That river is drying up. This winter, snow in the Rocky Mountains, which feeds the Colorado, was 70% lower than average. Last month, the US government calculated that two thirds of Arizona is currently facing severe to extreme drought; last summer 50 flights were grounded at Phoenix airport because the heat – which hit 47C (116F) – made the air too thin to take off safely. The “heat island” effect keeps temperatures in Phoenix above 37C (98F) at night in summer. 

Phoenix and its surrounding area is known as the Valley of the Sun, and downtown Phoenix – which in 2017 overtook Philadelphia as America’s fifth-largest city – is easily walkable, with restaurants, bars and an evening buzz. But it is a modern shrine to towering concrete, and gives way to endless sprawl that stretches up to 35 miles away to places like Anthem. The area is still growing – and is dangerously overstretched, experts warn.

“There are plans for substantial further growth and there just isn’t the water to support that,” says climate researcher Jonathan Overpeck, who co-authored a 2017 report that linked declining flows in the Colorado river to climate change. “The Phoenix metro area is on the cusp of being dangerously overextended. It’s the urban bullseye for global warming in north America.”

One of those plans is Bill Gates’s new “smart city”. The Microsoft founder recently invested $80m (£57m) in a development firm that aims to construct80,000 new homes on undeveloped land west of Phoenix, and a new freeway all the way to Las Vegas.

Another firm wants to build a “masterplanned community”, like Anthem, south of Tucson, and modelled after the hilltop towns of Tuscany. It envisages five golf courses, a vineyard, parks, lakes and 28,000 homes. The promotional video contains no details about where all the water will come from, but boasts: “This is the American dream: whatever you want you can have.”

What these cities want is water. The Phoenix area draws from groundwater, from small rivers to the east, and from the mighty Colorado. The Hoover Dam holds much of the Colorado’s flow in the vast Lake Mead reservoir, but the river itself is sorely depleted. That water has now dropped to within a few feet of levels that California, Nevada and Arizona, which all rely on it, count as official shortages.