• The northern plains are formed by the alluvial deposits brought by the rivers – the
  Indus, the Ganga and the Brahmaputra.
• These plains extend approximately 3,200 km from the east to the west. The average width of these plains varies between 150-300 km.
• The maximum depth of alluvium deposits varies between 1,000-2,000 m. From the
  north to the south, these can be divided into three major zones: the Bhabar, the Tarai and the alluvial plains. The alluvial plains can be further divided into the Khadar and the
• Bhabar is a narrow belt ranging between 8-10 km parallel to the Shiwalik foothills at
  the break-up of the slope. As a result of this, the streams and rivers coming from the
  mountains deposit heavy materials of rocks and boulders, and at times, disappear in this zone. South of the Bhabar is the Tarai belt, with an approximate width of 10-20 km where most of the streams and rivers re-emerge without having any properly demarcated channel, thereby, creating marshy and swampy conditions known as the Tarai.
• This has a luxurious growth of natural vegetation and houses a varied wildlife.
  The south of Tarai is a belt consisting of old and new alluvial deposits known as the
  Bhangar and Khadar respectively.

Source: NCERT- Indian Physical Geography