Correct Option
The correct option is 1 and 2 only.
Explanation
The Himalayan drainage system evolved through tectonic uplift, river dismemberment, and diversion of earlier longitudinal drainage.
The ancient Indo-Brahma river is considered to have flowed along the longitudinal extent of the Himalayas from the Assam region towards the Punjab and Sind region, indicating an east-to-west course. Its later dismemberment produced separate drainage systems such as the Indus, Ganga, and Brahmaputra systems.
- Statement 1: Correct. The Indo-Brahma river is described as an ancient longitudinal river flowing from east to west along the Himalayan foreland.
- Statement 2: Correct. Its dismemberment is linked to tectonic disturbances, including the Pleistocene upheaval in the western Himalayas, which affected the drainage pattern.
- Statement 3: Incorrect. The down-thrusting of the Malda gap helped divert the Ganga and Brahmaputra systems towards the Bay of Bengal, not the Arabian Sea.
Therefore, statements 1 and 2 are correct.
Key Takeaway: The Indo-Brahma river flowed east to west, but tectonic changes dismembered it and diverted the Ganga-Brahmaputra drainage towards the Bay of Bengal.