Flash mobs. We've all grown sick of them by now, haven't we? Well here's one with a worthy purpose. Thames21 are organising a series of mass participations with the aim of cleaning up Old Father Thames. The litter clearing events take place from 20th to 23rd August at the following locations: Bishop's Park, Fulham (Thursday), Hurlingham Park, Fulham (Friday), Woolwich (Saturday), Isle of Dogs (Sunday). No need to book, just turn up and get cleaning. Thames21 will provide all equipment including wellies and gloves. Full details available here. Image by Ray Wise in the Londonist Flickr pool.