The glaciers of the Himalayas gave birth to many great rivers : The Yangtze , Ganges, Indus and Mekong .
Over a billion people depend directly on the Himalayas for their survival .
Scientists report that the permafrost is melting , disrupting a major water source .
The swollen glacial lake Imja in Nepal faces the danger of a violent ‘ glacial lake outburst flood ’ (GLOF).