Fringe wetlands along rivers rarely pass
beyond an early successional stage,
because floods flush out accumulated silt and detritus. Similarly, flood waters
moving rapidly over flood wetlands shift sediment around and break down
vegetation. Rivers in areas flat enough to develop aquatic marginal wetlands will
also tend to change their course frequently, either by gradual erosion or cat-
astrophically in flash floods, unless artificially constrained, and will cut through
wetland areas. For these reasons, wetlands along river floodplains can be very
heterogeneous, but rarely progress to terrestrial or mire stages.
If flood waters rise slowly with little
kinetic energy, then physical changes are less
likely to occur; encroachment by terrestrial species will be suppressed but other
successional processes are unaffected. Succession will remain in an arrested
state, however, if the dry season allows the mineralisation of organic layers that
have built up during the flood. If a river's ability to flood is curtailed, by damming or
embanking, then terrestrial vegetation will quickly dominate, because the wetland
has been cut off from its supply of water.
Fringing marine wetlands may go through
cyclical changes in which succession
proceeds to a given stage and then is reset by storm activity. Dry land stages will
only be reached rapidly if the wetland is actively expanding, for example through
rapid siltation.
