Turning brown, Russian river, as is to be expected (7)
I believe the answer is:
'as is to be expected' is the definition.
(I've seen this before)
'turning brown russian river' is the wordplay.
'turning' shows that the letters should be reversed in order.
'brown' becomes 'tan' (tan is a kind of brown).
'russian river' becomes 'ural' (I've seen this in other clues).
'tan' back-to-front is 'nat'.
(Other definitions for natural that I've seen before include "Not made, normal" , "Without affectation" , "Unspoilt" , "Unaffected - musical character" , "Innate - neither flat nor sharp" .)