belt of the Midlands, lying at the base of the Great Himalayan Range. It includes the crystalline thrust sheets and the valleys encircled by them. This region is characterized by increased microseismic activity, with notable examples being the valleys of Jumla in western Nepal and Solu in eastern Nepal. This highland area, where altitudes can exceed 4,000 meters, is blanketed with thick snow during winter and receives some rainfall in summer.

 Great Himalayan Range

Nepal's Great Himalayan Range is a prominent feature, boasting eight summits that exceed 8,000 meters. This region is composed of metamorphic and crystalline rocks, topped with sedimentary layers and interspersed with granite intrusions.

Typically, the southern face of the Great Himalaya is steeper and shorter compared to its northern slope. A distinctive characteristic of this range is its role as a barrier to the humid air from the Indian Ocean, preventing moisture from reaching the Tibetan Plateau. Despite this, the range allows some deep trans-Himalayan rivers to pass through. These powerful rivers originate in the Tibetan marginal mountains, at altitudes below 6,000 meters, yet they traverse a range exceeding 8,000 meters.

The landscape is primarily shaped by rivers, glaciers, snow, ice, and wind. Glaciers serve as reservoirs of frozen water, supplying numerous large rivers in Nepal. Common landforms include cirques, lateral and terminal moraines, hanging valleys, and terraces of fluvio-glacial or lacustrine origin.

Additionally, numerous glacier lakes in the region pose a threat of outburst floods to communities living in the Midlands. Snow and rock avalanches, along with various other glacial disasters, have impacted this region.

Inner Himalayan Valleys

Due to the absence of a single continuous Great Himalayan Range, the intermediate areas are occupied by the Inner Himalayan valleys. These valleys are encircled by the Great Himalayan and Tibetan marginal ranges. The rivers flowing through these valleys create deep gorges as they break through the towering Himalayan barrier. These gorges allow the inflow of moisture-laden warm winds from the south, bringing some precipitation to the valleys. Examples of such valleys include Thakkhola, Manang, and Khumbu. They are characterized by deep gorges and unique climatic conditions due to their altitude and topography

Tibetan Marginal Ranges

The Tibetan marginal ranges form the northern boundary of the Inner Himalaya. The major Himalayan rivers that originate from these ranges flow towards the Indian subcontinent.