It’s two and a half years since Christo and Jeanne-Claude’s Gatesproject drew four million visitors to Central Park, New York.

The Gateswas a winter work, its saffron banners catching the low, slanting rays of sunshine over a snowy urban park. For their latest project, the artists who wrapped up the Reichstag and dressed Florida’s islands in skirts have headed into the countryside and swapped seasons. This time their raw materials are mountains, fabric, water and summer light.

For Over the River, Christo and Jeanne-Claude plan to suspend a canopy of silvery fabric panels above a 40-mile stretch of the Arkansas river in Colorado. Using six miles of fabric, there will be breaks for bridges. – or simply for the look.

Rafters – and this is the most rafted river in the United States, attracting 300,000 people each summer – will be able to glide beneath the panels, peering up at the sky through the loosely woven fabric. Highway 50 hugs the south bank, so those who don’t take to the water will have an alternative view. From the road, they will see shifting waves of silvery fabric, but they won’t be able to see through it.

I finger a small piece of the fabric (aluminium-coated polypropylene) and hold it up to the light, trying to imagine a summer’s day in the Rockies, not London gloom. It’s the same material they used for the Reichstag, but with a looser weave. “The fabric will mirror the light,” says Christo. “In the early morning it becomes rosy, in sunlight it’s platinum and at sunset warm and golden.”

Christo was born in Bulgaria in 1935 and went to art school in Sofia before defecting to the West in 1957. He met and fell for Jeanne-Claude in Paris a year later, while painting her mother’s portrait, and the pair have been making art together since 1961. As they conjure a vision of Over the River for me on the phone from their New York home, they brim with enthusiasm and sometimes talk at once: “There’s so much energy . . . the panels are in constant motion . . .”, “The banks are not always the same height, so . . .”, “the fabric is not always horizontal . . . sometimes it is inclined . . .”, “here in one direction, there in another . . .” If all goes to plan, Over the River will “go live” in the summer of 2011. But past experience has made the couple philosophical about the time it takes to realise their projects. The Reichstag took 25 years; The Gates 26. Obtaining planning permission for novel works is never quick. “There are precedents for how to build a skyscraper,” says Christo. “But there are no precedents for suspending fabric over water.”

Christo once told me that he and Jeanne-Claude like “to borrow space and gently disturb it for just a few days”. Over the River will last just two weeks, but when you borrow the Arkansas river, it’s not cheap.

The project is likely to cost about $42 million – twice as much as The Gates – with every penny coming out of the artists’ pockets. The pair refuse all donations and sponsorship, instead raising the money for each project through the sale of Christo’s works, which range in price from $25,000 for a small drawing to $3 million for a scale model. “We sell everything we have – except our son,” says Jeanne-Claude. As yet there are no scale models for Over the River, but some of Christo’s preparatory drawings and collages are on show at a gallery near Nice, in the South of France. Next year, a bigger exhibition will take place at the Phillips Collection, a museum of art in Washington DC.

As the latest step in the quest for permits, the couple have just submitted a 2,029-page document to the Bureau of Land Management, part of the US Department of the Interior, which ultimately owns the river and the land. The document will form the basis of a study of the impact of the project on the environment. The artists are hoping for a decision by early 2009.

When we speak, they have just returned from a round of meetings to win hearts and minds in Colorado. Local support is strong, they tell me, but the project has its noisy opponents and Christo and Jeanne-Claude are continually dreaming up ways to mitigate their objections. They will, for example, lay on helicopters round-the-clock in case a pregnant woman is prevented from getting to hospital.

In the 1990s Christo and Jeanne-Claude explored 14,000 miles of river before choosing their Colorado location. But the idea for Over the River was born in Paris in 1985, when the couple were wrapping the Pont-Neuf. They were standing on a barge under the bridge as their workers pulled on ropes to raise a panel of fabric from the deck. “As the fabric rose, Christo and I looked at each other with a gigantic smile,” recalls Jeanne-Claude.

Seven years later, out of the blue, the shared grin made sense. “We suddenly realised what that smile in Paris was,” she adds. “What did we see? We had seen fabric in mid-air, the sun shining through it, reflecting in the Seine . . . and that was Over the River.”

The show in France also highlights a second work in progress: the mastaba project for the United Arab Emirates – an enormous, pyramid-like structure involving almost 400,000 oil barrels. First mooted in 1977, it was put on hold during the Iran-Iraq war, but has recently reappeared on the radar. Not that Christo and Jeanne-Claude are giving much away. “The project was in limbo for many years, until, almost like a miracle, something happened,” says Christo. “Suddenly, there was somebody very eager to work with the project, somebody who builds things there. We are working with that somebody.”

So when might it happen? “Darling, to tell the truth: we don’t know,” says Jeanne-Claude. But Christo’s optimism takes hold. “If things go smoothly, like now, in 2012 it can be up!” Hopefully, Over the River will still happen first.

Christo et Jeanne-Claude: Deux Projets, Galerie Guy Pieters, Chemin des Trious, Saint Paul de Vence, France (www.guypietersgallery.com OO 33 493 320646) until Sept 15

ARE WE TIRED OF THEIR TRICKS?

In a joint career that has lasted almost five decades, Christo and Jeanne-Claude have done more wrapping than a team of shop assistants at Christmas. Anything from the Pont Neuf in Paris to the Berlin Reichstag has been swathed in fabric. Are we tired of the trick? Or will their Colorado project raise art world pulses?

In a sense, most artists through history have proved one-trick ponies. It is a fresh way of seeing the world that brings them to attention and makes them famous. But once they have developed a style or “look” , it becomes their trademark: look at Michelangelo’s muscular torsos, presented again and again in endless poses, or the chubby beauties of Rubens or Modigliani’s languorous lovelies.

In today’s era of brand recognition, we may be even more insistent on an artist’s fidelity to a style. The art market may well prefer familiarity to freshness.

Christo and Jeanne-Claude are players in this market. They make a huge amount of money by selling drawings, collages and models for their pieces. Much, though, goes towards funding their projects. No tickets are issued for the work itself.

Perhaps their projects should be treated more like voyages of exploration than finished objects. The artists who began by wrapping up bottles and magazines now surround entire islands. And we can’t help being struck by the monumentality.

But the scale of the conception is also part of the point – even the controversy. Thousands of people who might never enter a debate about art, take a position. Love it or loathe it – you can’t help but get entangled in the whole vast Christo process. RACHEL CAMPBELL-JOHNSTON

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