I am standing under Hammersmith Bridge looking at something I have known all my life as a Londoner but am beginning to realise that I don't have a clue about. The River Thames has been here a lot longer than the city itself, but it has been keeping its secrets well hidden beneath those familiar muddy tones of green and brown. On a grey afternoon such as this one, the surface of the river is particularly unforgiving, and a tentative dip soon brings me close to fast-running water. This is the sort of stuff that carries people away to a watery doom and I don't want any part of that. But I'm here with an open mind at this family beach party in Hammersmith, part of a series of events in South East Marine Week. It is not a beach party in the traditional sense, needless to say, more an opportunity to get a little gentle education. I had been vaguely aware, over the past few years, that the river was getting cleaner all the time. Its very appearance, it appears, is deceptive, because its colour is a result of the natural silts which are constantly disturbed from the bottom. From being a river that supported no fish at all, it can now boast more than 100 different types. I was aware that the Thames occasionally played host to a well- publicised dolphin or seal, but this diversity was news to me. It is all the result, I was informed, of the fact that the North Sea pours up the river twice a day, bringing with it all the teeming life of those salty depths. I took a deep breath and went for a light dredge with a net. The results didn't look like much at all, but when carefully sifted my sample was teeming with tiny shrimps, which are the basic foodstuff of the river, the tiny little fellows holding the key to the food chain, There were, thankfully, better fishermen here than me, and there was great excitement when someone captured a flounder. Granted, it was about an inch long, but the flounder was otherwise perfect in every detail. Further excitement was to follow, with the capture of the shell of a crab, but that did not last long. The shell belonged to a Chinese Mitten Crab-so-called because it appears to have mittens on its claws. Rachel Hill from the Environment Agency explained to me that it ate everything in its path, suffered no effective predators, and caused havoc by its habit of burrowing into the river banks, which are consequently being eroded. Furthermore, the fact that it was only a shell meant that somewhere not too far away the former occupant was going about its business only this time it would be bigger. This unwanted visitor, a delicacy in the restaurants of Chinatown, is here to stay. Further up the beach, enthusiastic volunteers were coping with another menace, this one of human making. The amount of rubbish on this relatively small stretch of the river was astonishing and depressing. There were the expected plastic bottles and hamburger cartons, tossed away carelessly by idiots. To my surprise, there was also the wheel from a car, complete with tyro. The most sinister items were also among the smallest: slim white sticks which looked as if they might have come from a child's lollipop but are, in fact, cotton buds. The thought occurred that thousands of Londoners must come to the banks of the Thames each morning to clean out their ears. By the end of the afternoon, all this rubbish had been cleared away in a quite astonishing number of black bags, but it would have been better had it not been there in the first place. There was, however, great cause for optimism in the behaviour of the kids who were present. They huddled excitedly round microscopes to look at tiny shrimps and gobies transformed into fearsome-looking creatures. They listened intently as it was explained to them how important it was to keep the river clean. Even the very smallest who were painting their fishy face masks might have gone away with the idea that fish are a good thing and wort