aerial view of terai plains near Biratnagar, Nepal

The Terai is a belt of marshy grasslands, savannas, and forests located south of the outer foothills of the Himalaya, the Siwalik Hills, and north of the Indo-Gangetic Plain of the Ganges, Brahmaputra and their tributaries. The Terai belongs to the Terai-Duar savanna and grasslands ecoregion. In northern India, the Terai spreads eastward from the Yamuna River across Himachal Pradesh, Haryana, Uttaranchal, Uttar Pradesh and Bihar. Corresponding parts of West Bengal, Bangladesh, Bhutan and Assam east to the Brahmaputra River are called Dooars.^[1] The lowland plains of the Terai lie at an altitude of between 67 and 300 m (220 and 980 ft). North of the Terai rises the Bhabhar, a narrow but continuous belt of forest about 8–12 km (5.0–7.5 mi) wide.^[2]

Etymology[link]

In Hindi, the region is called तराई tarāī meaning foot-hill.^[3] In Nepali, the region is called तराइ tarāi meaning the low-lying land, plain, especially the low-lying land at the foot of the Himālayas, south to the border with India.^[4] Nepalis may also call it Madhes (Nepali: मधेस), particularly when noting its ethnic compositions similar to adjacent India and unlike the ethnic makeup of the "hills" (Nepali: पहाड​, pahārd). The region's name in Urdu is ترائي tarāʼī meaning 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.^[5]

Geology[link]

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 km² 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 deposit 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.^[6]

The Terai region has a large number of small and usually seasonal rivers, 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]

As the Terai is increasingly deforested, drained and brought under cultivation, the connection with wetness fades in contemporary usage. Terai becomes a geographic, not a hydrographic term, for districts in the plains near or bordering the Siwaliks. A permeable mixture of gravel, boulders, and sand enables the water table to sink 5–37 m (16–121 ft) deep. The Terai zone is composed of less permeable layers of clay and fine sediments, bringing groundwater to the surface in springs and wetlands.

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. The 2008 Bihar flood was only an example of recurrent catastrophes in the Terai.

Climate[link]