Hello Chatra

The Kosi 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 Kosi system are – the Sun Kosi, the Indravati River, the Bhote Koshi, the Dudh Kosi, the Arun River, Barun River, and Tamur River. The combined river flows through the Chatra gorge in a southerly direction to emerge from the hills.^[1][2]
The Sun Kosi contributes 44 per cent of the total water in the Sapta Koshi, the Arun 37 per cent and the Tamur 19 per cent.^[3]
Out the rivers that form the Sapta Kosi, the three main tributaries, Sun Kosi, Arun and Tamur converge at Tribeni and enter Chatra Gorge.