Inner Terai Valleys of Nepal
The Inner Terai Valleys of Nepal comprise several elongated river valleys in the southern lowland part of the country. These tropical valleys are enclosed by the Himalayan foothills, viz the Mahabharat Range and the Sivalik Hills located farther south. The Inner Terai is called "bhitri tarai भित्री तराइ" in Nepali.
Malaria was prevalent until the late 1950s. Since its eradication, the area became a viable destination for large-scale migration of people from the hills, who transformed the area from virgin forest and grassland to farmland.
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The Himalayas were formed by the collision of the Indian sub-continent with Eurasia, which began about 50 million years ago and continues today. The oceanic crust in front of India slid under Eurasia, pushing up the Tibetan plateau. The Indian continental crust is also pushing under Tibet, but is partly compressed and thrust upward to form the Himalayan mountain range, extending for over 2400 km and rising as high as 8848 m at Mt. Everest Chomolangma.
The Himalayas have four tectonic subdivisions:
- The Indus Suture Zone, where the Indian Plate meets the Eurasian Transhimalaya or Karakoram-Lhasa Block.
- The Central Himalayan Domain, the high backbone of the Himalayas bounded on the south by a fault zone called the MCT (Main Central Thrust) that has elevated these peaks three or four thousand meters above lower ranges to the south.
- The "middle hills" below the MCT south to the Lesser Himalaya or Mahabharat Range which is bounded by another fault zone, the MBT (Main Boundary Thrust) that abruptly elevates the Mahabharats 1,000 to 2,000 meters above hills further south.
- The Sub-himalaya also known as the Muree, Chure Hills or Siwaliks, which are the southernmost foothills of the Himalayan Range and mainly composed of folded and overlapping sheets of sediment from the erosion of the Himalaya. They are bounded on the south by the HFT (Himalayan Frontal Thrust) elevating them about 500 meters above the Gangetic plain.
The Inner Terai valleys lie between the Siwalik and Mahabharat ranges or sometimes between different ranges in the Siwaliks. They hold flat plains with winding rivers that shift course from time to time, running northwest or southeast along the axis of the Siwalik ranges until they find a break and flow into the Outer Terai and Gangetic plain. Usually there is little difference in elevation between the Inner Terai valley floors and the plains of the Outer Terai.
The Terai has a humid, subtropical climate. The mean annual rainfall at the Rampur weatherstation in Chitwan was 2,214 mm (87.2 in) between 1995 and 2006. More than 80% of the total annual rainfall occurs during the monsoon season from June to September. Average temperatures ranged from 8.08 °C (46.54 °F) in January to 34.91 °C (94.84 °F) in June.
In the past, the inner and outer Terai were a formidable barrier between Nepal and potential invaders from India because marshes and forests were infested by anopheline mosquitos that transmitted virulent strains of malaria, especially during the hot spring and rainy summer monsoon.
Until the mid 18th century, the Terai was divided into several smaller kingdoms, and the forests were little disturbed. After the unification of Nepal in the late 1760s, the rulers granted large areas of fertiIe land and forest resources to members of the royal family, officials, priests and selected groups of the society. The beneficiaries of these grants had the right to collect revenues from cultivated land and forest products. They appointed tax collectors who were also responsible for reclamation of land and establishment of settlements. In the late 1920s, the Rana rulers ordered the clearing of forests and extraction of timber for export to India in order to collect revenues. Cleared areas were subsequently used for agriculture.
Tharu people have been living in the Terai for many centuries, and reputedly had an innate resistance to malaria. After malaria was eradicated using DDT in the mid 1950s, people from the hills migrated to the Terai. Timber export continued to 1969. In 1970, the king granted land to loyal ex-army personnel in the districts of Jhapa, Sunsari, Rupandehi and Banke, where seven colonies were developed for resettling about 7,000 people. They acquired property rights over uncultivated forest and waste land, thus accelerating the deforestation process in the Terai.
Both Inner and Outer Terai have become Nepal's richest economic regions, with fertile farms and forests because of the area's generally flat terrain that is drained and nourished by several rivers. The Terai also has the largest commercially exploitable forests.
The well-meaning malaria eradication campaign has had unexpected consequences by opening up the Terai region to human settlement. The Inner Terai valleys are home to a rich and diverse ecosystem. Since the early 1990s, however, the forests have been increasingly destroyed because of growing demands for timber and agricultural land This has led to concerns about the risk of losing many rare plant, animal and insect species.
The valleys also mitigate the severity of floods on the Gangetic plains. During heavy rainfall forests absorb water. During floods, rivers overflow their banks and flood adjacent forests. Later the forests gradually release water back into the rivers. Deforestation reduces this buffering effect. It also accelerates soil erosion, causing downstream rivers to silt up and overflow their banks.
The frequency and severity of flooding in the Gangetic plain and Bangladesh has steadily increased in recent years. Deforestation of the Terai appears to be one of the major causes. The Indian and Nepalese governments are cooperating in measures including construction of barrages and dams in the Terai, such as the Koshi Barrage. However, these efforts may have mixed results. They contain floodwater in the short term, but may increase the problem in the longer term by reducing water velocity in the rivers downstream, and thus accelerating silting and reducing the drainage capacity of the rivers.
The major Inner Terai Valleys are listed from west to east. Click on the terrain and satellite imagery links to see vegetation, rivers, topography, roads and towns.
The Surkhet Valley (Nepali: सुर्खेत उपत्यका) is situated in the Surkhet district, mid-western Nepal. The valley is about 700 m (2,300 ft) above sea level, forming an ellipse about 9 km (5.6 mi) east-west by 6 km (3.7 mi) north-south. It is drained by the Bheri River, a tributary of the Karnali. The district is the homeland of the Raji people. Tharu people from Dang settled in the valley since at least the 19th century. In recent decades there has been increased migration from surrounding mountain districts as well as other parts of the country. Birendranagar is the district's capital.
Dang and Deukhuri Valleys
Both valleys are located in the Dang Deukhuri District of the Rapti Zone in mid-western Nepal. The Dang Valley (Nepali: दाङ उपत्यका) lies between the Mahabharat Range in the north and the Churia Range in the south. It forms a nearly 1,000 km2 (390 sq mi) plain within a local drainage basin of less than 3,000 km2 (1,200 sq mi). It is drained by the Babai River, and is one of the largest Inner Terai valleys. Tulsipur is the administrative center of the Rapti Zone and has a fair-weather airport. Ghorahi is the largest city and the administrative, economical and educational center of Dang District.
The Deukhuri Valley (Nepali: देउखुरी उपत्यका) is southeast of the Dang Valley and extends about 60 km (37 mi) in WNW-ESE direction with a maximum width of 20 km (12 mi). It forms a nearly 600 km2 (230 sq mi) plain within a drainage basin of 6,100 km2 (2,400 sq mi). The valley is drained by the West Rapti River. It emerges from its gorge through the Mahabharat Range and flows WNW along the axis of the Siwalik Range between the Dundwa Range along the India border and the Dang subrange to the north. It flows 100 km (62 mi) in this direction until the Dundwas fall away near Banke, and the river can resume flowing SE toward the Ganges.
The Chitwan Valley (Nepali: चितवन उपत्यका) encompasses the districts of Nawalparasi, Chitwan and Makwanpur in the Narayani Zone of central Nepal. It is 150 km (93 mi) long and roughly 30–48 km (19–30 mi) wide, the largest Inner Terai valley. (Sketch Map) The cities of Bharatpur, Ratnanagar and Hetauda are in the valley. It is drained by the Rapti River flowing from the Mahabharat Range near Hetauda, then west down the valley to join the Narayani River west of Meghauli. The Narayani is also called Gandaki further upstream and Gandak in India.
The Chitwan National Park, Nepal’s first national park established in 1973, was declared a World Heritage site by UNESCO in 1984. It contains the largest and least disturbed natural Sal hill forest and associated communities. Its fauna comprises Bengal tiger, Indian leopard, great one-horned rhinos, wild Asian elephant, gaur, golden monitor lizard, gharial and marsh crocodile.
The Kamala Valley, also called Udayapur Valley (Nepali: कमला or उदयपुर उपत्यका) is in Udayapur district in southeastern Nepal (Sketch map). It is about 30 km (19 mi) long and between 2 km (1.2 mi) and 4 km (2.5 mi) wide. It is drained by the Triyuga river flowing east to join the great Koshi. This valley lies between the Mahabharat Range to the north and the Sivalik Hills to the south, with an average elevation of about 430 m (1,410 ft). Triyuga is the main town.
The mouth of the valley opens onto a 175 km (109 mi) rectangle of land where the Triyuga meets the Koshi river above the Koshi Barrage. It was designated the Koshi Tappu Wildlife Reserve in 1976, and is home to the last remaining population of wild Asian water buffalo in Nepal. The reserve is mostly wetlands, subject to seasonal flooding, but also includes some grasslands and small patches of riverine forest. It is a Ramsar Site.
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