Kali Gandaki

The Kali Gandaki Gorge or Andha Galchi is the gorge of the Kali Gandaki (or Gandaki River) in the Himalayas in Nepal. Gorge depth is difficult to define due to disagreement over rim height, but by some measures the Kali Gandaki is the deepest gorge in the world.
The upper part of the gorge is also called Thak Khola after the local Thakali people who became prosperous from trans-Himalayan trade. Geologically, it is a downfaulted graben.^[1]
The gorge separates the major peaks of Dhaulagiri (8,167 m/26,795 ft) on the west and Annapurna (8,091 m/26,545 ft) on 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 directly between Dhaulagiri and Annapurna I (7 km downstream from Tukuche) is at an elevation of 2520 metres, 5571 metres lower than Annapurna I.^[2] The river is older than the Himalayas. As tectonic activity forced the mountains higher, the river cut down through the uplift. This region is known for shaligram fossils, revered as one of five non-living forms of Lord Vishnu.
The Kali Gandaki river source coincides with the Tibetan border and Ganges-Brahmaputra watershed divide. The river then flows south through the ancient kingdom of Mustang. It flows through a sheer-sided, deep canyon immediately south of the Mustang capital of Lo Manthang, then widens as it approaches Kagbeni where high Himalayan ranges begin to close in. The river continues southward past Jomsom, Marpha, and Tukuche to the deepest part of the gorge about 7 km south of Tukuche in the area of Lete. The gorge then broadens past Dana and Tatopani toward Beni.
The Kali Gandaki gorge has been used as a trade route between India and Tibet for centuries. Today, it is part of a popular trekking route from Pokhara to Muktinath, part of the Annapurna Circuit. The gorge is within the Annapurna Conservation Area.<sup>[3]

source : http://en.wikipedia.org/wiki/Kali_Gandaki_Gorge</sup>