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 like Las Vegas and Los Angeles.
The BBC's Matthew Price is travelling along the river to investigate the scale of the problem Recession 'looming' for UK firms ...
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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.
Tomorrow Matthew Price will visit Lake Mead, the largest man-made lake in the US, to see how far the water has dropped, and he will ask farmers if they are worried about the drought.
(BBC)
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