The delta of the great Colorado River - where once it swept into the Gulf of California - used to be the most wonder-filled wetland in the whole North American continent.
Some 400 species of plants and animals - including jaguars, beaver and the world's smallest dolphin- thronged its 3,000 square miles of wetlands, lagoons and tidal pools. The local people made a good living fishing its teeming waters. Now it has become a forbidding desert of salt flats and giant heaps of dead clamshells. The fishing boats have been long since beached; the destitute people have to seek what work they can in wheat fields and tortilla factories far away.
The reason for the transformation is not hard to find. Not a drop of the mighty river which once carved the Grand Canyon now flows through the delta to the sea. It has all been used upstream - to slake the thirst of cities such as Tucson, Arizona, feed fountains in Las Vegas, green golf courses and irrigate farmland. Such water as remains in the delta has flowed in from the sea.
It is much the same story in that other great river of the American south-west, the Rio Grande. This does not merely fail to reach the sea: it disappears for much of its length. The atlases tell us it is one of the 20 longest rivers in the world, but in reality it stops some 800 miles inland at El Paso, Texas, which takes all its water. For the next 200 miles or so there is just a dribble of sewage in its old river bed, and even this often dries up in summer. Local people call it "the forgotten river". The dry channel does not come alive again until a relatively healthy tributary, the River Conchos, joins it from Mexico. For the rest of its length, as it forms the boundary between the two nations, it should, in justice, be called the Conchos, not the Rio Grande. But even this is quickly used up, mainly to irrigate farmland, and often fails to make it through to the Gulf of Mexico.
It is much the same story right across the world. China's Yellow River, the fifth longest in the world is in trouble at both ends. Its source in the Tibetan plateau is drying up - and for most of the past 35 years it has failed to reach the sea all year round.
Similarly, despite the words of the spiritual, the River Jordan is far from "deep and wide". In practice it ends at the Sea of Galilee, where Israeli engineers have blocked the outflow and piped the water to irrigate fields and supply Jerusalem and Tel Aviv.
Such water as flows down the Jordan valley again comes from a tributary, the River Yarmuk. But it cannot really do the job. In biblical times the valley carried a billion cubic metres of water every year; now it has to make do with less than a tenth of that. The ugly truth is that the river - sacred to Christians, Jews and Muslims - is now mainly made up of diluted sewage.
It is the same for river after famous river. The lower reaches of the Nile used to carry 32 billion cubic metres of water a year; now they are down to two billion. The Indus in Pakistan - "Asia's Nile" - similarly has lost 90 per cent of its water in the last 60 years. Australia's Murray River fails to reach the sea every other year.
Even in Europe, Germany's River Elbe has run so dry that it frequently becomes impassable to barge traffic for months at a time - and three years ago river traffic almost completely stopped on the Rhine. In Britain the Environment Agency regularly sounds the alarm about our chalk rivers and streams - which gave birth to the sport of fly-fishing. Dozens of them dry up every summer, and 40 of the 160 in the country are officially under threat.
The writer Fred Pearce, who has published a groundbreaking book on the crisis of the world's rivers, says: "The maps in an atlas no longer accord with reality. The old geography lessons about how rivers emerged from mountains, gathered water from tributaries and finally disgorged their bloated flows into the oceans are now fiction."
The UN-backed World Commission on Water for the 21st Century reported: "More than one half of the world's major rivers are being seriously depleted and polluted".
There are two main culprits; abstraction of water for rivers - usually after damming them - and global warming.
The world has, on average, built two giant dams a day, every day, for the past 50 years. Now 45,000 of them span the world's rivers. Every one of the world's 20 longest rivers is encumbered by them.
In many ways it all began on the Colorado, 70 years ago, with the Hoover Dam, the great symbol of Franklin Roosevelt's New Deal. Today, the dams intercept more than a third of the world's freshwater as it flows towards the sea and at any one time are holding back 15 per cent of it.
The UN's triennial World Water Development Report, published for a international conference in Mexico City this week, cautions that damming has "hugely changed the natural order of rivers worldwide." It goes on: "Humanity has embarked on a huge ecological engineering project with little or no preconception - or indeed full present knowledge - of the consequences. We have sought to redesign and impose a new order on natural planetary systems, built over aeons of time."
Dams waste massive amounts of water. In hot, dry regions, they lose about 10 per cent of their reservoirs to evaporation every year: much more is lost in irrigation. Global warming is making things even worse. The source of the Yellow River is drying out as glaciers retreat. And a great drought in the southwestern United States - so intense that even cacti are wilting - is exacerbating the crisis of the Colorado and the Rio Grande.
It is even endangering relatively healthy rivers. The Amazon, relatively unencumbered by great dams, was hit by the worst drought on record last year: water levels fell by 10 metres and boats were stranded. And salmon are endangered in Alaska's Yukon River because its waters are too warm.
This will only get worse as the world goes on heating up, making the desert delta of the Colorado just a foretaste of the rivers of the future.
Length: 4,000 miles
Famous as: Source of some of world's richest habitats
Problems: Depleted by a record drought last year. Widespread deforestation
Verdict: Largely undammed and rescuable
Length: 2,900 miles
Famous as: Carries most silt
Problems: Source is drying out and river now usually fails to reach the sea
Verdict: Attempts at rescue. Task immense
Length: 104 miles
Famous as: Holy river
Problems: Effectively ends below the Sea of Galilee. Site where Jesus was baptised now a pool of sewage
Verdict: Hardly exists, damage seems terminal
Length: 1,900 miles
Famous as: Border river
Problems: Now two rivers, split by 250-mile dry section
Verdict: Over-exploitedReuse content