Nineteen of the world's major rivers including the Brahmaputra, the Indus, the Ganges, and the Yangtze, originate in the Himalayas, and their composite drainage basin is home to an estimated 1.3 billion people. Physically, the Himalayas form three parallel zones of varying width, each having unique physiographic features and its own geologic history.
The Great Himalayas, the highest zone and the backbone of the entire mountain system, consists of a colossal line of snowy peaks with an average height exceeding 6,100 m/20,000 ft. It trends northwest-southeast from Jammu and Kashmir to Sikkim, an old Himalayan kingdom that is currently a state in India. East of Sikkim, the range goes east-west for another 420 km through Bhutan and the eastern part of Arunachal Pradesh as far as the peak of Kangto and finally turns northeast, terminating at Namcha Barwa.