Nearly everyone knows that the River Thames meanders its way through the heart of London (except, perhaps, for the London tube map makers).  The river features prominently in the rich history of the city, though today the merchant ships have been replaced with pleasure boats & rowers.    What is a surprise to many people when they come to visit is that the river is very tidal.  In fact, the swing can be as large as 18ft at Hammersmith near our house.  This makes for a dramatic difference in the look of the river, which you can see here: