it's childish. Dropping sticks off the side of a bridge and seeing which one passes beneath the fastest. A kids game. Yet it has an innocent charm that Xbox can't match.

Yes that is what you think it is pouring into the Thames and it aint pretty.Sadly, after visiting the Putney Bridge combined sewer overflow (CSO) the other day, I can honestly say the vision of pooh sticks bobbing along the Thames under bridges that I now hold in my mind, is far from innocent and is definately not pleasant.

Putney Bridge CSO discharges 34 times a year on average, accounting for 68,200 of the 39m tonnes of sewage that annually enters the river. Just 2 mm if rain is enough to set many of these overflows in action. Once that amount of pooh sticks enters the system, you just know something bad's going to happen.

That's why we're one of a growing number of organisations, including anglers, rowers, London Wildlife Trust and Thames 21, who've come together to form the Thames Tunnel Now coalition.

Tory minister Zac Goldsmith recently voiced concerns that conservation organisations like the RSPB are being too critical of government. We've said it before and I'll say it again, the RSPB is a great backer of smart development, but reserves the right to fight stupid development.

Zac says">Zac says: “London's existing sewer system is remarkable, given that it is 160 years old. But it needs an overhaul. We put enough raw sewage into the river to fill the Albert Hall 450 times, and that has to stop.” He added; “In addition to the environmental imperative, this project will create thousands of local jobs, and high quality apprenticeships, and contribute to getting us back on track economically.” He's smart.

It's not a bannana, it's a jaunty dinghyDespite the biting cold and unsavoury task, round the world rower Roz Savage took to a jaunty yellow dinghy to bob along the Thames at Putney. Luckily for her it wasn't raining, so the overflow was dry and crusty - pictured left.

The tunnel is being opposed by people living close to areas where construction will be above ground. Roz sympathises and says, "I quite understand their concerns, but sometimes you have to put up with short-term pain for long-term gain. I've rowed through some pretty grim stuff on my travels but the Thames is heart-breakingly returning to the open sewer it used to be 200 years ago."

The tide's out and the outlet's exposed, pinch your nose

Apart from the obvious damage being done to the Thames, its forseshore, London's reputation and our wildlife, this sewage has a ripple effect on the wider environment.

There were gulls, crows, mallard, geese, cormorant and even a grey heron at Putney Bridge. There are fish in the Thames. It is alive, despite the pollution. But it's a clogged and congested artery in desperate need of surgery. The tunnel is a big step, a once in a lifetime opportunity. We can rid the Thames of pooh sticks, and Captain's logs, by boldly stepping-up our collective action for nature to create a greener "future London".