Excerpt from When the Rivers Run Dry by Fred Pearce, plus links to reviews, author biography & more

They serve a strong brew at the Alamo coffee-house in Presidio, a small farming town near the US-Mexican border. They need to. Times are tough, says Terry Bishop, looking up from his second mugful. This land, next to the Rio Grande in Texas, has probably been continuously farmed for longer than anywhere in America, he says. Six hundred years minimum. It has been home to scalp-hunters and a penal colony; it has seen Comanche raids, Spanish missionaries, marauding Mexican revolutionaries and a population boom during a recent aliens amnesty. All that time, it has been farmed. But soon it will be back to sagebrush and salt cedar. Climbing the levee by the river at the end of his last field, Bishop shows me the problem. The once mighty Rio Grande is now reduced to a sluggish brown trickle.

In its middle stretches, the river often dries up entirely in the summer. All the water has been taken out by cities and farmers upstream.

"The river's been disappearing since the 50s," says Bishop, who has farmed here virtually all that time. There has not been a flood worthy of the name since 1978. For 300km upstream of Presidio, there is no proper channel any more, he says. They call it the forgotten river. Bishop's land brings with it legal rights to 10m cubic metres of water a year from the river - enough to flood his fields to a depth of more than a metre, enough to grow almost any crop he wants. But in recent years he has taken only a quarter of that. Even when he gets water, "it's too salty to grow anything much except alfalfa."

But that is all a bit academic now. Yields got so low that the farm went bust. Bishop lets some fields to tenants, but most of them lie idle these days. The land is gradually returning to desert. And Bishop drinks a lot of coffee. This is the way of things in Presidio. The town was once a major farming centre. It used to ship in thousands of Mexican workers to harvest its crops. Bishop's farm alone once employed 1,000 people. But that has all ended and the unemployment rate among the town's permanent residents is almost 40%.

The only profitable business now is desert tourism. An old silver mine a few miles up the road has been turned into a ghost town, and a fort at Cibolo Creek is now an upmarket resort attracting the likes of Mick Jagger. Harvesting tourists, that's the game now, says Bishop.

On the map, the Rio Grande is the fifth longest river in North America and among the 20 longest in the world. Its main stem stretches 3,000km (1,900 miles) from the snowfields of the Colorado Rockies to the Gulf of Mexico, via New Mexico and Texas. It drains one tenth of the continental US and more than two-fifths of Mexico. The hub of human exploitation of the Rio Grande is the Elephant Butte reservoir near El Paso in Texas (about 300km upstream of Presidio). It was built in 1915 and changed the river for ever. The wild, untamed flow - which obliterated villages and once rode right through downtown Albuquerque - was ended for good and its waters were corralled for irrigation.

Today, Elephant Butte and its downstream sister, the Caballo, all but empty the river to supply El Paso and nearby farmers. More than 9 million people in the basin rely on the Rio Grande's waters. But it is the farmers who make most use of it. Four-fifths of the water in the river is taken for irrigation - most of it to grow two of the thirstiest crops in the world: cotton and alfalfa, a grain fed to cattle. And the wastage is huge. Only about 40% of the water reaches the crops, while evaporation in the hot sun takes more than two metres of water a year from the reservoirs - a total of around 300m cubic metres from Elephant Butte alone.

Usually a trickle of water gets through to the sea. But since the mid-1990s, a decade during which drought has gripped the basin, the flow has been at record lows. In 2001, it ceased altogether. A sandbar 100m wide completely blocked off the river from the Gulf of Mexico. The bar lasted for five months before summer flows washed it away. And for much of the next two years it returned. You could drive a car across the beach between the US and Mexico. The Rio Grande had, literally, run into the sand.

This is an edited extract from When Rivers Run Dry, by Fred Pearce, published by Beacon Press.