On the basis of the river, Nepal is divisible into three regions.
(A) Koshi Pradesh (Region)
The area between Kanchenjunga Himal in the east and Langtang Himal (Gosaisthan) in the west is called Koshi Pradesh. Tamor, Arun, Likhu, Dudhkoshi, Tamakoshi, Sunkoshi and Indrawati form the Saptakoshi River. It is the largest river in Nepal. This river is 720 km long. The water flow capacity is 1,564 cubic meters per second. It is estimated that up to 22,000 MW of electricity can be generated from this river. The area of influence of Koshi Pradesh is about 60,000 sq. Km. The largest tributary of the Saptakoshi River is the Arun and the smallest is the Likhu River.
(B) Gandaki Pradesh (Region)
The affected area of Gandaki River from Langtang Himal in the east to Dhaulagiri Himal in the west is called Gandaki Pradesh. Seven major tributaries of the Gandaki River are Trishuli, Budhigandaki, Daraudi, Madi, Marsyangdi, Seti and Kaligandaki. The river is believed to have been named after Gandava Rishi. After coming to the Chitwan district (below Devghat) it is known as the Narayani river. This river flows through Central Nepal, with an affected area of about 38,000 sq km. This river is about 338 km. long. It has considered having an electricity generation capacity of about 21,000 MW. Among its tributaries, Kaligandaki is the largest river and Daraudi is the smallest. Gandaki is the deepest river in Nepal with an average of 1713 cubic meters per second water flow capacity.
(C) Karnali Pradesh (Region)
The affected area of Karnali River between Dhaulagiri Himal in the east and Byas Himal in the west is called Karnali Pradesh. The seven major tributaries of the Karnali River are Humla Karnali, Mugu Karnali, Thuli Bheri, Sani Bheri, Tila, Budhiganga and Seti. It is the longest river in Nepal. When this river reaches India, it is also known as Ghaanghara and somewhere Saryu. The river covers an area of about 42,000 sq. km. It is about 507 km Long. The electricity generation capacity from this river is estimated to be 32,000 MW. The average water flow capacity of this river is 1316 cubic meters per second.