Tall tales: Why everyone is dipping a toe in the Thames

It's a wonder that more people aren't following Times journalist Matthew Parris, the foolhardy Outdoor Swimming Society (OSS) or environmentalist Lewis Pugh by plunging into the Thames to cool off. The murky water, despite the occasional cadaver, is surely crying out to be explored since 125 species of fish, including salmon and sea trout, have now returned to set up home. Last year the river was even awarded the International Thiess River Prize for river management and conservation. So why not head to the top of Tower Bridge and do a Tom Daley?

'Not a great idea,' says Parris, who last year fulfilled a long-held ambition to swim across the river but misread the tides. It wasn't until he saw 'trees sliding by rather fast on the opposite shore' that it dawned on him he was drifting upriver. 'It wasn't really panic, so much as, "Oh, cripes, this isn't going to be straightforward!" I swallowed the odd mouthful and was surprised to find it salty.' When he eventually exited three-quarters of a mile upriver from where he set off in Limehouse, he was freezing, in shock and had 'pants full of mud'. After writing about his stunt, Parris incurred the wrath of the Port of London Authority. 'Frankly, swimming in the Thames is not only ignorant it is selfish, too,' said David Snelson, chief harbour master.

Swimming in the Thames wasn't always so frowned upon. In the 1800s families would take picnics to the sandy banks of the river at low tide and children would swim under Tower Bridge, despite the river being at its most polluted in history - one commentator called it 'little more than a fermenting sewer'.

So perhaps the OSS isn't as bonkers as it looks. This gang of 'wild swimmers' is taking on its sixth section - Buscot Lock to Grafton Lock in Oxfordshire - this Sunday. Jeremy Wellingham is one of the swim's organisers. He says: 'So far swans have been the greatest hazard but this should get easier when we reach the navigable Thames beyond Lechlade.'

Next up, David Walliams is attempting to swim all 215 miles of the river in September and hoping to beat Lewis Pugh who swam it in 21 days (although he started with a 19-mile run because the river at the source in the Cotswolds is just a trickle) in 2006. Matthew Parris has some advice for the Little Britain star: 'Remember that tide tables are in GMT not BST!'