Marshlands (3)
(Other definitions for fen that I've seen before include "Water-logged area of vegetation" , "Slough" , "Low marsh" , "One hundredth of a yuan" , "Low marshy land" .)
(Other definitions for swamps that I've seen before include "Flooded low lands" , "Waterlogged areas of dense vegetation" , "Bogs, marshes" , "Tracts of wet spongy land" , "Drenches; fens" .)