Q:

Why is a feminine adjective used in "El agua poco profunda"?

Why is "el agua poco profunda" and not "profundo"? Agua is masculine although it ends with an "a". And from all what i've learnt so far it should be profundo.

A:

Agua is feminine. 
The key here is singular feminine nouns that starts with stressed 'a' or 'ha'. Examples: el alma viva, el arma peligrosa, el hambre canina, el habla clara, el hacha pesada, el águila altiva, el ala blanca. 
Then the noun is feminine, and the adjective must be feminine although the article is masculine. And only when the article is immediately followed by the noun.
Better and deeper explanation (Spanish).

