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 THREE: LAKE MEAD
It's not often you see the general manager of a golf course hugging the man who's digging up the grass on his fairways and replacing it with something similar to desert gravel.
That though is exactly what Stephen Goldstein, who's in charge of the Black Mountain Golf and Country Club outside Las Vegas did, after bringing his golf cart to an abrupt stop on the way back to the club house.
Mr Goldstein is having 55 acres (22 hectares) of the grass on his course removed.
It's costing between $2m and $3m (£1m - £1.5m) to have done. It should save him more than $50,000 (£25,000) a year.
"The main reason was financial", says Mr Goldstein. "We knew that as time went on that water costs were going to increase."
"There was going to be a time when we would either be mandated to reduce our water consumption or obviously from a financial condition it would be difficult for us to water all the areas."
Water in Las Vegas is at a premium. One of the most important cities in the United States is slap-bang in the middle of one of the driest areas in the world.
How a Las Vegas golf course is cutting water consumption
The reason Mr Goldstein can afford to tear up some of his grass is because the Southern Nevada Water Authority is so keen to get people and businesses here to use less water that it's paying them $1.50 for each square foot of grass they remove.
So some gardens now no longer have grass and thirsty shrubs in them. They have gravel, desert landscaping, and cacti.
Indeed new housing developments are not allowed to put in gardens.
This part of the US has suffered almost a decade of drought. Las Vegas is at the sharp end, but all the states in the region are affected.
There's plenty of disagreement about what's causing the drought. Many here say it's just a dry spell, that the rains will return.
But the General Manager of the Southern Nevada Water Authority, Patricia Mulroy, says there's no doubt in her mind that man-made climate change is to blame.
"We're going to have to change our whole approach to water management in this part of the United States", she says.
It's such thinking that helps explain the water authority's push to stop any water being wasted.
Already they recycle all the city's used water. If it goes down a plug hole, or a drain it stays in the system.
But if a rogue sprinkler sprays the road, or a hosepipe is left running into a lawn, the water is lost.
Hence the water police.
On patrol every day, they are chasing the source of every little dribble of water.
This community [in Las Vegas] is realising … that they need to adapt to the desert
Patricia Mulroy Southern Nevada Water Authority
So when Scott Comstock stops his patrol vehicle by the side of the road, and starts filming a small puddle of surface water on the pavement, it looks almost comical.
The water's coming from inside the grounds of a school, dribbling out from a leaky irrigation pipe.
"It does indeed seem like a very small amount when you consider the fact that in a day this city can burn through 600m gallons of water", he says.
"If this is not checked and something done about it," Mr Comstock says, pointing at the water, "this will get worse all the time, it will get worse."
These and other measures do seem to be working.
Between 2002 and 2007, the population of Las Vegas grew by 400,000 people.
In the same time, water consumption dropped by 15bn gallons, according to the water authority.
That's roughly an 18% fall.
Patricia Mulroy admits that for a long time people here acted as if they could change the environment.
She now believes attitudes are changing.
"With the onslaught of the drought and the rude wake-up call that global warming and climate change is bringing to the entire western United States, this community [in Las Vegas] is realising … that they need to adapt to the desert", she says.
"It's a cultural shift that we are going through first, but every other western community will have to go through."
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.
Water lapped at the underside of this pontoon just seven years ago
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.
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.
Captain Bob takes Matthew Price on a tour of Lake Mead
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.
The reservoir, and the Colorado River which serves it, is still the principal 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.
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.
Seven years ago you could have had a picnic right by the lake
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.
Trees along the riverbanks soak up a lot of water
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.
Dams have brought changes to the river's ecology
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.
Drew Grim says vegetation along the riverbanks is part of 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.
The Colorado River has ecological, commercial and symbolic importance
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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