The men who make water music

What are Judi Dench and some dancers and musicians doing floating down the Thames? A new opera, silly.

When Max Couper looks out of his bedroom window and across the water in the morning, he can see the outline of a man who captured the imagination of millions with his learned and carefully organised vision of a new world. Standing on the north bank of the Thames, a few metres away from Battersea bridge, the 16th century figure of Sir Thomas More has struck a curious chord with this 21st-century artist who, by contrast, wants to capture the imagination of a mere few thousand with his version of a new world.

When Max Couper looks out of his bedroom window and across the water in the morning, he can see the outline of a man who captured the imagination of millions with his learned and carefully organised vision of a new world. Standing on the north bank of the Thames, a few metres away from Battersea bridge, the 16th century figure of Sir Thomas More has struck a curious chord with this 21st-century artist who, by contrast, wants to capture the imagination of a mere few thousand with his version of a new world.

For two nights only, people walking down the Thames path by Battersea or Westminster will be diverted by an opera set on two floating barges, with its own language, its own brightly coloured aesthetic and its own mind-boggling soundscape. As the voices ring out in yelps and howls across the river and bright sails billow from the 80ft masts, you wonder what exactly Sir Thomas More would have made of this exotic and arresting Thames-based Utopia.

The opera is the latest in the "String of Pearls" year-long festival celebrating the millennium. The string, for those unfamiliar with the idea, is the River Thames, and the pearls are historic buildings and institutions which mark its course. It may all sound a bit precious, but it has led to some dynamic and unusual collaborations, which have allowed people into the unexplored crevices of public institutions ranging from Lambeth Palace to the Goldsmiths' Livery Company.

As one pearl, Couper's art collection has been linked with another pearl, the Royal Opera House, and the result is a venture which includes Dame Judi Dench, the composer Trevor Wishart and the Royal Ballet choreographer Tom Sapsford.

Couper owns a flotilla of five barges, which is approached via a metal gangway leading down from the bank. On the Friday before the first performance, when the summer heat seems to surge from the stone buildings in the city, a cool breeze blows off the river, and this - along with the gentle rhythm of the swelling tide - induces a sense of hedonistic ease.

Detached from the strains and stresses of "real" London life, this idiosyncratic and slightly haphazard arrangement of boats seems like a quirky metaphor for the acts of imagination that take place here. Inside three of the barges lie twenty years' worth of art-works, including huge abstract steel sculptures, models inspired by the sea and giant fictional navigation charts. The final two barges have been specially adapted to create the stage for Fleeting Opera.

When I visit for the first time, Couper is waiting on the third barge, which can only be reached via a precarious obstacle course of narrow through-ways and carefully balanced pieces of wood. He is sitting with Wishart and Dean Leslie, whose roles for the project strangely combine being winchman and lawyer, sorting out some of the final details.

On water, in a variable climate, there are all kinds of extra logistical challenges for staging an opera. These include making sure the acoustics are well balanced enough to come across to the audience on the bank in any weather conditions, stabilising the barges so the dancers won't fall over and the highly prosaic problem of finding heavy music stands which won't blow away.

And the differences do not end there. "There is no reason at all why an opera on water should have any relationship to land opera," declares Couper. "This is opera for the people, it's on their river and no-one has to pay to come and see it. We have decided that it is not going to be a narrative piece - instead it is going to create a sense of ritual reminiscent of medieval pageants. Opera originated as a very experimental form, and we also want to recreate that sense of exploring different mediums."

Like all the best rituals, the theme of Couper's work centres on fertility. He explains: "For thousands of years rivers have been revered as centres of fertility. We have imagined a pregnant matriarch, Angelica (mezzo-soprano Kate McCarney), who is being wooed by two admirers, Capo (baritone Jonathan Fisher) and Diablo (tenor Nicholas Heath). Throughout the performance the two barges will separate and come together again, illustrating the patterns of attraction and repulsion that occur during the courtship."

The experimental aspect of the opera comes through most obviously in the music. Wishart is a leading sonic composer which, to the uninitiated, means recording sounds, like the clapping of hands or machinery noise, and transforming them via sampling and computer programming into an organised pattern.

For example, an elastic band is tied to a metal pole and held taut. Then it is plucked and the vibrations are recorded. Fed into a sampler, the vibrations take on a whole new life as they are magnified and transposed up and down the scale. Your mind strains to capture the sound with a word - in one form it could be the frantic lapping of the water, in another, as Wishart himself suggests, "something poetic like flocks of birds".

The two collaborators, Couper and Wishart, cut strikingly different figures as they stand discussing their opera. Couper's lean frame and tight black curls speak loudly of his Italian descent - even though he was actually born in Sussex - while Wishart's balding, slightly manic exterior, suggests a mad and brilliant professor. Wishart loves the anomaly that in his late middle-age he is still composing music which certain trendy clubs consider to be cutting-edge. "I was 42 when I last went to a disco, and I didn't enjoy it very much," he says. "Now I hear I'm being played in places where I couldn't begin to imagine myself fitting in."

There will be three performances in all, one tonight between the Albert and Battersea Bridges, and two tomorrow at Westminster, by the private MPs' terrace at the Houses of Parliament and across the river where the real people can get at it. Dame Judi Dench, who approached Couper through a friend when she heard about the project, will only be performing on the second night. Although the singers will be singing in a new phonetic language, designed by Wishart to fit in with his polyrhythmic, harmonically-complex music, Dench will be narrating in English.

The Sunday before the opera starts, I go down to the river again, where passers- by are being diverted by scenes including four people lifting a double-bass from one barge to another, struggling not to drop it into the water. Wishart is up by the controls of a portable music studio, dancing in time to the dizzying collection of sounds which whirl from the PA sytems set up on the boats, and shouting instructions down to the live musicians.

Sir Thomas More is standing placidly on the other bank of the Thames, watching as the cold wind sets further challenges for the musicians and dancers. It is difficult to know what he is thinking, except, perhaps, that the Thames has never seen or heard anything like this before.

Free performances of 'Fleeting Opera' take place tonight by the Peace Pagoda in Battersea Park and tomorrow at Albert Embankment opposite the House of Commons, both at 9.30pm