Rising brown river is untreated (7)
I believe the answer is:
natural
'untreated' is the definition.
'rising brown river' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'brown' becomes 'tan' (tan is a kind of brown).
'river' becomes 'ural' (I've seen this before**).
'tan' written backwards gives 'nat'.
'nat'+'ural'='NATURAL'
'is' is the link.
(Other definitions for natural that I've seen before include "Reason for death when not suspicious" , "Accidental" , "Normal; not man-made" , "Additive-free" , "One regarded as sure to succeed in some field" .)