The Terai Region

The Terai or Madhesh region begins at the Indian border and includes the northermost part of the flat, intensively farmed Gangetic Plain called the Outer Terai. This is culturally an extension of northern India with Hindi, Awadhi, Bhojpuri and Maithili spoken more than Nepali, however it was annexed to Nepal by conquest and by treaty with the British.
The Outer Terai ends at the first range of foothills called the Siwaliks or Churia. This range has a densely forested skirt of coarse alluvium called the bhabhar along its base. Below the bhabhar, finer, less permeable sediments force groundwater to the surface in a zone of springs and marshes. In Persian, terai refers to wet or marshy ground. Before the use of DDT it was dangerously infested with malaria. Nepal's rulers used it as a defensive frontier called the char kose jhadi (twelve kilometer forest)
Above the bhabhar belt, the Siwaliks rise to about 700 metres (2,297 ft) with peaks as high as 1,000 metres (3,281 ft), steeper on their southern flanks because of faults known as the Main Frontal Thrust. This range is composed of poorly consolidated, coarse sediments that do not retain water or support soil development so there is virtually no agricultural potential. Hillside vegetation is limited to scrub forest and the area functions as a deserted buffer zone allowing the development of distinctive cultures in valleys and hills further north. In several places beyond the Siwaliks there are dun valleys called Inner Terai with productive soil. Among these are Dang and Deukhuri in western Nepal and the Rapti Valley (Chitwan) in central Nepal. Population in these valleys was historically limited by malaria and mainly limited to the Tharu ethnic group that had developed genetic resistance. Around 1960 DDT came into use to suppress mosquitos and the way was open to settlement from the land-poor hills to the detriment of Tharus.