Climatologist Steph McAfee of the University of Nevada, Reno, says the U.S. west has always been something of an improbability.
"The average precipitation in Las Vegas is something like four inches (10 centimeters) a year," she told AFP.
"And to make it possible to have cities like Las Vegas and Phoenix and Los Angeles we rely on water that falls in the mountains as snow in parts of the West that are obviously much, much wetter."
The last two decades of drought are not, McAfee says, actually that unusual in climatic terms, according to tree ring reconstructions.
But "what's going on now is that we're having a drought, and temperatures are much warmer and when temperatures are high, things dry out faster.
"That is a consequence of climate change... driven by human greenhouse gas emissions."
On Lake Mead, boat seller Jason Davis manoeuvers his craft towards Hoover Dam, where thousands of tonnes of concrete loom over the water in graceful modernist lines, and a ring of mineral deposits shows where the water level used to be.
For him, the lake is not just a battery for the huge generators in the dam, but a waterscape whose beauty and peacefulness are worth protecting.
"You know, people who haven't been here don't appreciate it," he says as a sunset rages in the desert sky above.
"It's like, out of sight, out of mind. Hey, we're using too much water.
"Well, if you if you haven't seen these rings, you don't quite comprehend.
"Hopefully it's not too late."
