It is located 40km outside Las Vegas and supplies 90 per cent of its water. But over the past decade, as the city's population has grown by 400,000 to 2 million, Lake Mead has slowly been drained of more than 15 billion cubic metres of water and is now well under half full. Barnett predicts it may be a "dead pool" that provides no water by about 2036.
The lake currently looks as if someone has removed a giant plug from it.
Tom Merrit, 51, who has fished on the lake for years, pointed to the top of a faraway hill and said: "My boat used to be right up there. We've had to keep moving down and down as the water recedes."
Gesturing to a newly emerging island about 100m long, he added: "That rock never used to be there. It's really sad because this used to be a great lake. But if they don't do something soon it'll be gone."
Lake Mead's water level is currently at 330m above sea level. There are two pipes, known as "straws", that take water from it to Las Vegas.
The first extracts water at an elevation of 320m and is likely to be sucking at air, rather than water, soon. The second straw is at 305m. Lake Mead is expected to fall a further 6m towards that critical point by the end of this year.
Beneath the ground, a mammoth effort is already under way to complete a new, lower straw which will be able to draw the last of the water from the lake. But it is a painfully slow process as a giant drill the size of two football pitches advances at a rate of 2.5cm per day.
That rescue project is costing US$817 million ($930.5 million) and is expected to be complete by late 2015, but it is not viewed as a long-term solution.
Las Vegas also wants to build a separate US$15.5 billion pipeline that would pump 102 million cu m of groundwater a year from an aquifer 420km away in rural Nevada.
But a judge has refused permission after environmentalists sued on the basis that it would adversely affect 220ha of meadows, 55km of trout streams and 52,600ha of habitat used by sage grouse, mule deer, elk and pronghorn, an antelope-like creature that is endangered in the region. The court heard that 25 species of Great Basin springsnails would be pushed toward extinction.
Rob Mrowka, a Las Vegas-based scientist at the Centre for Biological Diversity, which brought the legal case against the pipeline, said: "It's a really dumb-headed proposition. It would provide a false sense of security that there's plenty of water and it would delay the inevitable decisions that have to be taken about water conservation and restricting growth.
"The drought is like a slow spreading cancer across the desert. And as the water situation becomes more dire we are going to start having to talk about the removal of people [from Las Vegas]."
Mrowka cited Lake Las Vegas, a mega-resort where stars including Celine Dion live, as one of the "most egregious examples" of wasting water.
He said: "It's a community for the rich and famous and it has a 320-acre [129.5ha] lake filled with three billion gallons [11.4 billion cubic metres] of water from Lake Mead. That's three billion gallons of drinking water, and each year they take millions more to keep it from stagnating and smelling."
Las Vegas gets just 10cm of rain in a good year. In the first four months of 2014, there has been just 3cm.
The entire state is now classified as in "severe drought" and rivers are so low that 27 million young migrating salmon are having to be taken to the ocean in trucks.
Mrowka said: "The Colorado is essentially a dying river. Ultimately, Las Vegas and our civilisation in the American southwest is going to disappear, like the Indians did before us."
