The south-western US is suffering its eighth consecutive year of drought. There are concerns that the Colorado River, which has sustained life in the area for thousands of years, can no longer meet the needs of the tens of millions of people living in major cities such as Las Vegas and Los Angeles.
The BBC's Matthew Price is travelling along the river to investigate the scale of the problem and is sending a series of diary items from there.
DAY TWO: LAKE MEAD
It takes slightly longer to get to Lake Mead these days.
That's because for the past few years, the largest man-made lake in the US has been shrinking.
So you have to drive further round to hit the water's edge, towards the famous Hoover Dam, which was built back in the 1930s to help regulate the flow of the Colorado River and to guarantee a constant water supply to Las Vegas and other cities in the American south-west.
Eventually you arrive at a launch ramp. "American Angler Captain R W 'Bob' Wood" (that's what his card says) is waiting on the pontoon with his fishing boat.
"We used to launch the boat clear back where you turned off on the highway. It's probably close to a half mile away," he says.
In just six or seven years, the water level has dropped significantly. Where we are now bobbing on the surface is 102ft (31m) below the old water level.
Bathtub line
We idle out of the little harbour - the marina used to be here, but a few months ago they towed it away because the water level had made it difficult to get the boats in and out. It's now further round the lake, in deeper water.
To the left there's a water intake pump, way, way above our heads. It used to be on the water's surface.
There are towering rock formations, rising up above the land, that just a few years ago were islands in the lake.
The most telling sight though is what they call the bathtub line.
We speed towards one section of the lake shore, where the distant white line gets larger and larger, until the boat is alongside a vast towering cliff face, perhaps 600ft or 700ft high.
The rock is naturally black, but where the water used to be it has turned white, because of the calcium in the water. The bathtub line is 100ft high here.
This matters.
The reservoir, and the Colorado River which serves it, is still the principle water source for millions of Americans. Some argue that one day there may not be enough water to feed the cities here.
There are disagreements as to why water levels in the lake are dropping.
Many environmentalists argue climate change is to blame.
Richard Seager from the Lamont Doherty Earth Observatory told me that they expect global warming to reduce the amount of water in the area by 10% to 15% by the middle of the century.
"It's comparable to the 1930s dust bowl drought, but instead of that lasting just a few years… it will become the new drier climate that will be there for ever more," he says.
Others - Captain Bob included - do not believe climate change is to blame, arguing that weather is cyclical. He is certain the lake will regain its normal levels one day.
Desert
In a sense, it does not matter what is causing it. Both sides of this debate agree on the fact that humans are using too much water in this part of the US.
"Down in San Diego, there are people watering the grass - and it's raining! Too many people using the water, yes," Captain Bob says.
It is not just people watering their gardens - indeed most scientists say that is almost a minor problem.
It is estimated that around one tenth of the water taken from reservoirs like Lake Mead is used by the urban area - by households, hotels and businesses.
Most of the water that comes down the Colorado River, around 90%, is used by agriculture, and most of that is in vast farms, which help keep America in fruit and veg.
Others are smaller concerns, like that of Robert Houston who farms up near the north rim of the Grand Canyon.
Like farms across this desert area, he sprays his crop of alfalfa with a series of sprinklers.
"Water resources are very scarce," he agrees.
Are they being overused, I ask.
"I guess there's always that chance. The whole world's getting overused. All our resources are getting overused. [There are] more and more people."
But, he adds: "We might as well use the water rather than just letting it flow into the ocean."
Back on Lake Mead, that bathtub rim of white flashing past our heads as we speed back to the pontoon, Captain Bob admits that they are now relying on the weather. They need more snow up on the Rocky Mountains, he says, to feed the river and the lake.
Still, he says, "Sooner or later water is going to become a major, major problem.
"We have to do things to curb water usage."
It's not exactly a radical idea, considering the fact that we're in the middle of a region dominated by deserts and naturally low precipitation.
Do though people who live here realise that they are living in a desert?
"I don't think they do," says Captain Bob. "I think the older people realise it. I think the people who move here from [elsewhere] haven't a clue. They have no idea that it's dry."
Tomorrow Matthew Price rides along with the water police in Las Vegas, one of the ways city officials are trying to reduce the water that people use.
DAY ONE: PAGE, ARIZONA
It takes several hours to get to Page, Arizona. From anywhere.
The drive, though, is far from dull. It's one of those journeys that can make you feel incredibly insignificant.
Vast landscapes dwarf everything made by man. The cars and trucks speeding along the desert highways appear as small as model vehicles.
You could stick the skyscrapers of Manhattan, from where I flew in a few hours earlier, next to the immense rock formations, and they would look like Toy Town.
In places the landscape falls sharply away into canyons, in others it rises up towards plateaus, and everywhere the geological history of the place is obvious.
Today, as I walked alongside the Colorado River just outside the town of Page, I saw two prints in the red Navajo sandstone, each with three "toes". It was the fossilised footprint of a dinosaur which had stood at the same spot many thousands of years before me.
This land is sacred to the Native Americans who live here.
Shana Watahomigie is a park ranger with the National Parks Service. She is also a member of the Havasupai tribe, which still lives alongside the Colorado River in the Grand Canyon.
Havasupai means "people of the blue-green water", and, as we dip our toes in the chilly river, Ms Watahomigie tells me the Colorado is part of her history.
"It's my lifeline, my bloodline. We have a lot of respect for the river, water and the earth."
The Colorado River springs up from the snowy peaks of the Rocky Mountains to the north. It winds through the south-western states, towards Mexico and the ocean.
Source of life
The river helped form the Grand Canyon. It is one of planet Earth's most iconic rivers.
It has also been the source of life for this region and the people living here for hundreds of years, which is now a problem, as far as people like Shana Watahomigie are concerned.
Two vast dams were built along the river in the last century. They have caused changes to the ecology of the river.
The lakes behind the dams supply water to agriculture, to industry and to tens of millions of people living in the south-western US.
"We are compromising [the river] by controlling it," says Ms Watahomigie. "The plants have suffered. The wildlife has suffered, as well as human beings. Now the water isn't reaching them."
On the river though, as I get onto a raft with Drew Grim, a local river guide, it is hard to see the problem.
It is a haven. Our boat glides almost silently past the terracotta red canyon walls that rise up high on either side above us.
The river seems to be flowing well, there are plenty of other tourist boats out on the river, there are fishermen angling for trout.
It is what is happening behind the dams on the waterway that is the problem.
"Lake Powell [the lake formed by the Glen Canyon Dam] has been less than half full for a number of years and it didn't show signs of going up," says Mr Grim.
"This year we did get some good snow, and levels rose, quite dramatically, eight inches at a time, but this is just one year."
Trees of salt
For the best part of a decade the water levels have been falling rapidly.
The tens of millions of water users downstream, in huge sprawling cities like Las Vegas and Los Angeles, are simply using too much water.
So too, Drew Grim says, is some of the vegetation along the river. Tamarisk trees - otherwise known as salt-cedars - were brought to the region from overseas decades ago to try to stop erosion elsewhere in the south west.
Their seeds spread, and now the plants line much of the river.
"The problem is the amount of water they drink. They drink a tremendous amount," says Mr Grim. "This adds to the problem of water, as now we have our fresh water source being sucked dry by these invasive trees."
He says they are trying to remove the tamarisk trees. Once they were seen as no particular threat.
"Now it's becoming more of an issue, since water's such a concern. Anything we can do to save water, we need to try."
This though is a pretty minor problem, compared to the vast amounts of water used by households, and, more importantly, by agriculture.
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