El agua - not again!
We all know it's "el agua" and not "la agua". It's been asked to death and we always explain that it is because of not wanting the two "a" sounds next to each other.
Then why is it "mucha agua" and not "mucho agua"?? 
4 Answers
Because you aren't telling the whole story about why it is el agua.
The purpose is to avoid the two stressed "a's" together. In combinations like la agua the "a" in la is stressed and so it the "a" in agua. In mucha agua only the "a" in agua is stressed.
from toothpaste ch's thread:
Hi,
It's "mucha agua". You can see this discussion in this thread.
"Agua" is feminine, but since it begins with a stressed "a", when you add the definite article, you say "el" so that two stressed "a's" are not together. In "mucha agua", although there are two "a's" together, only the "a" from "agua" is stressed.
Thats actually a really good question! This thread may help you:
it states that agua is called "a tónica." If a feminine word begins with an "a" and the accentuation is on the first syllable, then the masculine definite/indefinite article is used, but it is still feminine:
There was a question about "el agua" asking why it is not "la agua", as "agua" is feminine. Two of the answers suggested that "la" is a stressed syllable. I didn't see anyone disputing this later. I am wondering if it is true that articles are stressed in Spanish, as they are not in English.
Probably because "Much-" is distinctive even when the "-a a.." blurs.
With la agua you would hear lagua, while with mucha agua you hear muchagua which you can more readily pick apart.
Just thinking out loud....
