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 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.
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.
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.
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