The phrase Inner Terai Valleys or Bhitri tarai (भित्री तराइ) refers to various elongated valleys in Nepal lying between the southernmost Himalayan foothills, the ~600 metre Chure or Siwalak Range and the 2-3,000 metre high Mahabharat Range further north, whereas Outer Terai refers to plains extending south of the Chure/Sivalik Hills to the border with India.
Major examples in Nepal include the Chitwan Valley southwest of Kathmandu and the parallel Dang and Deukhuri Valleys in western Nepal.
In the past the inner and outer Terai regions contained thick, malaria-infested forests, forming a frontier between Nepal and the British Raj (1858-1947) in India. The indigenous people, Tharu had a degree of inherited resistance to malaria and populated these areas. After an malaria eradication campaign people migrated to the Terai from the mountains and from neighbouring India. Today more than half the population are migrants, and many towns have developed.
The Inner and Outer Tarai's are Nepal's richest economic regions, with fertile farms and forests. Tarai residents have more available agricultural land than other Nepalese because of the area's generally flat terrain, which is drained and nourished by several rivers. As well, the Terai has the largest commercially exploitable forests. In the early 1990s, however, the forests were being increasingly destroyed because of growing demands for timber and agricultural land.
The Tarai is still Nepal's main source of food, and a migration destination for landless hill peasants.
The Inner Terai includes several valleys, including:
| Region | Valley | District | Rivers | Centers |
|---|---|---|---|---|
| East | Kamala | Udayapur | Kosi, Triyuga | Gaighat (or Triyuga), Katari |
| Central South | Marin Khola | Sindhuli | Sun Koshi, Kamla (Tributaries of the Koshi) | Kamalamai |
| Central | Chitwan | Chitwan | Narayani (Gandaki), East Rapti | Bharatpur |
| Mid-West | Surkhet | Surkhet | Bheri | Birendranagar |
| West | Deukhuri | Dang Deokhuri | West Rapti | |
| West | Dang | Dang Deokhuri | Babai Royal Bardia National Park | Tribhuvannagar |
| Far West | Jogbudha | Dadeldhura | Mahakali | Dadeldhura |