River formations (6)
I believe the answer is:
deltas
'river formations' 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 "Estuaries" , "some flowers form them" , "Final sections of River" , "Triangular islands at mouths of rivers" , "Triangular areas where rivers divide before entering the sea" .)