The American southwest is suffering from a 23-year-long drought.

A grave water shortage resulting from the overuse of the main water supply and global warming reducing the inflow of new water, has led to cutbacks in the amount of water being allocated to the states that depend on the Colorado River.

That in turn is hurting humans, animals and the industries that thrive in the west – agriculture, high technology and tourism.

Water, the stuff of life, is disappearing fast. I headed north out of Phoenix, the city in the desert, to see for myself. Passing through the high ground around Flagstaff in Arizona, we saw the first snowfall of winter. A strange business, this climate change.

Under the timeless gaze of Navajo mountain, the dark blue water of Lake Powell's serpentine form coils its way through the arid redness of Utah and Arizona - until it runs into the massed concrete wall of the Glen Canyon Dam.

It was the dam that created Lake Powell back in 1972.

Joleen Sharatt saw it being built. Her father took the family on weekend trips from their home in Utah to watch the transformation of the Colorado River.

"I was a little kid the first time we were here when they were building that down. My father, he just loved this country down here and he would come and we would watch the dam being built. The concrete was a constant pour," she said.

Half a century on, we met Jolene on a clifftop beside the road to Wahweep, a marina and resort on the lakeshore. The clifftop view over the bottom end of the lake and the dam that interrupted the course of the Colorado.

"We took the last boat ride down the Colorado River before they closed it for the dam," she told me.

Drought, overconsumption, and climate change, are main factors dissipating the amount of Colorado River water

But now she was back witnessing another transformation of the lake and the river that feeds it – its disappearance.

"It's absolutely gorgeous when it’s full, and it's very shocking to see it so low," she said.

"And its shocking to see how low it is on the dam, because when it gets so low, they can't pump it out of the dam."

Lake Powell – the biggest man-made reservoir in the United States – is three quarters empty. It is the most obvious manifestation of the West’s growing water crisis.

One of the original aims of the dam was flood control. Now the big concern is deadpool – when the water level drops below the level of the water intakes on the dam.

That means no water passing beyond the manmade barrier, flowing through the Grand Canyon and re-filling lake Meade, the other great lake on the Colorado waterway – the one held in place by the Hoover Dam.

So, what has gone wrong with the Colorado iver, the lakes and the dams that control the water supply to seven southwestern states in the US, and Mexico?

In short, overuse of the resource, and undersupply of water due to climate change.

In Phoenix, one of the fastest growing big cities in the US, we met Sarah Porter, a lawyer who runs Arizona State University’s Kyle Centre for Water Policy.

Sarah Porter says the original allocation of water rights has resulted in too much demand

"In 100 years of the Colorado Compact, where seven states agreed to share water in the Colorado River, this is the worst it's ever been," she said.

"We're in a perilous place right now because water levels are so low in the two big reservoirs that hold water for California, Arizona, Southern Nevada and Mexico.

"Those water levels are so low that we're approaching deadpool, which is the point at which water can't be delivered off the reservoir."

And deadpool would have devastating consequences for agriculture in the region, which has thrived only with assistance from manmade irrigation.

"The reason Arizona was ever settled was for agriculture - cotton and cattle were big industries in Arizona historically," Sarah says.

"And as part of that, the federal government worked with farmers and ranchers to build big water infrastructure, like the kind of infrastructure that you see on the Colorado River.

"Those industries have been around for a long time and of course, it takes a lot of water to grow food and fibre.

"In Arizona, we have a big agricultural region in the southwestern part of the state, Yuma County. Yuma produces almost all of the winter lettuce - leafy greens that are eaten not only in the US, but in Britain between November and March.

"It's a really important agricultural region. And, of course, across the Colorado River is Imperial Valley, California. Another extremely important region for growing fruits and vegetables and wheat and other products.

"Those are the places that would be the most dramatically impacted should water stop flowing in the Colorado River."

Sarah says the original allocation of water rights has resulted in too much demand, taking water out of the river basin, while climate change has drastically reduced the supply of water going into the system.

"When the water was divvied up from the Colorado River to the seven states, the people at the table were over optimistic," she said.

"Some people say deliberately over optimistic about how much water we can reasonably expect from the Colorado system. So, it's over allocated.

"Then, on top of that, we are seeing impacts of climate change. We're in the longest drought that's been seen in this region for over 1,200 years. We're looking at years where we have normal snowpack, and yet only a fraction of that snowpack turns into flows in the Colorado River, because instead of snow melting slowly and going into the ground and eventually making its way into the river, what we're seeing is that snow is essentially evaporating quickly.

"The ground, the area, the region is hotter and drier and so the ground absorbs more snow melt than it used to."

The lake was named for a 19th century explorer of the Colorado River, Major John Wesley Powell. A plaque on the dam says Mr Powell’s enlightened policy ideas on land use and water conservation led to the creation of the Bureau of Reclamation in 1902 – the government agency that undertook vast public works to shape nature in the American west to bring water and agriculture to the deserts of Arizona and California.

The first big project was the Hoover Dam and Lake Meade. The Glen Canyon dam was built in the 1960s. The lake filling up by 1972 and becoming a tourist attraction in this arid region that NASA uses to test lunar vehicles in.

Hoover Dam depends on its recharging on the water passing through the Glen Canyon dam

One of the big draws are the houseboats that tourists can rent at two marinas along the lakeshore. They have to keep building new slipways to launch and recover the houseboats, because the water level in the lake keeps dropping, and the houseboats are left further and further from the amenities on the shore.

The channels for safe navigation are closing up too.

Mike Chamberlain has been coming to Lake Powell for decades. A retired weatherman and keen photographer, we came across him at his holiday trailer, parked up on Lone Rock beach on the Utah side.

We have driven down from the original beach level to where Mike is parked up. He told us that where we are now used to be underwater.

From there, we are looking down a steep drop to the lake bottom – now a sandy expanse covered with low bushes. to the right, small birds wade in ashore from the edge of lake Powell. It’s the point where the lake ends and turns back into desert.

Mike Chamberlain has been coming to Lake Powell for decades

Mike takes a long-term view of the situation. He has seen various weather cycles down the decades, which bring drastic changes to the level of the lake – both up and down.

"When they allocated the water rights, they used a 30-year reference period. And the period they used happened to be a pretty wet 30 years," he said.

"But now that we are getting back to normal levels or rainfall – and below normal, because this region has been in a drought for I don’t know how many years – but it doesn’t allow the lake to recharge."

In a desert, wearing sunglasses, we discussed snowfall in the region, and rainfall in the desert.

"When you consider the size of the (Colorado) basin – there's the upper basin of Colorado, Wyoming, New Mexico and Utah, and the lower basin states of Nevada, Arizona and California – even though we had a decent monsoon season this summer, it didn’t make a difference.

"What makes a difference is winter snowpack.

"And it can't just be in Colorado. It can't just be in Utah. When we’re talking about the Colorado Basin, we’re talking an incredible size.

"So it takes a good snowpack in the entire area. And when we get snowpacks that exceed 100% - and I’ve seen it reach 130%, 150%, 160% - those years are big years, and the lake will come up 50 or 60 feet."

But climate change is having an effect on those snowpacks, and right now, the lake is at its lowest level since its creation.

Lone Rock is indeed a lone rock, an isolated lump that looks like a huge office tower on a flat plane. As recently as two years ago, it was an island, popular with kayak tours.

Now, it resembles the scenery from a John Ford western.

A white high-water mark is clearly visible a little over half way up. Mike tells us its 173 feet – about 54 metres – above the desert floor.

It might rise up again with the right snow conditions, but as Sarah Porter points out, the rise in global temperatures means more snow is evaporating, not melting into river basins.

A buoy that reads 'No Boats' lays on cracked dry earth where water once was at Lake Mead

The course of the water distribution has changed. Meanwhile, demand continues in the same fixed places.

"We are not getting the water to fill the reservoirs," says Mike.

"And there is an allocation that has to be met, so much water has to go down the river to places like Phoenix, and LA and Las Vegas. The lower basin states and Mexico. There’s a treaty."

There may be a treaty, I say, but if there is no water, then that’s that, is it not?

"Well, that’s the thing that people are grappling with right now", says Mike.

"How do we deal with the shortage, because somebody somewhere is going to have to say we can’t use as much water."

This is not a problem with an easy fix. Farmers in the southwest have already lost productive land as water allocations are cut back.

This year, the allocation cut in Arizona was just over 20%. Cities are spending billions on infrastructure to conserve and recycle water: a nuclear power plant on the outskirts of Phoenix is cooled by waste water from the sewage system.

For Ms Porter, there are three things to be done.

"I don't think it's reasonable for us to expect that the Colorado River is going to recover and produce the amount of water that water users have been expecting over the last few decades," she said.

"The most immediate imperative is that we leave more water in the system to allow those reservoirs to recover and get out of this danger zone, where we're looking at no hydropower production or reaching deadpool and no water coming off the systems.

"Other solutions out there are to boost the amount of water conservation that we're doing. We have the capacity, in our cities especially, to use less water.

"And there's some ability for agriculture to use less water and still be productive, make money, grow crops.

"And then, finally, there are some solutions out there for bringing new water supplies into Arizona and other parts of the Southwest. That water would be very expensive. So, we probably don't want to start working on those solutions until we've made sure that we're being as efficient as we can with the water supplies we have to hand."

Oh yes, the hydro power. Both the Glen Canyon Dam and the Hoover Dam produce electricity – the cleanest, greenest type – hydroelectric power.

But if lake Powell drops to deadpool, then the water cannot get through the dams' intake system to drive the turbines that generate the power.

The same is true of the Hoover Dam, which depends on its recharging on the water passing through the Glen Canyon dam.

We drive south out of Page, the town that grew up around the Dam project, on a road that takes us between the Grand Canyon and sacred Navajo mountain.

In the last of the golden hour before dusk, the low sunlight sets the red sandstone canyons and rock formations aglow.

Giant cactus plants form strange silhouettes in the landscape.

And we wonder if this can be fixed? Can it fix itself? Or will it just get worse?