The Koshi or Sapta Kosi drains eastern Nepal. It is known as Sapt Koshi because of the seven rivers which join in east-central Nepal to form this river. The main rivers forming the Koshi system are – the Sun Koshi, the Indravati River, the Bhote 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.[2][3]
The Sun Koshi contributes 44 percent of the total water in the Sapta Koshi, the Arun 37 per cent and the Tamur 19 percent.[4]