How are himalayan rivers older than himalayan mountains?

A river rising on the southern sides of Himalayas will have steeper gradients and since the eastern Himalayas enjoy heavier rainfall it is possible to assume that this river by headward erosion would have pushed back to its origin to the otherside and captured the upper portion of the Tsangpo. read more

That situation exists because the major Himalayan rivers, such as the Indus, the Brahmaputra, the Sutlej, and at least two headwaters of the Ganges—the Alaknanda and the Bhagirathi—are probably older than the mountains they traverse. read more