Koshi River in the context of Arun River, China-Nepal

The Indian Himalayan Region (abbreviated to IHR) is the section of the Himalayas within the Republic of India, spanning thirteen Indian states and union territories, namely Ladakh, Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, West Bengal, Arunachal Pradesh, Nagaland, Manipur, Mizoram, Tripura, Meghalaya, and Assam. The region is responsible for providing water to a large part of the Indian subcontinent and contains various flora and fauna.

Physiographically, the Himalayas start from the foothills of south (Sivaliks) and extend up to the Tibetan plateau of the north (Trans-Himalaya). Three major geographical entities, the Himadri (greater Himalaya), Himachal (lesser Himalaya) and the Sivaliks (outer Himalaya), extend almost uninterrupted throughout its length and are separated by major geological fault lines. Huge but older waterways like the Indus, Sutlej, Kali, Kosi and Brahmaputra cut steep gorges through the main Himalayan range to escape into the Great Plains, establishing their antecedence.

Mahālangūr Himāl (Nepali: महालङ्गूर हिमाल, Mahālaṅgūra himāla) is a section of the Himalayas in northeast Nepal and south-central Tibet of China extending east from the pass Nangpa La between Rolwaling Himal and Cho Oyu, to the Arun River. It includes Mount Everest, Lhotse, Makalu, and Cho Oyu — four of Earth's six highest peaks. On the Tibetan side it is drained by the Rongbuk and Kangshung Glaciers and on the Nepali side by Barun, Ngojumba and Khumbu Glaciers and others. All are tributaries to the Koshi River via Arun River on the north and east or Dudh Kosi on the south.

Mahalangur Himal can be divided into three subsections:

The Tamur River is a major river in eastern Nepal, which begins around Kanchenjunga. The Tamor and the Arun join the Sun Koshi at Tribenighat to form the giant Saptakoshi which flows through Mahabharat Range on to the Gangetic plain.

Koshi Province (Nepali: कोशी प्रदेश, romanized: Kōśī pradēśa) is an autonomous province of Nepal adopted by the Constitution of Nepal on 20 September 2015. It covers an area of 25,905 km (10,002 sq mi), about 17.5% of the country's total area. With the industrial city of Biratnagar as its capital, the province includes the towns of Birtamod, Sundar Haraincha, Damak, Dharan, Itahari, Triyuga Municipality and Mechinagar, and Mount Everest, Kangchenjunga and Ama Dablam. Koshi River, the largest river of the nation, forms the province's western boundary. Under the First-past-the-post voting system issued by the Constituency Delimitation Commission, Nepal, the province hosts 28 parliamentary seats and 56 provincial assembly seats.The province is bordered by the Tibet Autonomous Region of China to the north, the Indian states of Sikkim and West Bengal to the east, Bihar to the south, and Bagmati Province and Madhesh Province to the west. According to the 2021 Nepal census, there are around five million people in the province, with a population density of 190 per square kilometre. In the 2011 Nepal census, the province had approximately 4.5 million people.

The Arun River (Nepali: अरुण नदी) (Limbu: ᤕᤢ᤺ᤰᤑᤠ᤺ ᤛᤠᤶᤘᤠ, Yukpha: Saamwaa, in Classical Limbu) is a trans-boundary river that is part of the Koshi or Sapta Koshi river system in Nepal. 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 Sunkoshi, also spelled Sunkosi, is a river that is part of the Koshi or Saptkoshi River system in Nepal. Sunkoshi has two source streams, one that arises within Nepal in Choukati, and the other more significant stream that flows in from Nyalam County in the Tibet region of China. The latter is called Bhote Koshi in Nepal and Matsang Tsangpo in Tibet. Due to the significant flows from Bhote Koshi, the Sun Koshi river basin is often regarded as a trans-border river basin.