Mouth features (6)
I believe the answer is:
deltas
'mouth features' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for deltas that I've seen before include "Triangular patches of land at mouths of rivers" , "Final sections of River" , "Lasted like triangular areas at river mouths" , "Triangular islands at mouths of rivers" , "Coastal features" .)