Conquest of Camelot - Riddle stone

I'm playing again Conquest of Camelot and there is a riddle at the force field barrier at the 5 stones that I don't know the answer. Here it is:

Bright as diamond,
Loud as thunder,
Never still,
A thing of wonder.
What am I?

If you know the answer, please let me know because that's the only riddle missing to go thru the barrier.