Rowing clubs were racing on the Thames at Henley Reach for many years, so eventually the town council decided to create an annual event. Henley Regatta was started in 1839 when it was raced on a single afternoon, and when it was attended by a member of the Royal Family it became Henley Royal Regatta. It has since expanded to a 5 day event and is now the largest and most famous regatta in the world.
There are colour photos elswhere here
This is the best guide site I know of,
it's funny in a very English, tongue-in-cheek way, but still completely true. Definitely worth a read.