The Sun Kosi's headwaters are located in the Zhangzangbo Glacier in Tibet. Its upper course, the Bhote Koshi, is known as Poiqu in Tibet. Both river courses together form one basin that covers an area of about 3,394 km2 (1,310 sq mi).
The Indravati meets the Sun Kosi at Dolaghat, up to where it is followed by the Arniko Rajmarg. From there, the Sun Kosi flows eastwards through the valley formed between the Mahabharat Range and the Himalayas. Tamakosi, Likhu, Dudhkosi, Arun, and Tamor are its left tributaries and Indravati is the right tributary.
The average annual flow is 22 x 109 m3. The average sediment load is 54 x 106 m3.
The Tamur and the Arun rivers join the Sun Kosi at Tribenighat to form the Saptkoshi, which flows through the Chatra Gorge across the Mahabharat Range on to the Gangetic plain.
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