---------- The water will get saltier and saltier as it evaporates and they take out fresh water. They would need a return flow to keep the salt constant.
But that would take centuries -- and meanwhile, the inflow would generate lots of electricity -- and it would make Las Vegas a seaport city. Before it gets too salty the Snake River could be diverted from Twin Falls (elev 3745 ft) into Nevada -- there's more electricity.
How long it took the lake to get salty would depend on the inflow rate & size of the lake.
Besides, the greens would never let you flood Death Valley, even if it would save thousands of human lives - you might kill some horned toads or such.