Southwestern channel (6)
I believe the answer is:
arroyo
'southwestern channel' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arroyo that I've seen before include "Rocky ravine" , "Dry river gully" , "Dry gully; yo roar (anag.)" , "Dry watercourse" , "dry course" .)