A river runs through it

Come the millennium, could riverside London be the modern art centre of the world? So much seemed promised by presentations made this weekend at Spaced Out, an ICA conference on millennial projects for the visual arts. The great curve of the Thames from Millbank to Southwark's Bankside will become a brilliant highway of light connecting the old Tate to its spectacular new Museum of Modern Art. Between those two points it will be fronted by Richard Rogers's festival renovation of the South Bank and the Hayward Gallery and, if things go right, bridged by Will Alsop's stunning new Institute of Contemporary Arts.

The ICA evidently knew what it was doing when it commissioned Alsop to find a new location and make proposals for a new building. An artist himself and an architectural innovator of great flair and imagination, Alsop is involved in several art-centred projects here and abroad. Architect- in-residence at Riverside Studios for several years in the early Eighties, he has an insider's knowledge of the requirements of a multi-dimensional, multi-purpose arts complex. And Alsop is passionate about the Thames. (At Riverside his first proposal was to build a pier and turn the face of the studios to the river.) No surprise, then, that he should have looked most intently in that direction for the ICA.

Of the two sites he recommends, Jubilee Gardens, on the South Bank between Hungerford Bridge and the old County Hall, is high-profile and obvious. The Tate considered this before what now seems its inevitable choice of Bankside. Ever since the dismantling of the 1951 Festival of Britain buildings, it has been an unhappy space waiting for an identity fit for its superb location. Alsop's proposed buildings would occupy less than a quarter of the space, but their presence would reactivate the whole area, turning it into a park that would function as a dynamic cross-axis between Waterloo, the South Bank and, by means of a new footbridge on the sunny side of Hungerford Bridge, Trafalgar Square and Covent Garden. The south end of the bridge would lift elements of the ICA above the river before simplifying to a span that would terminate in a new square on the north bank.

Alsop's other proposal, however, is more startlingly original. It is to build the new ICA as a bridge across the river, using the cast-iron- clad column piles left by the now dismantled Dover and Kent railway bridge just east of Blackfriars Bridge. Spanning the four-lane embankment road would be a swelling moulded form containing theatres, cinema, seminar rooms, and housing at the bridgehead a set of galleries, what Alsop calls "light-modulators" or "boxes of delight".

The galleries on the ICA bridge would be mounted on rails, like carriages in a station, and moved at will out across the water, joining to create spaces of variable dimensions and flexible utility, and leaving within the shell a great occasional space for other activities. Spanning the river and supporting their tracks would be a hollow beam structure containing offices, restaurants, bars and bookshops.

Alongside this beam a footbridge would link the ICA to Bankside, curving back and eastward from the South Bank as the final stretch of the riverside walk to the new Tate from Millbank. With the Mermaid Theatre just behind the ICA, and the new Globe Shakespeare Theatre next to the Tate, the elements would be in place for the beginnings of a lively cross-river cultural nexus.

Alsop's projected ICA, whether it is sited at Blackfriars or at Jubilee Gardens, is truly something special. Having no collection, it operates as a kind of research institute, on the model (as in its conception) of a scientific laboratory. It is a place of no fixed assumptions as to what art is, what it may effect, or where it might be found: it exists in order to facilitate redefinitions. Alsop's designs reflect this perfectly. The building at Blackfriars looks indeed like a giant retort, its bowl containing space for chemical reactions and interactions, its rounded outer form declarative of the organically continuous structural tension of the shell or bulb. It might be seen, too, as a progenerative womblike form; or like a head generating an infinity of ideas. "Our heads are round," said Picabia, "so that thought can change direction."

At Jubilee Gardens this shape recurs, with others as elegantly expressive, similar components in a very different configuration. With the advantage of a larger site, Alsop is able to distribute administrative, artistic and recreational functions to a set of smaller buildings sat above a glass- roofed sunken plane. From this meeting place, a foyer with bar, which is in effect a covered piazza the size of a football pitch, there will be the entrances up to the cinema, theatres and talks rooms, and to the restaurant, bars and shops on the bridge.

At either location the ICA will come to brilliant life, more accessible, more flexible, at a juncture of energy and movement, freed from the Crown Property restrictions of its present building. Its great co-founders, Herbert Read and Roland Penrose, anarchist knight and surrealist knight, would be well pleased at these brilliant prospects.