Ther most famous river in England, the Thames runs for over 200 miles, the lower half of which are tidal. The tidal fluctuation at London Bridge is 3.5 meters. As you have no doubt guessed, it passes through London. Boats can travel up to Oxford and small craft can get as far as Lechlade¹, which is near the river's source.
The Magna Carta was signed on an island in the Thames² in 1215.
Possibly the Thames was a part of the Rhine before the British Isles decided, geographically speaking, to go their own way.
It's pronounced "temms" you 'murrikan heathens! Try saying it like the Queen of England would. Or better yet the Queen Mum. Take a nip of gin and tonic and tuck your tongue back to the middle of your mouth, say "hmm hmm hmm" and then say "temms".
At time of writing you could see (with patience) the river live at http://www.livesights.com/cgi-bin/nph-VideoPages.plx?title=live
1. Thanks to Albert Herring.
2. Thanks to Gorgonzola.