Mr Smith goes to . . . the Thames Barrier

The Thames Barrier looks like a line of stepping stones, or perhaps conch shells. On most days, there is clear water between the shells, to let shipping pass. Today, they are threaded by a chain of metal, the barrier's movable gates, and there are raindrops on the screen of my computer. (A note to the reader: there are bold experiments with the idea of the author-in-residence at the time of writing, which happens to be 10.45 on a Sunday morning. Fay Weldon has checked into the Savoy for three months. Will Self is installing his escritoire in a Liverpool tower block. Both intend to write their new surroundings into the pages they produce. Taking its cue from these writers' roadshows, this account of visiting the Thames Barrier is being written at the barrier itself. Today is the day when this prodigious watergate is raised, in its annual test. The test I have set myself is to finish before the dummy run is concluded, in about six hours' time; or before the battery of my laptop runs flat, whichever is the sooner.)

Dozens of people have come to Woolwich Reach to watch engineers prove that the Thames's silver bangle is practical as well as elegant. It's quite a thing to see the gates rising from the waves, pushed by rocker beams, the tense yellow legs of the giant grasshoppers who seem to be nesting in the shells. The environmental picture is uncertain - the river is forecast to rise by 60 centimetres in the coming century. You wonder how long this eye-catching feat of engineering will be able to fulfil its job description. The builder's spec was to meet the challenge of floods until 2030. But right now (Sunday afternoon), the situation can be put in a conch shell: the barrier rules the waves. It tames the Thames. Downriver of it, the briny estuary is dammed, until it laps greenly at the riverbank, splashing on to the walkway which leads from one side of the beautiful obstruction to the other. If you follow the walkway, a tiny portion of the Thames Path, you find yourself looking in astonishment not at a mighty river but at listless shallows, the leeward side of a monstrous lock gate. Fish are practically flapping on the mud.

It's raining again. Families and foreign exchange students are marooned in the cafe, which has some of the slowest service on the river. It sells a Thames Barrier pencil sharpener: a tiny slide lifts to reveal the blade. Doing his best to keep spirits up is a giant cat wearing a sou'wester and walking upright in red gumboots. This mascot is "Inspector Down Pour" (sic), according to a label pinned to his oilskin. One of the dads is talking to the moggie, wanting to know how you can apply to go on to the barrier. He averts his eyes as the cat advises him. He's embarrassed to make eye contact with an outsize tabby. On my computer, the battery light ebbs like the Thames upriver of the barrier . . .