Nepalese rivers can be grouped into three categories on the basis of their origin
(1) antecedent to Himalaya,
(2) after the Mahabharat. and
(3) after the Churia range.
Antecedent rivers belong to the period prior to the rise of the Himalaya. These rivers added their tributaries during or after the Himalayan origin alongwith the developement of monsoon climate. After the formation of Mahabharat hills, the antecedent rivers changed their courses as Mahabharat stood as a barrier. As a result, most of the rivers changed their courses either to the east or west. Most of these rivers were responsible to deposit the sediments in the Churia basin.
The major river systems namely the Koshi, the Karnali and the Gandaki belong to the antecedent group. Rivers originating from the Mahabharat range and cutting through Churia hills come under the second group, these include Kankai, Bagmati, Kamala etc. The third group of rivers originate from the southern face of the Churia hills. For the purpose of commercial rafting, the following rivers are in use.
1. Saptakoshi River System (East Nepal)
2. Narayani or Saptagandaki River System (Central Nepal)
3. Karnali River System (West Nepal)
Eleven rivers in the three river systems are open for tourists for rafting. The rivers are:
The Trishuli (Trisuli-Narayanghat)
The Kali Gandaki (Kusma-Tiger Tops)
The Bheri (Birendranagar- Chisapani)
The Seti (Damouli-Tiger Tops)
The Sunkoshi (Baseri-Chatara)
The Karnali (Karaleghat-Chisapani)
The Tama Koshi (Busti-Chatara)
The Arun (Tumlingtar-Chatara)
The Budhi Gandaki (Arughat-Trishuli)
The Bhote Koshi (Kodari-Lartza Bridge)
The Marshyangdi (Ngadi-Bimalnagar)