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 . 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
2008 Bihar flood
2008 Bihar flood
Terai