Sometimes a small scale example helps, my home town of 100K people has a very small semi-navigable river pass through the middle of it, maybe a hundred feet wide and maybe head depth at most, that flows over a hundred cubic meters per second on average per wikipedia or 30K gallons per second.

Some back of the envelope engineering estimates imply 30 kilogallons per second, if somehow shoved thru plumbing, equates to around or over two billion low flow toilet flushes per day.

There's a lot of thirst on the west side of the Mississippi, not so much on the east.

Of course water treatment to drink that river water isn't free, nor sewage treatment to keep it drinkable downstream, but still, that's a lot of water for a river.

There are obvious analogies to income inequality. Humanity has more water than it knows what to do with, but simultaneously there are thirsty individuals.