The river Brahamputra drains not only the southern part of the Himalayas but also the northern slopes and the watershed is much further north than the main axis of the great Himalayan range. Such a drainage is called

Antecedent drainage refers to a river that has maintained its course and direction despite the rise of mountain ranges around it. The Brahmaputra flows northward through the Himalayas (which rose later), meaning it predates the main Himalayan orogeny and has an antecedent drainage pattern with its watershed north of the main range.