This lowland belt is characterised by tall grasslands, scrub savannah, sal forests and clay rich swamps. In northern India, the Terai spreads eastward from the Yamuna River across Himachal Pradesh, Haryana, Uttarakhand, Uttar Pradesh and Bihar. The Terai is part the Terai-Duar savanna and grasslands ecoregion. Corresponding parts in West Bengal, Bangladesh, Bhutan and Assam east to the Brahmaputra River are called 'Dooars'.[1] In Nepal, the Terai lies at an altitude of between 67 and 300 m (220 and 984 ft) and comprises more than 50 wetlands. North of the Terai rises the Bhabhar, a narrow but continuous belt of forest about 8–12 km (5.0–7.5 mi) wide.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 as alluvial deposits mainly of 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 Siwalik 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.[2] 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.The reduction in slope as rivers exit the hills and then transition from the sloping Bhabhar to the nearly level Terai causes current to slow and the heavy sediment load to fall out of suspension. This deposition process creates multiple channels with shallow beds, enabling massive floods as monsoon-swollen rivers overflow their low banks and shift channels. Many areas show erosion such as gullies. 20 of the 75 districts of Nepal are situated in this region.
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