Some of the world 's major river systems arise in the Himalayas .
Some 's combined drainage basins are home to some 3 billion people ( almost half of the Earth 's population ) in 18 countries .
Some of the world 's major river systems arise in the Himalayas , and their combined drainage basins are home to some 3 billion people ( almost half of the Earth 's population ) in 18 countries .