Never noticed that the Thames is tidal?

London by London is always worth a quick read, if only for some of the daft questions that get asked. Take this from today's issue:

Every day on my trudge across London Bridge I look down at the murky water of the Thames below and frown because the damn thing's flowing in the wrong direction. Is this the first sign of a psychosis or has anyone else noticed how the Thames appears to flow up stream? Please come to my rescue with a plausible explanation before I jump in to test the currents.