That's fossil water and when it's gone, it's gone. If you really are glacier-fed in a warming world, you're up the creek without a paddle, no pun intended.
It's especially clear that regions in Asia and South America are headed for a water supply crisis because once that fossil water (glacier) is gone, it's gone.
Mother nature is not going to act like a reservoir as it has in the past and when the water comes out all at once, there isn't enough capacity to contain it.