The Gandaki River (also, the Kali Gandaki, the Narayani in Nepal, and the Gandak in India) is one of the major rivers of Nepal and a left bank tributary of the Ganges in India. The headwaters of the river are formed at an altitude of 3,900 m (12,795 ft) in the Mustang region of Nepal, which borders Tibet. The river is notable for the deep gorge through which it flows and for a large hydroelectric facility in Nepal. It has a total catchment area of 46,300 sq km, most of it in Nepal, which lies between the Kosi system to the east and the Karnali system to the west.

The Kali Gandaki rises in the Tibetan plateau north of the Himalayas in the Mustang District of Nepal, near the border with Tibet. The two headwaters of river, Nup Chhu and Shar Chhu ("West River" and "East River") meet near the town of Lo Manthang in upper Mustang. The river then flows southwest with the name of Mustang Khola. In Kagbeni a major tributory, Kak Khola, coming from Muktinath, meets Mustang Khola; from there, the river is called the Kali Gandaki.

The river then flows southward through a steep gorge known as the Kali Gandaki Gorge, or Andha Galchi, between the mountains Dhaulagiri (8167 m) to the west and Annapurna (8091 m) to the east. If one measures the depth of a canyon by the difference between the river height and the heights of the highest peaks on either side, the Gorge is the world's deepest. The portion of the river between the Dhaulagiri and Annapurna massifs is at an elevation of between 1300 metres and 2600 metres, 5500 to 6800 metres lower than the two peaks. The river is older than the Himalayas. As tectonic activity forces the mountains higher, the river has cut through the uplift.

South of the gorge, the river is joined by Rahught Khola at Galeshwor, Myagdi Khola at Beni, Modi Khola near Kushma and Badigaad at Rudrabeni. The river then takes a right-angle turn and runs east along the northern edge of the Mahabharat Range. The largest hydroelectricity project in Nepal is located along this stretch of the river. Turning south again and breaking through the Mahabharats, Kali Gandaki is then joined by a major tributary, Trishuli at Devighat, then by the (East) Rapti draining the Inner Terai valley known as Chitwan. The Gandaki then crosses the outermost foothills of the Himalayas into the Terai plains of Nepal. From Devighat, the river flows southwest and is known as Narayani or Sapt Gandaki. The river later curves back towards the southeast as it enters India.

The river flows southeast across the Gangetic plain of Bihar state, eventually merging with the Ganga near at Hajipur, or Patna.The entry point of the river at the Indo-Nepal border is also the confluence of Gandak, known as Triveni, with rivers Pachnad and Sonha also descending into India from Nepal.Pandai river flows into Bihar (India) from Nepal in the eastern end of the Valmiki Sanctuary and meets Masan. its drainage area in India is 7620 sqkm. After the river enters India at Triveni, it travels for a further length of 300 km before it joins the Ganga, just downstream of Hajipur at Sonepur, near Patna.Bfore joining the river Ganga nar patna, it flows through Champaran, Sarang and Muzaffarpur districts. The total length of the river is 630 km out of which 330 km lies in Nepal and Tibet.

It is said that the Gandak River has shifted about 80 km to the east due to tilting in the last 5000 years. The Gandak plains, called the Gandak Megafan comprises Eastern Uttar Pradesh and North Western Bihar and lies in the Middle Gangetic Plains.

The Gandaki river basin is reported to contain 1025 glaciers and 338 lakes.

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