The Terai or Tarai is a lowland region in northern India and southern Nepal that lies south of the outer foothills of the Himalayas, the Sivalik Hills, and north of the Indo-Gangetic Plain. This lowland belt is characterised by tall grasslands, scrub savannah, sal forests and clay rich swamps. In North India, the Terai spreads from the Yamuna River eastward across Haryana, Uttarakhand, Uttar Pradesh, Bihar and West Bengal. The Terai is part of the Terai-Duar savanna and grasslands ecoregion. The corresponding lowland region in West Bengal, Bangladesh, Bhutan and Assam in the Brahmaputra River basin is called 'Dooars'. In Nepal, the term is applied to the part of the country situated north of the Indo-Gangetic Plain. Nepal's Terai stretches over , about 23.1% of Nepal's land area, and lies at an elevation of between . The region comprises more than 50 wetlands. North of the Terai rises the Bhabar, a narrow but continuous belt of forest about wide.
Etymology
The Urdu word tarāʼī means "lands lying at the foot of a watershed" or "on the banks of a river; low ground flooded with water, valley, basin, marshy ground, marsh, swamp; meadow". In Hindi, the region is called 'tarāī' meaning "foot-hill". In Nepali, the region is called 'tarāi' meaning "the low-lying land, plain" and especially "the low-lying land at the foot of the Himālayas". It has been described as "low, marshy ground".
Geology
The Terai is crossed by the large perennial Himalayan rivers Yamuna, Ganges, Sarda, Karnali, Narayani and Kosi that have each built alluvial fans covering thousands of square kilometres below their exits from the hills. Medium rivers such as the Rapti rise in the Mahabharat Range. The geological structure of the region consists of old and new alluvium, both of which constitute alluvial deposits of mainly sand, clay, silt, gravels and coarse fragments. The new alluvium is renewed every year by fresh deposits brought down by active streams, which engage themselves in fluvial action. Old alluvium is found rather away from river courses, especially on uplands of the plain where silting is a rare phenomenon.
A large number of small and usually seasonal rivers flow through the Terai, most of which originate in the Sivalik Hills. The soil in the Terai is alluvial and fine to medium textured. Forest cover in the Terai and hill areas has decreased at an annual rate of 1.3% between 1978 and 1979, and 2.3% between 1990 and 1991. With deforestation and cultivation increasing, a permeable mixture of gravel, boulders and sand evolves, which leads to a sinking water table. But where layers consist of clay and fine sediments, the groundwater rises to the surface and heavy sediment is washed out, thus enabling frequent and massive floods during monsoon, such as the 2008 Bihar flood.
Geography
In India, the Terai extends over the states of Haryana, Uttarakhand, Uttar Pradesh, Bihar and West Bengal. These are mostly the districts of these states that are on the India–Nepal border: Haryana: Panchkula district Uttarakhand: Haridwar district, Udam Singh Nagar and Nainital districts Uttar Pradesh: Pilibhit district, Lakhimpur Kheri district, Bahraich district, Shravasti district, Balrampur district, Gorakhpur district, Siddharthnagar district and Maharajganj district Bihar: West Champaran district, East Champaran district, Sitamarhi district, Madhubani district, Supaul district, Araria district, Kishanganj district West Bengal: Siliguri subdivision of Darjeeling district, Jalpaiguri Sadar subdivision of Jalpaiguri district
Inner Terai
The Inner Terai Valleys of Nepal consists of five elongated valleys located between the lower Himalayan Range and Sivalik Hills. From north-west to south-east these valleys are: Surkhet Valley in the Surkhet district, north of the Kailali and Bardiya Districts; Dang Valley in the Dang Deokhuri district; Deukhuri Valley located south of the Dang Valley; Chitwan Valley stretching across the Chitwan and Makwanpur Districts; Kamala Valley, also called Udayapur Valley, in the Udayapur District north of the Siraha and Saptari Districts.
Outer Terai
The Outer Terai begins south of the Sivalik Hills and extends to the Indo-Gangetic Plain. In the Far-Western Region, Nepal, it comprises the Kanchanpur and Kailali Districts; in the Mid-Western Region, Nepal, Bardiya and Banke Districts. Farther east, the Outer Terai comprises the Kapilvastu, Rupandehi, Nawalparasi, Parsa, Bara, Rautahat, Sarlahi, Mahottari, Dhanusa, Siraha, Saptari, Sunsari, Morang and Jhapa Districts.