The Koshi or Sapt Kosi drains eastern Nepal. It is known as Sapt Koshi because of the seven rivers which join together in east-central Nepal to form this river. The main rivers forming the Koshi system are – the Sun Koshi River, the Indravati River, the Bhola Koshi, the Dudh Koshi, Arun River, Barun River, and Tamur River. The combined river flows through the Chatra Gorge in a southerly direction to emerge from the hills.
The Sun Koshi contributes 44 per cent of the total water in the Sapta Koshi, the Arun 37 per cent and the Tamur 19 per cent.