Bihar is divided into three physiographic units on the basis of physical and structural conditions:
- Shiwalik Range
- Bihar Plain
- Southern Plateau Region.
Shiwalik Range
This range shadows the state from Northern part of West Champaran district over an area 32 km long and 6-8 km wide and it is divided into sub-divided into three parts on the basis of variation:
- Ramanagar Doon
- Someshwar Range
- Harha Valley
Bihar Plain
The 
plains of Bihar, adjoining Nepal, are drained by a number of rivers that have their catchments in the steep and geologically nascent Himalayas. Kosi, Gandak, Burhi Gandak, Bagmati, Kamla Balan, Mahananda and Adhwara Group of rivers originates in Nepal, carry high discharge and very high sediment load and drops it down in the plains of Bihar. This plain is sub-divided into two parts on the basis of characteristics:
Northern Plain
It is located in East & West Champaran (Terai area with higher elevation), and Chaurs of Samastipur, Begusarai, Saharsa and Kathihar districts. Region is drained by Saryu, Gandak, Burhi Gandak, Bagmati, Kamla-Balan, Kosi and Mahanadi and their tributaries.
Southern Plain
It is narrow than northern plain of Bihar and triangular in shape because many hills are located in this region such as hills of Gaya, Rajgir, Giriak, Bihar Sharif, Sheikhpura, Jamalpur and Kharagpur hills.
Southern Plateau Region
It is located between Kaimur districts in the West to Banka in the East. It is made up of hard rock’s like gneiss, schist and granite. This region blessed with many conical hills which are made up of batholim like Pretshil, Ramshila and Jethian hill.
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