ART / The watercolour world: 'Go with the flow' is the recipe for self-made painting. Rosie Millard reports on two artists collaborating with nature

'It was a two-week adventure in London,' said Mary Lemley as she surveyed her car covered in mud and tarpaulins, and her boots caked with Thames silt. Lemley is not some Girl Guide type, traversing rivers at midnight just for the fun of it, but an artist; her exhibition, 'These Fragments We Have Shored against Our Ruins', currently hanging in east London, is the result of 14 days on - and sometimes in - the river Thames. Described as 'an interdisciplinary work', 'Fragments' is possibly the only exhibition to be made possible by tidal flow.

Lemley hung a different 40ft piece, every day for 14 consecutive days, up and down the river during low tide, collecting each one exactly 12 hours later. The end result is a display of the Thames via its grimy waves; a wash of mud, shrimps, silt and pollution. A seam of iron oxide pigment, stitched into a long pocket down each cloth, adds a rusty rinse. Hung together to dry in a vast, vaulted warehouse, the final effect is not unlike a kind of muddy cathedral.

Unfortunately, some life forms in the river were not too happy at being part of Lemley's temporal record. Mary and the 'Fragments' team were so busy battling with one 40ft cloth on the Wandsworth embankment at five o'clock one morning (a challenge reminiscent of a Generation Game Christmas special) that they hardly noticed a tarpaulin-covered boat bobbing on the water beside them. That is, until the owner appeared, clad in red striped pyjamas and incandescent with rage. Captain Mo of the Svend Knud has lived in his boat on the Thames for 21 years, and seemed to be unaware of the progession of Land Art.

'What the bloody hell's going on here?' he demanded. On being told that this was Art, the Captain glared at the team as if they should be keelhauled, and retired to his cabin. Twelve hours later, he was slightly more forthcoming. 'Art on the river?' he said, helping to pull the cloth from the muddy bank. 'Well, the only other thing I know of that kind was a man a couple of years ago who jumped off every bridge on the Thames. I think he was being sponsored or something.'

Lemley had not set out to disturb the Captain; she had planned to hang her cloths specifically at places where the Thames' tributaries flow into the river, one of which happened to have been shared by his mooring. Seven on the north bank, seven on the south; night after night, Lemley ranged up from Putney's refined Beverley Brook to the evil-looking Neckinger in Southwark, where Bill Sykes is supposed to have fallen to his death. And the times of action didn't exactly correspond with normal working hours; low tide on the Thames is highly inconsiderate.

'I am just exhausted,' she said one night as she dragged Cloth No 11 to the entry of Earl's Sluice in Deptford. 'My whole body is being ruled by the tides. I feel like some form of ancient river- worker.'

But she surveyed the cloth, the sheet billowing out from the stone pier, with satisfaction: 'I've always had this fondness for the Thames; working on all its tributaries makes me feel more attached to it, even though half of them have now turned into sewers.'

Various customers from a nearby pub wandered past and leaned over the river to see what was going on. 'I've been to art college and I know what this sort of thing is all about,' one man said. A more favourable reception came from the inventor Sir Clive Sinclair, on his way back to his taxi after a dinner party. 'I think this is rather wonderful, poetic,' he said. 'Very calming.' Mary beamed at him. 'They make wonderful paintings, and the great thing is that you don't have to sit around and do it yourself,' she said. 'The river does it for you.'

Indeed, from Putney's rowers to dockers at Limehouse, people have been drawn to the goings-on. For some, it has meant an awareness for the first time of London's most obvious landmark. 'Well, if the Thames was a motorway, it wouldn't make much difference to me,' said Leo Cummins, a demolition worker in Greenwich. He peered over at Cloth No 13. 'But this is good.' 'I feel like doing an art-work here myself,' said his mate John. 'I could do something with this silo here, get some music going, and perhaps hang a few cloths up.' His motives, though, were rather more market- orientated than Lemley's. 'You could charge everyone a fiver, and make a couple of grand,' he said excitedly.

Entry is free, however, to see 'Fragments' at Trinity Wharf. All 14 cloths are displayed with bound volumes containing poetry inspired by the river; each one, marking a particular tributary, hangs in its proper geographical order. 'Standing here, you're in the middle of the Thames,' says Lemley. 'There were moments,' she continues, 'when I was hanging off ladders and it all seemed frightfully important. But then dragging these sheets around the Thames became just so comic. I don't buy all this putting of paint on to canvases as heroic. I don't buy climbing in and out of the river as heroic; for me, it's just something that allows you to check into something rather mysterious.'

The artist is currently suffering from Thames-induced flu; but for others, her round-the-clock watery perambulations have borne fruit. 'I'd like more artists to come here,' said Greenwich John, standing amid his riverside demolition site. 'Instead of things being destroyed, it would make use of the place . . . instead of wasting it.'

'Fragments . . .', Building H, Trinity Wharf, Orchard Pl, London E14. Weekends only by appointment (071-709 0731)

(Photographs omitted)