The common name is a contradiction since mangroves grow in seawater.
Freshwater Mangrove is a small tree that grows by (freshwater) streams and in (freshwater) swamps where the soil is supersaturated for much of the year – I guess that’s the allusion in the term “mangrove”.
See also http://www.pbase.com/donfranklin/image/162175108 and the next image for more photos of the flowers of this species.