Arun

The Arun is the eastern Himalaya's great trans-mountain river. It rises as the Phung Chu (Bum-chu) near Gutso in Tibet's Nyalam County and drains a huge sweep of the dry plateau before turning south and slicing clean through the main chain of the Himalaya between the Makalu and Kanchenjunga massifs — a valley some 5,000 m deep, among the deepest on Earth. It enters Nepal at Kimathanka in Sankhuwasabha district.

Geologists read the Arun as a classic antecedent river: it was already flowing south before the Himalaya rose, and kept cutting its gorge as fast as the mountains came up beneath it. Its hydrology is just as distinctive — because most of its catchment lies in the rain shadow of the plateau, the Arun drains more than half of the entire Sapta Koshi basin yet supplies only roughly a quarter to a third of the system's water.

Inside Nepal the river is joined by the Barun, draining the Makalu Barun country, and the Sabha and Sankhuwa kholas. It passes below the broad river terrace at Tumlingtar before meeting the Sun Koshi and Tamor at Tribeni, where the Sapta Koshi proper begins.

Steep fall plus steady trans-Himalayan flow make the Arun corridor Nepal's biggest hydropower prize. The 900 MW Arun-3 — under construction by India's SJVN since 2018, with a 70 m-high dam and 308 m of gross head — will be Nepal's largest plant when commissioned, and the Upper Arun and Lower Arun projects are planned as a cascade on the same reach.