One of my kids had a bit of homework about the Thames Barrier - that amazing bit of rotating technology that keeps Spring Tides from flooding London. The whole concept of a river being tidal seemed to confuse them. So I took some pictures.
The first picture was taken at 08:30 am on 17th September 2010, as high tide was turning. This is taken from just outside the Globe Theatre, looking North-East across the river towards Cannon Street station and The City of London (you should be able to see the Gherkin). The thing in the foreground is the jetty for the river-bus.
The lower picture was taken about 2:15pm just before low tide, at the same place. A handy pigeon asked to pose for the picture, and you can see the river-bus too. The tidal range at London Bridge is about 5 metres (and can be as high as seven metres). That's an unbelievably huge amount of water that washes in and out twice a day.