The drainage pattern was already present before a period of uplift and folding that formed the present structure.

• As the uplift took place, the rivers were able to cut down at approximately the same rate and so maintain their courses.
• This process is called antecedence and the drainage system thus developed is called antecedent drainage.
• Many of the Himalayan Rivers have antecedent origin i.e. these rivers existed even before the Himalayan ranges were uplifted.
• These rivers originate in the Tibetan side beyond the mountain ranges of Himalayas.
• The Indus, Satluj, Alaknanda, Gandak, Kosi, Brahmaputra all has an antecedent origin.
• Since these rivers are antecedent, they run transverse to the mountain ranges cutting deep V-shaped, steep-sided valleys (deep gorges).

Source: NCERT XI – India Physical Environment