They need to shut those water-wasting fountains off as well.
The Bellagio fountains pull their water from a private well underneath the resort, which was dug to feed the predecessor property's even thirstier golf course. The water is then processed and sent back out to Lake Mead. To your point though, 12 million gallons a year are needed to replenish the water that evaporates in the 110° dry summers.
In any case, only 7% of Lake Mead's water supply is used by the hotels and casinos (including their fountains).
That's actually quite a lot given how low the water supplies already are.
