Inner Terai Valleys of Nepal (Bhitri Madhesh / Dun Valleys)

The Inner Terai Valleys, known in Nepali as Bhitri Madhesh or Bhitri Tarai (भित्री मधेश / भित्री तराई), are a series of broad, low-lying valleys that lie between the outer Siwalik (Churia) hills to the south and the higher Mahabharat Range to the north. Because they resemble the flat plains of the Terai yet sit "inside" the first line of hills, Nepali usage calls them bhitri (inner) Madhesh. In geological and geographical literature they are also called dun valleys, a name borrowed from Dehradun (Doon) in the Indian sub-Himalaya, where identical valleys occur.

Structurally, these are longitudinal valleys formed where softer rock between the Siwalik and Mahabharat ranges was eroded and then partly filled with alluvium washed down from the surrounding slopes. The Siwalik or Churia rim typically rises to about 600–900 m, while the Mahabharat wall to the north climbs to roughly 2,000–3,000 m, so the valley floors — mostly between about 150 m and 700 m — sit like sheltered troughs behind the outer hills. This position gives the inner terai a warm, humid, monsoonal climate much closer to the plains than to the hills that surround it.

Under the Land Resource Mapping Project (LRMP, 1986) — the standard classification that divides Nepal into five physiographic belts (Terai, Siwalik, Middle Mountains, High Mountains and High Himal) — the inner terai dun valleys are generally grouped within the Siwalik physiographic zone rather than the true Terai plain along the Indian border. In effect they are pockets of Terai-like lowland that the geology of the Himalayan foothills has pushed north into the hill country.

Running from north-west to south-east, geographers recognise a series of inner terai valleys, each enclosed by its own stretch of Siwalik and Mahabharat hills and drained by a river that eventually cuts south through the Churia to reach the Ganges plain. The best known, in order, are listed below.

Alongside these, smaller inner terai pockets are sometimes named separately, including the Kamala and Marin plains around Sindhuli and the Tinpatan and Trijuga flats, but they belong to the same dun-valley family and share the same geology and history.

For most of recorded history the inner terai and the outer Terai were shielded by the Char Kose Jhadi (चार कोशे झाडी), literally the "four-kos-wide forest." A kos is a traditional unit of about 3 km, so char kose describes a dense belt of sal (Shorea robusta) forest and tall grassland roughly a dozen kilometres deep that once ran almost continuously along the foot of the hills. This jungle, and the malarial fevers that haunted it, kept the lowlands thinly settled.

The forest was more than an accident of nature. Nepal's rulers valued the Char Kose Jhadi as a defensive frontier against invasion from the south and as a royal hunting reserve stocked with tiger, rhinoceros and elephant. Because malaria (aulo) was lethal to outsiders through much of the year, the hill population could not easily colonise the valleys, and much of the forest was left intact well into the twentieth century.

The chief exception were the Tharu, the indigenous people of the Terai and inner terai. Having lived in these forests for centuries, the Tharu carried a relative resistance to malaria; medical studies later found malaria incidence in Tharu communities several times lower than among neighbouring groups, a gap researchers attributed to genetic adaptation. The Tharu therefore farmed and herded in valleys such as Dang, Deukhuri and Chitwan long before the great hill migrations began.

The inner terai was transformed in the mid-1950s (about 2011–2013 BS). With support from the World Health Organization (WHO) and the United States (through what became USAID), the Government of Nepal launched a malaria-control programme that sprayed DDT across the Terai and inner terai jungles. Within a few years the disease was brought under control, and land that had been almost uninhabitable became some of the most sought-after farmland in the country.

What followed was one of the largest internal migrations in modern Nepali history. Pahari (hill) communities — Bahun, Chhetri, Newar, Magar, Gurung and others — moved down from the crowded mid-hills into the newly safe valleys, clearing forest and grassland for paddy, mustard and other crops. Planned resettlement schemes, most famously the Rapti Valley Development Programme in Chitwan from the mid-1950s, accelerated the change.

The migration reshaped the population and often dispossessed the original inhabitants. As forests were cleared and land titling favoured newcomers, many Tharu and other indigenous families lost customary land and were pushed into tenancy or the bonded-labour (kamaiya) system, especially in the western valleys of Dang, Deukhuri and Surkhet. The inner terai today is therefore ethnically mixed, with Tharu, Danuwar, Hayu and Bote communities living alongside a large hill-origin population.

The floors of the inner terai valleys are built largely of recent alluvium — fertile silts, sands and gravels washed off the Mahabharat and Siwalik slopes and spread across the valley by rivers such as the Babai, West Rapti, Rapti, Narayani and Triyuga. Away from the coarse, gravelly Bhabar zone that fringes the Churia, these deep, well-watered soils are highly productive, and Dang, Deukhuri and Chitwan rank among Nepal's important grain and oilseed producing areas.

Climatically the valleys are tropical to subtropical, with a hot pre-monsoon season, a heavy summer monsoon (roughly June to September) and a dry, mild winter. Rainfall is substantial: at the Rampur station in Chitwan, mean annual rainfall was about 2,214 mm over the period 1995–2006. Sheltered behind the Churia, the valley floors are generally warmer and more humid than the Mahabharat hills immediately to the north.

Because they combine flat, fertile land with river access and improving road links, the inner terai valleys now hold fast-growing towns — Birendranagar in Surkhet, Ghorahi and Tulsipur in Dang, Bharatpur in Chitwan, Kamalamai (Sindhulimadi) in Sindhuli and Gaighat in Udayapur — and are threaded by major routes such as the East–West (Mahendra) Highway spurs, the BP Highway and the Rapti and Karnali corridors.

The same forests that once made the valleys dangerous now make them globally important for wildlife. Chitwan Valley holds Chitwan National Park, gazetted in 1973 (2030 BS) as Nepal's first national park and inscribed as a UNESCO World Heritage Site in 1984. Its sal forest, grassland and floodplain shelter the greater one-horned rhinoceros, the Bengal tiger and the Asian elephant, and it anchors Nepal's nature-tourism industry.

Other inner terai and neighbouring valleys carry their own reserves. Banke and Bardia National Parks flank the Dang–Deukhuri lowlands to the south-west, while the Kamala/Trijuga valley in Udayapur lies close to the Koshi Tappu Wildlife Reserve, established in 1976, Nepal's key wetland for the wild Asian water buffalo (arna) and for migratory birds. Together these protected areas make the inner terai a cornerstone of Nepal's biodiversity conservation.