If Ganga is coming from heaven, then why is it drying up?

From: Krishna Dhan Das

River Ganges originated from karana ocean (Vamana deva pastime). Does river ganges till receives that water?

If yes, then how do ecologists say that if pollution and melting of ice caps continues with same pace ganges is going to dry by 2050? If no, then is it that ganga receiving water from Karana ocean is one time event? (it challenges the purity of ganges) Please clarify.