Saptakoshi River - Geography

Geography

In Nepal, the Koshi basin is bordered on the west and the south-west by the basins of the Gandaki and the Burhi Gandak. The Bagmati river (draining Kathmandu Valley) sub-basin forms the south-western portion of the overall Koshi basin. The Kanchenjunga Himalayas are its eastern watershed, with the Singalila Ridge forming the border with the Teesta basin. Its basin contains 6 of the 14 mountains in the world taller than 8,000m (including 5 of the 6 highest). The 6 peaks are Mount Everest, Kangchenjunga, Lhotse, Makalu, Cho Oyu and Shishapangma. Mount Everest is the highest point surrounding the Koshi basin. The Koshi has seven major tributaries: Sun Kosi (Nepali: सुनकोशी), Tamakoshi (तामाकोशी) or Tamba Koshi, Dudh Kosi (दुधकोशी), Indravati (इन्द्रावती), Likhu (लिखू), Arun (अरुण) and Tamur (तमोर).

The Dudh Kosi joins the Sun Kosi at the Nepalese village of Harkapur. At Triveni the Sunkoshi from the west is joined by the Arun from the north (the Dudhkoshi and Arun encircle Mount Everest), then the Tamar from the east. The combined river becomes the Saptakoshi (सप्तकोशी), literally "Seven Koshis". The Saptakoshi crosses the Lesser Himalaya or Mahabharat Range in a deep gorge some 10 km (6.2 mi) long. At Barāhkṣetra in Nepal it emerges from the mountains and becomes the Koshi. After flowing another 58 km (36 mi) it crosses into Bihar, India, near Bhimnagar and after another 260 km (160 mi) joins the Ganges near Kursela. The river has a total length of 729 km (453 mi).

The Kosi alluvial fan is one of the largest in the world, and extends from Barāhkṣetra across Nepalese territory, covering northeast Bihar and eastern Mithila to the Ganges, 180 km (110 mi) long and 150 km (93 mi) wide. It shows evidence of lateral channel shifting exceeding 120 km (75 mi) during the past 250 years, via at least twelve major channels. The river, which flowed near Purnea in the 18th century, now flows west of Saharsa. A satellite image shows old channels with a confluence before 1731 with the Mahananda River north of Lava.