In a report, the WWF says India, China and Nepal could experience floods followed by droughts in coming decades.
The Himalayas contain the largest store of water outside the polar ice caps, and feed seven great Asian rivers.
The group says immediate action against climate change could slow the rate of melting, which is increasing annually.
"The rapid melting of Himalayan glaciers will first increase the volume of water in rivers, causing widespread flooding," said Jennifer Morgan, director of the WWF's Global Climate Change Programme.
"But in a few decades this situation will change and the water level in rivers will decline, meaning massive eco and environmental problems for people in western China, Nepal and northern India."
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