Physiographic Divisions of Bihar

Bihar can be broadly divided into three physiographic regions:

1. Shiwalik (Sub-Himalayan) Range

• Location: North-western Bihar (West Champaran).
• Area: Length ~32 km, width 6–8 km.
• Subdivisions:
  • Ramnagar Dun (Harha Valley): Small hills (~240 m), southernmost part, area ~214 sq km.
  • Someshwar Ranges: Highest point – Someshwar Fort (874 m), forms part of India–Nepal border, area ~75 sq km.
  • Doon Valley (Harha Valley): Lies between Ramnagar Dun and Someshwar Range, drained by Harha River, area ~643 sq km.
• Geology: Formed from Tertiary rocks; part of recent geological epochs (Pliocene–Pleistocene).

2. Bihar Plains (Indo-Gangetic Plain)

• Area: ~90,650 sq km, about 95% of Bihar.
• Divisions: Split by the Ganga River into North Bihar Plains and South Bihar Plains.

North Bihar Plains

• Formation: Made of alluvial deposits brought by northern tributaries of the Ganga, such as Ghaghara, Gandak, Bagmati, Burhi Gandak, Kosi, and Mahananda.
• Geology: Quaternary rock system.
• Location and Extent: North of the Ganga, covering Tirhut, Saran, Darbhanga, and Kosi divisions; from Ghaghara–Gandak Doab (west) to Mahananda valley (east).
• Main Rivers: Ghaghara, Gandak, Bagmati, Kamla, Kosi, Mahananda; characterized by oxbow lakes (Chuar formation).

Major Doabs and Features:

1. Ghaghara–Gandak Doab

• Districts: Saran, Siwan, Gopalganj
• Main crops: Paddy, maize, wheat, sugarcane, oilseeds, pulses
• Industries: Agriculture-based; sugar industry prominent
• Key industrial centers: Gopalganj, Chhapra, Siwan, Mirganj, Mehraura

2. Gandak–Kosi Doab

• Prominent industries: Dairy (Barauni, Muzaffarpur)

3. Kosi–Mahananda Doab

• Districts: Purnia, Araria, Kishanganj, Madhepura, Khagaria, Saharsa
• Features: Flood-prone, underdeveloped in agriculture, industry, and transport
• Industries: Sugar and fruit processing\
• Key centers: Chanpatia, Sugauli, Samastipur, Motihari, Bagaha
• Main crops: Paddy, maize, wheat, barley, pulses, oilseeds; cash crops: sugarcane, tobacco, red chilies
• Region-specific produce:
  • Darbhanga – Mango
  • Muzaffarpur – Litchi
  • Hajipur – Banana
  • Barauni – Fertilizer factory, oil refinery, thermal power station

Subdivisions of North Bihar Plains:

• Ghagra Plain: Westernmost part – Siwan, Gopalganj, Saran
• Gandak Plain: Between Bagmati and Ghaghara plains
• Bagmati Plain: Contains Chaurs (oxbow lakes), spread over Sitamarhi, Muzaffarpur, Samastipur, East Champaran, Sheohar
• Kamla Plain: Central part, surrounded by Kosi (east), Bagmati (west), Indo-Nepal border (north), Ganga (south); Chaurs formed due to river course changes
• Kosi Plain: Spread over Supaul, Saharsa, Madhepura, Madhubani, Darbhanga; flood-prone, river changes course frequently
• Mahananda Plain: Easternmost part, bordered by Indo-Nepal (north), Ganga (south), West Bengal (east), Kosi (west)

3. Southern Plateau Region

• Description: Hard-rock plateau extending from Kaimur to Banka districts.
• Geology: Composed of gneiss, schist, granite; rich in minerals.
• Hills and Ridges: Pretshila, Ramshila, Jethian, Rajgir, Kharagpur, Brahmayoni
• Rivers: Son, Punpun, Koel, Panchane, Karmanasha flowing northwards from the plateau
• Subdivisions:
  • Western Part: Extension of Chota Nagpur Plateau – Kaimur, Rohtas, Aurangabad, Gaya, Nawada
  • Eastern Part: Continuation of Rajmahal Hills – Banka, Munger, Jamui, parts of Bhagalpur