The Terai region is composed of a 26 to 32 km wide broad belt of alluvial and fertile plain in the southern part of the country. This belt extends from the westernmost part of the country to the eastern limit and covers about 17% of the total land area.
In complete topographic contrast to the Mountain and Hill regions, the Tarai Region is a lowland tropical and subtropical belt of flat, alluvial land stretching along the Nepal-India border, and paralleling the Hill Region. It is the northern extension of the Gangetic Plain in India, commencing at about 300 meters above sea level and rising to about 1,000 meters at the foot of the Siwalik Range. The Tarai includes several valleys ( dun ), such as the Surkhet and Dang valleys in western Nepal, and the Rapti Valley (Chitwan) in central Nepal.
The word tarai , a term presumed to be derived from Persian, means "damp," and it appropriately describes the region's humid and hot climate. The region was formed and is fed by three major rivers: the Kosi, the Narayani (India's Gandak River), and the Karnali. A region that in the past contained malaria-infested, thick forests, commonly known as char kose jhari (dense forests approximately twelve kilometers wide), the Tarai was used as a defensive frontier by Nepalese rulers during the period of the British Raj (1858-1947) in India. In 1991 the Tarai served as the country's granary and land resettlement frontier; it became the most coveted internal destination for land-hungry hill peasants.
In terms of both farm and forest lands, the Tarai was becoming Nepal's richest economic region. Overall, Tarai residents enjoyed a greater availability of agricultural land than did other Nepalese because of the area's generally flat terrain, which is drained and nourished by several rivers. Additionally, it has the largest commercially exploitable forests. In the early 1990s, however, the forests were being increasingly destroyed because of growing demands for timber and agricultural land.
In between the Chure hills, rising abruptly to the north of the southern plains and the Mahabharat range, are a number of low valleys called "Duns". The Duns resemble the Terai in relief and climate and are also called the Inner Terai. The Chure range running east west across the country is shaped like a hedge. The hills of this range are rather sparsely forested, averaging 600 to 1220 meters in altitude and 8-16 km in breadth. The soil is immature and dry.