It originates in the Tibet Autonomous Region of the People's Republic of China where it is called the Phung Chu or Bum-chu.

The Tingri county occupies the upper reaches of the Bum-chu and the lateral valleys formed by its tributaries, the foremost of which are Lolo-chu, Shel-chu, Rongpu-chu, Trakar-chu, Kharda-chu, Ra-chu Tsangpo, and Langkor Gya-chu.

The force of its accumulated waters carves its way, south of Drengtrang, through the main chain of the Himalayas directly between the mountain massifs of Makalu and Kangchenjunga into Nepal.

The Arun drains more than half of the area contributing to the Sapta Koshi river system but provides only about a quarter of the total discharge.

The landscape south of the border tends to be steep with less than 15 percent of the area having a sustained slope of less than 15° and is strongly dissected by stream channels.

The northern third of the Nepalese portion of the Arun basin supports a rich, though human-modified, forest of mixed hardwoods, Chir pine, fir, and rhododendron at elevations of over 1,000 metres (3,300 ft).