Most of the water sources are runoff from lakes and tarns which, of course, are replenished each year by snowmelt. Just like anywhere else, the higher you go the better (cleaner) the water. That said, with the hordes of people on this trail all water should be purified.
If you're asking about availability of water, there is plenty until you hit Trail Camp. Later in the season, it can be pretty dry from that point up to the crest and on to the summit. Make sure you refill at Trail Camp.
