[I have no wish to tread upon any toes, but a Front Page Feature surely deserves a thread of it's own.]
I stuck what is the title of this thread into Google and in half a second got about 42,000 hits. Many regarding the closures and re-openings and many many pictures.
Also there is mention of a competition for suitable covers to hide the Fountain during the 2012 Olympics...
http://ucreative.ac.uk/news/2011/December/archfountain#.T64p3WYRgUM
I have not found anything as yet on the when or why there is a fountain at all, perhaps someone here knows.