The Kali Gandaki rises along the border with Tibet at an altitude of 3,900 m (12,795 ft) in the Mustang region of Nepal. . The two headwaters Nup Chhu and Shar Chhu (“West River” and “East River”) meet near Mustang’s ancient captial Lo Manthang. The river then flows southwest with the name of Mustang Khola. In Kagbeni a major tributary Kak Khola descends from Muktinath and from this point 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 — Sivalik Hills — 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 (also known as Harihar Kshetra), 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.